Bulletin IS V.29 no. 3 cop. 6 NATURAL HISTORY SURVEY ILLINOIS NATUl^AL HISTORY SURVEY Bulletin Printed by Authority of tho Stilo of llllnoff Hybridization of Four Species of Sunfisfies (Centrarchidae) WILLIAM F. CHILDERS HMURAl mm\ SiiuiitY DEC 13 1967 UBVT( STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION NATURAL HISTORY SURVEY DIVISION Urbana, Illinois ILLINOIS NATURAL HISTORY SURVEY Bulletin Volume 29, Article 3 September/ 1 967 •''• 5'"'« ">' minois Hybridization of Four Species of Sunfishes (Centrarchidae) WILLIAM F. CHILDERS STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION NATURAL HISTORY SURVEY DIVISION Urbana, Illinois STATE OF ILLINOIS DEPARTMENT OK REGISTRATION AND EDUCATION BOARD OF NATURAL RESOURCES AND CONSERVATION John C. Watson, Chairman; Thomas Park, Ph.D.. Biology; L. L. Sloss, Ph.D., Geology: Roger Adams, Ph.D., D.Sc, Chemistry : Robert H. Anderson, B.S.C.E., Engineering; Charles E. Olmsted, Ph.D.. Forestry ; \V. L. EvERiTT. E.E,, Ph.D., Representing the President 0} the I'niversity of IlHtinij ; Roger E. Beyler. Ph.D., Repre- senting the President oj Southern Illinois l' niversity. NATURAL HISTORY SURVEY DIVISION, Urbana, Illinois SCIENTIFIC AND TECHNICAL STAFF George Sprugel, Jr.. Ph.D.. Chief Herbert H. Ross. Ph.D.. Assistant Chief Robert O. Watson, B.S., Assistant to the Chief Alice P. Campbell, B.A., Secretary to the Chief Section of Economic Entomology William H. I.ucrmann, Pli.D.. Entomologist and Head Willis N. Bruce, Ph.D.. Entomologist Wayne L. Howe. Ph.D.. Entomologist Ronald H. Meyer. Ph.D.. Associate Entomologist James E. Appleby, Ph.D.. Associate Entomologist Robert D. Pausch, Ph.D.. Assistant Entomologist Ralph E. Sechriest. Ph.D.. Assistant Entomologist Delmar B. Broersma. Ph.D.. Assistant Entomologist Joseph V. Maddox. Ph.D.. Assistant Entomologist Edward J. Armbrust, Ph.D.. Assistant Entomologist Clarence E. White. B.S., Research Assistant Ranu Banerjee. B.A.. Technical Assistant Douglas K. Sell, B.S., Technical Assistant Sue E. Watkins. Junior Scientific Assistant Howard B. Petty, Ph.D., Entomologist. Extension Stevenson Moore. Ill, Ph.D.. Entomologist. Extension RoscoE Randell, M.S.. Technical Assistant. Extension Stanley Rachesky, M.S.. Technical Assistant. Extension Donald E. Kuhlman, M.S.. Technical Assistant, Extension Amal C. Banerjee, Ph.D., Research Associate Jean G. Wilson, B.A., Research Associate Margaret J. Jensen, M.S., Research Assistant Keun S. Park. M.S.. Research Assistant Kftirah RriNnoLD. M.S. Research Assistant Section of Faunistic Surveys and Insect Identification IIerheri II. Ross. Ph.D.. Assistant Chief and Head Milton W. Sanderson. Ph.D., Taxonomist Lewis J. Stannard. Jr.. Ph.D., Taxonomist Philip W. Smith. Ph.D.. Taxonomist Wallace E. LaBerge. Ph.D.. Associate Taxonomist Donald W. Webb, M.S., Assistant Taxonomist John D. Unzicker, Ph.D., Assistant Taxonomist Bernice P. Sweeney, Technical Assistant Bess C. White, A.B.. Technical Assistant Section of Aquatic Biology George W. Bennett. Ph.D., Agnatic Biologist and Head William C. Starrett, Ph.D., Aquatic Biologist R. Weldon Larimore. Ph.D., Aquatic Biologist D. Homer Buck. Ph.D., Associate Aquatic Biologist Robert C. Hiltibran, Ph.D., Associate Biochemist Donald F. Hansen. Ph.D., Associate Aquatic Biologist William F. Childers. Ph.D.. Associate Aquatic Biologist Dennis L. Doolev, Technical Assistant Mary Frances Martin. Technical Assistant C. Russell Rose, field Assistant Charles F. Thoits, III, B.A., Research Associate Steven 0. Lounsberry. Project Assistant Jon Stelter. Project Assistant Section of Applied Botany and Plant Pathology j. Cti.Ric Carter. Ph.D.. I'lant Pathologist and Head Ju.MUs L. FoRSBERG. Ph.D.. Plant Pathologist Robert A. Evers. Ph.D.. Botanist R. Dan Neely. Ph.D.. Plant Pathologist Eugene B. Himelick, Ph.D.. Plant Pathologist Donald F. Schoeneweiss, Pli.D.. Associate Plant Pathologist Walter Hartstirn, Ph.D., Assistant Plant Pathologist CJe.ne E, Reid, Technical Assistant Betty S. Nelson, Technical Assistant Section of Wildlife Research Glen C. Sanhersun, PhD., Wildlije Specialist and Head Frank C. Bellrose. B.S., U'lldlife Specialist Harold C. Hanson. Ph.D., iVildhje Specialist Richard R. Graber, Ph.D.. U'lldlije Specialist Ronald F. Labisky. M.S., Associate IVitdlife Specialist William R. Edwards, M.S., Associate Wildlife Specialist William W. Cochran, Jr., B.S.. Assistant IVildtife Specialist Robert E. Greenberg, M.S., Research Associate Helen C. Schultz, M.A.. Technical Assistant Carolyn S. Evers, B.A., Technical Assistant Robert D. Crompton, Field Assistant Mary Ann Johnson. Technical Assistant Ronald E. Duzan, Technical Assistant William L. Anderson, M.A.. Research Associate James A. Bailey, Ph.D.. Research Associate Jack A- Ellis. M.S.. Research Associate Stanley L. Etter. M.S.. Research Associate William J. Francis, Ph.D.. Research Associate G. Blair Joselyn. M.S.. Research Associate Ronald L. Westemeier. B.S., Research Associate Gerald L. Storm. M.S.. Field Ecologist Jeffrey C. Hanson, M.S., Research Assistant Robert E. Hawkins, B.S.. Research Assistant Keith P. Thomas. M.S.. Research Assistant Eleanore WiLSdN, Project Asiiitaiit Section of Publications and Public Relations Owen F. Glissendorf. M.S., Technical Editor and Head Robert M. Zewadski, M.S., Associate Technical Editor Phyllis K. Bonfield. B.J., Assistant Technical Editor Wilmer D. Zeiir. Technical Photographer Richard M. Sheets. Technical Illustrator Technical Library DoKis F. Dnnns, B A,. M.S.L.S.. Technical liln Administration and Service Robert O. Watson. B.S., Assistant to the Chief Grace C. Finger. B.S., Financial Records Melvin "E. Schwartz, Property Control. Trust Accounts J. William Lusk, Mailing and Distribution Services James B. Curtis. Greenhouse Superintendent Robert O. Ellis, Garage Superintendent CONSULTANTS: Herpetology, Hobart \L Smith, Ph.D., Professor of Zoology. University of Illinois; Parasi iologv, Norman D. Levin e. Ph.D., Professor of I'elertnary Parasitology and I' elen nary Research. University of Illinois ; Wildlife Research. Willard D. Klimstra. Ph.D., Professor of Zoology and Director of Co-operative Wildlife Re- search. Southern Illinois University ; Statistics, Horace W. Norton. Ph.D., Professor of Statistical Design and Analysis. University of Illinois. CONTENTS Acknowledgments 159 The CENTRARCHroAE 160 Lepomini Evolution 161 Reported Natural Lepomini Hybrids 162 Species Selected for Study 162 Geographic Distribution 162 Diagnostic Morphological Characters 162 Habitat Selection 162 Reproduction 163 Time of Spawning 163 Location of Nests 166 Spawning and Care of Young 167 Duration of Fertility of Gametes 167 Hybridization Experiments 170 Stripping Experiments 170 Methods and Materials 170 Results and Discussion 171 Isolation Experiments 181 Methods and Materials 181 Results and Discussion 181 Hybrids Reared in Ponds 184 Sex Ratios 184 Fecundities 185 Hybrid Vigor 187 Rate of Growth 187 Electrophoretic Patterns of Hemoglobins 189 Vulnerability to Hook-and-Line Capture 189 Hybrid Sunfishes for Sport Fishing 189 Summary 190 Literature Cited 192 Appendix 194 Index 211 Color Plate, parent and hybrid sunfishes following page 184 This report is printed bij authority of the State of Illinois, IRS Ch. 127, Par. 58. 12. It is a contribution from the Section of Aquatic Biology of the Illinois Natural lli.stonj Survey. (39846—r.OOO—9-67) Frontispiece,-A green sunf.sh x blucg.ll F. ^^^l!^^^'^^ ^,^::'f:'u^^'^e:^''^ female bluegills in a pond containing no other 3' estimates that there are 15.(KK) and 17.000 lU-cent fish nd Slastenenko (1957:76-91) 167 known natural interspe- cific fisllivbrids oi the world. C'rossman 1 1965:1261) suggest the pos- three additional Inbrids in f the Ksc dae. ApproximateK iX) pircent 170 h\brids were found in rs; the rest occurred in marine sh water environments, .\bout s of the freshwater hybrids nd in .North .\merica. Ilubbs ,18-19) pointed out that hy- m has probabK- been most in North .\merican freshw aters le existing fish fauna became ed only as recentb as the Mio- ocene. and Pleistocene epochs. Iso remarked that a consider- 1 iy of circumstantial e\ idence 1 that introgressi\e h\bridiza- been a significant factor in m in the tribe Lepomini of the Centri hidae. The erm liybrUl has been variously define (Darxvin 1897:1-2; Darlington 195S;. ; Stebbins 1959:231). Unless othen nts. Sacramento perch Roanoke bass Rock bass Mud sunfish Black crappie White crappie Flier Banded sunfish Bluespotted sunfish Blackbanded sunfish Warmouth Bantam sunfish Green sunfish Bluegill Orangespotted sunfish Pumpkinseed Redear sunfish Spotted sunfish Dollar sunfish Redbreast sunfish Longear sunfish Largemouth bass Smallmouth bass Redeye bass Suwannee bass Spotted bass Guadalupe bass September, 1967 Childers: Hybridization of Sunfishes 161 nia. In the last 100 years many species have been widely introduced into fresh- waters throughout North America and in other parts of the world. Lepomini Evolution On the basis of the fossil record, current natural geographic distribution, and comparative morphology, the evo- lution of the Lepomini can be hypoth- esized as follows: 1) The Centrarchidae date from the early Cenozoic and are closely related to the sea basses (Ser- ranidae) (Miller 1958:199). 2) The Mississippi River basin was probably their center of origin (Branson & Moore 1962:88). 3) A relative abundance of extinct centrarchids in Miocene and Plio- cene rocks of Oregon, Nevada, and Utah indicates that the Cen- trarchids' range was much larger then than it is now (Miller 1958: 193, 199). 4) The Rocky Mountain uplift, be- ginning in the Miocene or early Pliocene and increasing to the end of the era ( Schuchert & Dun- bar 1941:386) isolated west coast Centrarchids from those east of the Rocky Mountains. 5) Fossils of the extant species war- mouth and black crappie have been found in middle Pliocene deposits in Logan County, Kan. (Branson & Moore 1962:96). 6) Late Pliocene to early Pleistocene deposits in southern Idaho and eastern Oregon contain a fossil- ized sunfish which is probably of the genus Lepomis (Miller 1958: 194). 7) During the Pleistocene the west Lepomini hybrids known to occur in nature. Kind of Hybrid Warmouth x Pumpkinseed Warmouth x Redbreast sunfish Warmouth x Green sunfish Warmouth x Bluegill Green sunfish x Bluegill Green sunfish x Pumpkinseed Green sunfish x Longear sunfish Green sunfish x Redbreast sunfish Green sunfish x Red-ear sunfish Green sunfish x Orangespotted sunfish Bluegill x Red-ear sunfish Bluegill x Pumpkinseed Bluegill X Orangespotted sunfish Bluegill X Longear sunfish Bluegill X Redbreast sunfish Pumpkinseed x Orangespotted sunfish Pumpkinseed x Redbreast sunfish Pumpkinseed x Longear sunfish Longear sunfish x Orangespotted sunfish Warmouth x Red-ear sunfish Bluegill X Spotted sunfish Reference RadcliflFe (1914:27)^ McAtee&Weed (1915:13)i McAtee & Weed (1915: 13 )i Hubbs (1920:102)2 Bailey & Lagler ( 1 938:.588-604 )2 Bailey & Lagler (1938:588-604 )2 Cross & Moore (1952:410-411)- Raney (1940: 364 )i Trautman ( 1957:501 )i Hubbs & Ortenburger ( 1929:42 )i Cross & xVIoore (1952:411)- Bailey & Lagler (1938:588-604)- Cross & Moore (1952:411)- Cross & Moore (1952:411)- Bailey & Lagler (1938:577)' O'Donnell (1953:487)' Greeley & Bishop (1933:101)' Hubbs (1926:72)' O'Donnell (1935:487)' Childers ( unpublished ) Stinauer & Childers (unpublislied) 'Cont.iins no description. ^Contains description. 162 Illinois Natural History Survey Vol. 29, Alt. 3 coast species were probably re- stricted to a southern coastal dis- tribution and are represented today by one relic species, the Sacramento perch. 8) During the Pleistocene northern species east of the Rocky Moun- tains withdrew in a southeasterly direction or became extinct. 9) Speciation in the genus Lcpomis probably has proceeded at a rapid rate during the Recent epoch. sunfish, a more southern species, was successfully introduced into this area in 1946 (Bennett 1958:177). Diagnostic Morphological Characters Forbes & Richardson (1920:245-251, 257-259) and Trautman (1957:496- 504, 516-518) give good morphological descriptions of the four experimental species. Certain key morphological characteristics of the four species are presented in Table 1. Reported Natural Lepomini Hybrids It is theoretically possible for the 11 species of Lepomini to hybridize in 110 different Fi combinations; however, since it appears impossible morpholog- ically to differentiate between hybrids of reciprocal crosses ( Hubbs & Hubbs 1932:433), only 55 morphologically dif- ferent Lepomini Fi hybrids could be identified. Of this number at least 21 have been found in nature. In the list of naturally occurring Lepomini hy- brids (page 161), an attempt has been made to give credit to the author of the first published description of each kind. SPECIES SELECTED FOR STUDY Four species of sunfishes in the tribe Lepomini (red-ear sunfish, bluegill, green sunfish, and warmouth) were selected as experimental species be- cause of local availability; importance to sport fishing; taxonomic relation- ships; and similarities and differences in their morphology, habitat selection, and reproductive behavior. Geographic Distribution The natural geographic ranges of the foiu- species greatlv overlap one another (Tautman 1957: 497, 500, 504, 517). All four species are sympatric in east-central Illinois, and they are quite abundant in a number of lakes and ponds in this area. Bluegills, green sunfish, and warmouths are indigenous to east-central Illinois, and the red-ear Habitat Selection Larimore (1957:2), in discussing the distribution of the warmouth in Illinois, stated that although the warmouth is principally a pond and lake fish, it occurs in the Rock, Mississippi, and Illinois rivers and is reported as com- mon in small, sluggish streams in the southern part of the state. In east- central Illinois \\'annouth are only oc- casionally found in streams. Many of the creeks and some larger streams in this area have been dredged and are unsuitable for most species of fishes. The undredged portions of these streams are probably unsuitable for warmouths because their current \'eloc- ities are greater than \\'armouths can tolerate. Trautman (1957:498) reports that in Ohio The Warmoutli Sunfish ^xas most mimcrous in lakes, ponds, oxbows, marshes, and streams of base or very low gradients which had s'lt-free water, an abundance of aquatic vegetation, and a mucky bottom which was often covered with organic debris. The species was present only in small numbers in weedless oxbows and ponds which liad a >'ellow-silt bottom, and al- though its colloquial name was "Mud Bass" it seemed to be less tolerant to turbidity and siltation than was the Green Sunfish. The green sunfish is abundant in creeks and small rivers in east-central Illinois (Forbes & Richardson 1920:250; Larimore & Smith 1963:325). This spe- cies is adept at ascending small tem- porary streams formed by overflow waters from lakes and ponds. Green sunfish are prolific and frequently gain access to a new lake or pond before other species of sunfishes. When this September, 1967 Childers; Hybridization of Sunfishes 163 occurs, they commonly produce such large populations that the individuals become stunted. Green sunfish are usu- ally unable to compete successfully with other species of sunfishes which typically inhabit clear-water lakes and ponds. Trautman (1957:501) stated that the green sunfish is more tolerant of tmbidity and siltation than other sunfishes except the orangespotted; however, the largest populations were found in clear-water habitats under conditions of low competition with other sunfish species. In east-central Illinois the largest populations of bluegills and red-ear sunfish occiu- in lakes and ponds which have relatively clear waters. Forbes & Richardson (1920:258) found that the bluegill occurred throughout Illinois, but it was generally limited to the larger streams and their principal trib- utaries and was common in north- eastern glacial lakes. During the past 30 years bluegills have been stocked in thousands of Illinois lakes and farm ponds by federal and state agencies (Bennett 1962:104). In 1951 the Illinois Department of Conservation obtained red-ear sunfish breeding stock from Dr. G. W. Bennett of the Illinois Natural Histor\' Survey. These adult fish were offspring of the red-ear sunfish which were introduced into east-central Illinois from Indiana in 1946 (Lopinot 1961:3). From 1951 to 1964 the Illinois Department of Con- servation stocked 1,383 lakes and ponds with red-ear sunfish, and this species has been widely distributed throughout the state (W. J. Harth, personal com- munication). Trautman (1957:518) remarked that wherever the red-car sunfish has been introduced into waters which are north of its natural range, it has essentially inhabited nonflowing waters which were relatively clear and contained at least some aquatic vegetation. Traut- man also stated that at Buckeye Lake, Oliio, the red-ear sunfish seemed to re- quire as much as, or more aquatic vegetation than, did the bluegill, and that although both species frequented open water, the red-ear congregated about brush, stumps, and logs more than the bluegill. Reproduction The reproductive activities of the four kinds of sunfishes were observed over a 7-year period, from 1958 through 1964, in a number of lakes and ponds within 50 miles of Urbana, 111. The most frequent observations were made in Big Pond ( owned by William Utterback and located 5 miles south- east of Gibson City, 111.) and Lake Italy (owned by the Material Service Cor- poration and located 3 miles south of Fairmount, 111.) Big Pond contained bluegill, red-ear, and green sunfishes, and Lake Italy contained all four species. Time of Spawnl\g.—For all four spe- cies, males in spawning condition were first collected each year during late April or early May. The first ripe fe- males were collected during the 2nd or 3rd week of May. The first fish to become ripe were invariably large indi- viduals. Ripe individuals from stimted populations of bluegills and green sun- fish were first collected 2-4 weeks later than from nonstunted populations. Ripe males and females of all four species were collected each month, May througli August; however, ripe indi- viduals were much less abundant dur- ing July and August than during May and June. The latest observed fall spawning occurred in Big Pond during the 1st week of September, 1960. Big Pond is 7iaturally divided into three areas which arc connected by two short, narrow, shallow channels. On August 24, 1960, the three areas were separated by placing heavy canvas bar- riers across both channels. Two areas were treated with rotenone. On Sep- tember 5 both treated areas were in- spected to determine if any fish had 164 Illinois Natural History Survey Vol. 29, Art. 3 -a o E njin u — September, 1967 Childebs: Hybridization of Sunfishes 165 C o 3 ,£ 166 Illinois Natural History Survey Vol. 29, Art. 3 survived the rotenone treatment. The fish kill appeared to be complete in one area, but in the other approximate- ly 30 male bluegills were occupying nests. All nests contained either eggs or larval young. A careful inspection of the untreated area failed to reveal a single nesting sunfish. Swingle (1956:865) suggested that certain species of fishes secrete or ex- crete a hormone-like substance which acts as a repressive factor and inhibits reproduction in ponds containing dense fish populations. Apparently the rote- none treatment with its resulting dras- tic reduction of the fish population stimulated the few surviving bluegills to reproduce within about 9-11 days during a period which was somewhat later than their normal spawning season in east-central Illinois. In Alabama, red-ear sunfish spawned in the spring when surface water reached a temperature of about 24° C.(75° F. ), reproduced sparingly or not at all during the summer, and again spawned heavily in the early fall (Swingle 1949:299). I have observed no extensive fall spawning of red-ear sunfish in any east-central Illinois lakes and ponds. In Fork Lake, 111., during 1939, the bluegills of both sexes had gonads in spawning condition during June, July, and August. Males matured earlier than females and large males became sexu- ally mature earlier than smaller males. Nests were first observed on May 28 when the water temperature at 3 feet was 25° C.(77° F.). Occupied nests were last observed on September 18 (Bennett, Thompson, & Parr 1940:17- 18). In the Gardner Ponds at the Univer- sity of Wisconsin Arboretum the spawning season of green sunfish com- menced in late May or early June when the water temperature reached about 21° C.(70° F.), continued through June and July, and terminated in early August. Apparently larger males spawned earlier and more frequently than smaller males (Hunter 1963: 16-18). In Park Pond, Vermilion County, 111., warmouth spawning was initiated dur- ing the 2nd week in May, 1949, when the water temperature at 12 inches was approximately 21° C.(70° F.). Gonad- al weight-body weight ratios indicated that most spawning was completed by early July. Warmouths of less than 89 mm (3.5 inches) total length failed to spawn. Males ripened earlier in the season than females and large fish spawned earlier than smaller ones (Larimore 1957:31-35). Location of Nests.—The first evi- dence of reproductive activity in the spring was the movement of males into shallow water. As the length of the photoperiod and the temperature of the water increased, males constructed nests (saucer-shaped depressions in the substrate) which they defended with great vigor. All four species usually nested in areas where the water was less than 3 feet deep. Red-ear sunfish, bluegills, and green sunfish normally nested in colonies, on firm substrates, and often in locations exposed to the sun. \Var- mouths were more solitary in their nest site selections. They frequently nested on soft substrates even \\'hen firm substrates were available. Lari- more (1957:40) reported that war- mouths were not as consistent in selecting a particular type of substrate as they were in selecting a spot near a stump, rock, root, clump of vegeta- tion, or some similar object, and that nests were never found on an area of bottom completely exposed, such as was usually chosen by the bluegill. In Utterback's Big Pond and Lake Italy, red-ear sunfish males and bluegill males were frequently found nesting together in the same colony. Green sunfish males were less commonly found nesting with males of the other species; however, this difference may September, 1967 Childers: Hybridization of Sunfishes 167 have been related to the smaller num- bers of green sunfish in both bodies of water. Wannouth males were never observed nesting in colonies. In colonies of nests occupied by more than one species, males of the minority species formed a subcolony within the larger group. Spawnixg and Care of Young.— In general the four species are remark- ably similar in their spawning and parental behavior. During spawning a pair slowly swims side by side in tight circles over the male's nest. Fertiliza- tion is external and the demersal eggs adhere to the material forming the bottom of the nest. After spawning, the female leaves or is driven from the nest by the male, and the male fans the eggs and larval young until they become free-swimming fry. During fanning, the male hovers over the nest while un- dulating his body in such a way that currents of water are directed down- ward into the nest. Fanning can best be described as stationary swimming. The water currents thus produced are prob- ably important in cleansing and oxy- genating the developing embryos. During this period, the male also pro- tects the eggs and young and will viciously attack predators much larger than himself. Duration of Fertility of Gametes. —Since three of the four species select- ed for study sometimes nest in mixed colonies, the functional life spans of gametes could be very important in controlling hybridization between these species. If gametes arc capable of fer- tihzing and being fertilized over long periods of time, sperm driftage could result in the production of hybrid individuals. Experiments were con- ducted to determine the functional life spans of bluegill, green sunfish, and warmouth gametes. In one set of experi- ments, both sperm and eggs were aged for various periods of time prior to fer- tihzation; in another group of experi- ments only eggs were aged. Ripe male and female bluegills, green sunfish, and warmouths were cap- tured by seining and trapping in local ponds. 1 These fish were moved into the laboratory and separated in aquaria according to species and sex. Fish were held in these aquaria for one-half hour to 2 hours before gametes were stripped from them. Care was taken to avoid any temperature shock to the fish prior to their use in the experiments. In the experiments in which both sperm and eggs were aged prior to fertilization, the method was: Five clean glass petri dishes were individu- ally numbered from 1 to 5. Each dish was then partially filled by adding 20 ml of water. All of the water used in these experiments was obtained from the well on Parkhill's Lake Park Sub- division Number Two. The water was moved into the Ilhnois Natural History Survey laboratory and stored in a 210- gallon aquarium. It was aerated and filtered through activitated charcoal for at least 1 week prior to its use. A partial chemical analysis of tliis water is presented in Table Al in the appendix. Starting with dish 1 and ending with dish 5, eggs from a ripe female war- mouth were stripped into each of the five dishes. Immediately after eggs were stripped into a dish, the dish was gently shaken to scatter the eggs over the bot- tom. Eggs were stripped into consecu- tive dishes at approximately 7-second intervals, and so the entire egg-strip- ping process was completed in about 30 seconds. During the next 15 seconds, one-half ml of seminal fluid was strip- ped from a ripe male warmouth and diluted with 10 ml of water. One ml of this solution was then added to the water and eggs in dish 1, and to the *A11 .siinfishcs used in these experiments were ob- tained from these IMinois ponds; bhiegilis and green sunfish frum Pifers Pond, about 3 miles southeast of SuUivan, and Utterback's Biff Pond, 5 miles southeast of Gibson City; blueffills from Redhead's Pond. 4 miles east of Homer; warmouths and green sunfish from Lake of the Woods, 2 miles northeast of Ma- homet; warmouths from Taylor's Pond, 3 miles south- west of Fairmount. 168 Illinois Natural History Survey Vol. 29, Art. 3 otlier dishes in sequence after intervals of 2.5, 5.0, 7.5, and 10.0 minutes. From 5 to 10 minutes after the gametes were mixed in each dish, the zygotes were washed three times (by decanting and refilling each dish with clean water) and then enough water was added to each dish to cover the eggs. Water in each dish was changed several times during incubation, and dead eggs and embryos were removed. Newly hatched larvae were transferred to clean, numbered dishes. The number of eggs in each dish at time of fertiliza- tion, the number of eggs which hatched, and the number of larvae which developed into normal-appearing swim-up fry were recorded. The in- cubation temperature was recorded with an air thermograph located di- rectly alongside the petri dishes. The same procedure was followed in measuring the functional life spans of bluegill and green sunfish sperm and eggs, except that six egg samples were stripped from each female and the time interval separating the mixing of gametes in the sequence of dishes was 1.0 minute instead of 2.5 minutes. In the second group of experiments only the eggs of the three species were aged prior to fertilization. The pro- cedure of the first group of experiments was used, except that two drops of undiluted seminal fluid were stripped directly on the eggs after they had been aged for 0.5, 30.0, 60.0, 120.0, and 180.0 minutes. The results of individual experi- ments concerning the functional life spans of activated gametes of bluegills, green sunfish, and wamiouths are pre- sented in Tables A2-A13 of the appen- dix. Data from experiments for each species were pooled and are presented in Tables 2-4. Under the conditions of these experi- ments "average functional lives" ( length of time gametes were aged that resulted in a 50-percent reduction in fry viability) of warmouth, bluegill, and green sunfish eggs were interpo- lated to be 94, 60, and 47 minutes, respectively. Specific differences may have been the result of variation in the physiological state of the mature eggs and the exposure of the various samples to uncontrollable environmen- tal difi^erences. All of the eggs in an individual exper- Table 2.—Duration of fertility of activated warmouth gametes. Data from experiments W1-W4 are combined. Age of Eggs in Minutes September, 1967 Chiluers: Hybridization of Sunfishes 169 Table 3.—Duration of fertility of activated bluegill gametes. Data from experiments BI-B4 arc combined. Age of Eggs in Minutes 170 Illinois Natural History Survey Vol. 29, Art. 3 oxygen tensions were probably variable and more critical in dishes where mor- tality was high. Chance contamination of some samples by bacteria and pro- tozoans also may have resulted in en- vironmental differences in various samples. The average functional life spans of sperm (based on the age of the spemi in the experiments in which both eggs and sperm were aged) from war- mouths, green sunfish, and bluegills were interpolated to be 1.1, 1.0, and 1.0 minutes, respectively. Specific dif- ferences in the results of these experi- ments were probably not valid because of the factors previously described. The average functional life span of eggs from the three species was 67 minutes, and for sperm it was 1 minute. Functional life spans of gametes from red-ear sunfish were not investigated; however, they are probably similar to those of warmouths, bluegills, and green sunfish. The brief functional life spans of the spermatozoans of these species are un- doubtedly very important in reducing hybridization caused by sperm drifting from nest to nest. HYBRIDIZATION EXPERIMENTS Two types of experiments were used to produce hybrid sunfishes. In the first, referred to as "stripping experiments," gametes were stripped from ripe adults and manually mixed. \Vith this method it was possible to determine species isolation due to incompatibilities be- tween sperm and eggs ( primary genetic isolation). In the second type, desig- nated "isolation experiments," one or more pairs of fish composed of a male of one species and a female of another were isolated in small ponds to deter- mine if they would hybridize when mates of their own species were absent. In this paper R refers to red-ear sunfish, B to bluegill, G to green sunfish, and W to warmouth. Matings between individuals of different species are des- ignated as P, crosses, and the resultant hybrids are designated as Fi hybrids. F:; hybrids are those produced by mating an Fi male with an Fi female. Tlie male parent species is always given first; tlius, the Pi cross of a male bluegill and a female green sunfish is desig- nated B X G and the resultant hybrids are designated BG Fj hybrids; GB Fi designates the reciprocal hybrids. Stripping Experiments Sperm and eggs stripped from the four parent species were paired in 16 different combinations to produce zy- gotes representing the four parent spe- cies and 12 hybrids. These experiments were designed to allow comparisons of rates of embryological development and the extent of viability of Fj hybrids and their maternal parent species. Methods and Materials.—Ripe males and females of the four species were brought into the laboratory from nine local ponds. Laboratory treatment of these fish was the same as for those used in experiments concerned with functional life spans of gametes. A ripe female of one of the four species and one ripe male of each of the four species were used in each experi- ment. No individual fish was used more than once. Fish selected for an experi- ment were individually isolated for at least 30 minutes before gametes were stripped, and the person doing the stripping rinsed and dried his hands after handling each fish. Twelve clean petri dishes were indi- vidually marked and 20 ml of aged, filtered well water were added to each dish. A sample of eggs from one ripe female was stripped into each of the 12 petri dishes, and the eggs were scat- tered by gently shaking the dishes. Two drops of milt were then stripped into each dish. Milt from one male of each of the four species was used to fertilize the eggs in three dishes. An entire strip- ping program for the five fish was completed in less than 5 minutes. Ap- proximately 10 minutes after the strip- ping was completed, the zygotes were September, 1967 Guilders: Hybridization of Sunfishes 171 washed three times by decanting and refilUng each dish with clean water. During incubation the amount of water in each dish was regulated so that the de\'eloping embryos were always cov- ered with a thin layer (2-8 mm) of water. Dead embryos were removed, and the water co\'ering the lixing embryos was changed several times during each experiment. The frequency with which dead embryos were removed and water was changed was varied according to the incubation temperature. In the ex- periments with the highest (28.6° C.) and the lowest (22.3° C.) mean incu- bation temperatures the intervals were approximately 5 and 24 hours, respec- tively. Larvae were transferred to clean, numbered dishes within 1 hour after hatching. An air thermograph was used to re- cord temperatures adjacent to the dishes containing the embryos. The maximum range of fluctuation of air temperatme during any one experiment was 3- C. Hourly air temperature fluc- tuations never exceeded 0.7"' C. Since the petri dishes contained relatively small amounts of water and since air temperature fluctuations were slight, water temperatures were considered to be the same as air temperatures in these experiments. Records were made of the number of eggs in each dish at the time sperm and eggs were mixed, the number of eggs that hatched each hour, and the number of larvae that developed into morphologically normal-appearing swim-up fry. Upon termination of each experiment, all living fry were killed with a 4-percent aqueous solution of formaldehyde and stored in a 1-perccnt solution. Total body lengths of 25 morphologically normal-appearing fry of each kind of viable fry from each experiment were measured to the nearest 0.03 mm with an ocular micrometer. A total of 11 stripping experiments was conducted: Eggs from three red- ear sunfish, three bluegills, three green sunfish, and two warmouths were ferti- lized with sperm from males of all four species. The temperatures at which these experiments were conducted were well within the range of temperatures that embryos of the four species are subjected to under natural conditions. Nine of the experiments were ter- minated when the zygotes developed into swim-up fry. The other two ex- periments, both of which were con- ducted with red-ear sunfish eggs, were terminated shortly after the fry became free swimming. Results and Discussion.—The per- centages of eggs that hatched and the percentages of eggs that developed into morphologically normal-appearing fry were calculated for each of the 132 samples of the 11 experiments (Tables A14-A24 of the appendix). These per- centages were transformed into degrees of a right angle to minimize bias inher- ent in using weighted percentages in an analysis of variance ( Fisher & Yates 1963:74-75). A 7094 digital computer was used in analyzing these data. Data from the 11 stripping experiments were condensed and are presented in Tables 5-8. Preliminary tests revealed that high percentages of eggs hatched in some petri dishes containing as many as 500 eggs; however, mortality was higher in dishes containing 400-500 eggs than in dishes containing 200-300 eggs. Since the number of eggs per sample was a variable in these experiments, the num- ber was purposely kept low (mean number of eggs per sample was 65) to minimize the effect of crowding. An analysis of variance revealed that there was no significant correlation between the number of eggs per sample and the percentage that hatched. Consecjuent- ly, the number of eggs per sample was used as a statistical weight in the anal- ysis of the viabilities of the 16 dif- ferent kinds of zygotes. Data from the 11 stripping experi- ments pertaining to the percentages of eggs that hatched and tlie percentages 172 Illinois Natural History Survey Vol. 29, Art. 3 I m -o c Ea o J3 E 5 ^ o en inO CO CD OO r-( ^ - ^ t^ '^ ^ be 1= E S;- £q Si tic Q "I ^te = •»- lo in Lo o (M in CD CO 00 September, 1967 Childers: Hybridization of Sunfishes 173 _3 J3 E fO 1- !3.2 o £ f3 ^ fc f3 B,a U IM 00 rf t~ O * 00 c-l 00 01 <}< TJ< lo lo in o ci oi oi CD O — 1 O in t^ * O —I TJI^ lO CO 00 00 o ro 05 o in in in o oi c-i c-i CO CO CO o 00 05 00 CO ro in t~ g J) 2^2 (a S ? to d lO tr~ C! Ol C^ O t^ COo o o •-^ c c d d 05 01-^0 t-- CD t- CD TJ^ tT "^ ^ in >D ID lO CM (M (N CO t~ CO t~ t~ O] C-l Ol Ol
i> i> in c o o o •Tl t- ^ ol TT c CO CD in CO « oi ^ Ol Ol p p in t~7 d 00 d od in -^ -^ ^ CD CO d 'a U S (u CO CO "^ Tt^M t^ CO t- 00 3 O CO -^ ol Ol -G in CO 00 in o o o u >! X( X ^ O PC M ^ o 00 'c .§ 0; a 00 in o COO C75 Ol O; in i> Tt< 00 — ; rt —; d CO CO CO Ol CD i-H -^ ^H» 00 t~ 00 CO CO Ol TPO O C:5 -H CO Ol ^ Ol O O O U X K X >< O a cQ ^ t ^ ^ oq 00 O O 00^ Ol ol rt d d d d ^ tt ^ ^ in in in in Ol Ol Ol Ol CO f- CD t- r~ t~- CO in Ol -—I -H Ol CO CO CO CO d d d d U in d CO CD CD CD _ l-^ l> t-^ 1^ S ol O! Ol Ol n in m t~ CO -^ t^ ^ in Tf o OO — I TfH O O d f-^ 00 d Oj Ol Ol Ol Ol Ol t^ Ol 00 00 t~ t~ CD o 00 05 X t~ i> in Ol t- in .-H ^^ Ol Ol CO X X X ^ O « m ^ i: tot; o u-^ ^ cS 0; llg-l a " Seg E j: " «.S = « *j 'n « N c :e So.* 3% S-Se II M— rt u c: September, 1967 Childers: Hybridization of Sunfishes 175 o s > ZS lip 3 tl y = = I 6CQ CO c; o ci t_ ^ ^ C] c c o d 1/3 CO M" CO t^ CO t-; t^, ^ TT Tf •^' in in >n in oq o^ CM ol in CO in in Tt< Tf TT CO CO CO CO CO CO CO CO CO Tj< Cj5 ^ COO 05 -»< O C-T -< ^ O 1> 05 in in in »n c-i Ol '. The mean hatching time and stand- ard deviation were calculated for each of the 132 samples of eggs in the 11 experiments (Tables A14-A24 of the appendi.x). A statistical weight was calculated for the mean hatching time of each sample by dividing the number of eggs that hatched by the variance of the mean hatching time. A 7094 digital computer was used in the analysis of variance of these data. Tlie statistical analysis, in which the weigjited mean hatching time of each kind of hybrid was compared with that 178 Illinois Natural History Survey Vol. 29, Art. 3 Fig. 2. — Bluegi 1 1 fry (above) and WB F, hybrid fry (below) selected to show the range of morphological deformities of the hybrid fry. All fry were produced from eggs from one female bluegill. Both kinds of zy- gotes were allowed to de- velop simultaneously under similar conditions for 113 hours before being sacrificed. The mean hourly tempera- ture was 26.9° C. (80.4° F.), standard deviation, 0.48° C. (0.86° F.). of its maternal parent species, revealed that WB Fi hybrid zygotes hatched sig- nificantly sooner than pure bluegill zygotes when both kinds of zygotes were incubated at the same tempera- tures. Although WB zygotes hatched in less time, the newly emerged WB Fi larvae were not as advanced in their development as the unhatched pure bluegill embryos. WR F, liybrids were not significantly different from pure red-ears in hatching time; however, the newly emerged WR larvae were not as advanced in their development as the pure red-ear larvae. There were no statistically significant differences in the time of hatching between the other 10 kinds of hybrids and their respective September, 1967 Childers: Hybiudization of Sunfishes 179 maternal parent species, and differ- ences in the degree of development between the hybrids and their respec- tive maternal parent species were not pronounced. The mean body length and standard deviation were calculated for each kind of viable fry from each experiment (Tables 5-8). An analysis of variance in which the mean body length of each kind of viable hybrid fry was compared to that of its maternal parent species revealed that BR and GR were sig- nificantly (0.05 level of probabiUty) longer bodied than the pure red-ear fry. The other eight kinds of hybrids were not significantly different from their respective maternal parent species fry in total body length. The alpha temperature threshold of development (Shelford 1927:357) and the mean number of developmental units (degree-hours of effective tem- perature) necessary for 50-percent hatching were calculated for each of the 16 kinds of zygotes. The statistical method was designed by Dr. H. W. Norton'^ to determine the linear cor- relation of two variables (T and R) when both variables are subject to error. Given: Equation I (T-A)t = U where T = mean hourly incubation tem- perature A = alpha threshold of develop- ment t = mean hours of incubation necessary for 50-percent hatching U = number of developmental units necessary for 50-percent hatching Then: Equation II _ (W + WjW-) (T-UR)~ (W + V/U2) 'Professor of Statistical Design and Analysis, Ani- mal Science Department, University of Illinois, Ur- bana. where W = 4 ( estimated on the basis of the accuracy with which the thermograph could be adjusted and read ) v= '^"'^' nvb+3vw (three times the number of zygotes which hatched, times the fourth power of the mean hours of incubation, di- vided by the number which hatched, times the pooled between variance of the hatching times, plus three times the within variance of the hatching times ) R = 1/t (reciprocal of the mean hours of incubation ) Then: Equation III 2[(W+V/U^) R (A-t-UR-T) -V/U-'(A-fUR-T)-] =0 T, V, R, and W values were calcu- lated for each kind of larvae for each of the 11 experiments. A 7094 digital computer was programmed to select all T, V, R, and W values for one kind of larva and to determine by a trial- and-error method the U value that best satisfied equation III. An estimate of the goodness of fit of T and R values to the linear regression line U was determined by this equation : S=(W+V/U=) (A+UR-T)2 Only two experiments were con- ducted using warmouth eggs, and the mean incubation temperatures of these two experiments differed by only 0.3° C. Consequently, the alpha thresholds and the numbers of developmental units necessary for 5()-percent liatching of warmouth and tlie three kinds of hy- brid zygotes i^roduced from warmouth eggs are not reliable. The alpha thresh- olds, numbers of developinental units necessary for 50-percent hatching, and S values for the other 12 kinds of zy- gotes are presented in Table 10. The / test comparisons revealed that 180 Illinois Natural History Survey Vol. 29, Art. 3 Table 10.—Alpha temperature thresholds of development and numbers of developmental units (degree-hours of effective temperaturel necessary for 50-percent hatching of red-ear sunfish, bluegill, green sunfish. and nine different kinds of hybrid sunfish zygotes. I See Tables 5-8 for mean incubation temperatures.) Patent Species" September, 1967 Guilders: Hybridization of Sunfisiies 181 In the 11 experiments reported here, a high positive correhition between liigh mortality prior to hatcliing and premature hatching should be apparent if environmental dissohed oxygen levels became critical enough to affect the time of hatching. An inspection of the data revealed no such correlation; consequently, in these experiments dif- ferences in the sizes of eggs from dif- ferent females appear to be a more likely source of experimental error than low levels of environmental dissolved oxygen. Isolation Experiments Thirty-two isolation experiments were conducted from 1957 through 1963. Sixteen of these experiments, all of which involved intrageneric Lcpomis matings, were reported by Childers & Bennett (1961:6). VIethods and Materl,\ls.—Males of one species and females of a different species were isolated in small earthen ponds (0.02-0.90 acres). Each of the 12 possible hybrid-producing combina- tions was tested in one or more ponds. Shortly before the ponds were stocked with the parent species, each pond was drained or treated with rotenone to eliminate any fish which might be pres- ent. Parent species were stocked during late May or early June and the ponds were checked at approximately monthly intervals throughout the summer. The following August or September each pond was drained, seined, or treated with rotenone, and small fish (if present) were identified. Three of the .32 ponds had become contaminated with other fish, and two ponds dried up during the late summer. No hybrids were found in these five ponds. Results of the other 27 experiments are re- ported in Table 11. Results and Discussion.—R x G, G X B, and W x G pairings successfully hybridized each time they were tested. R x G and G x B were each tested in two ponds and W x G was tested in one. The B X G cross, not included in Table 11, was attempted in two ponds. Both ponds became contaminated with male green sunfish, and large numbers of green sunfish young were produced in both ponds. Consequently, both ex- periments gave no test of hybridization between bluegill males and green sun- fish females. The R X B cross was attempted in four ponds. No hybrids were produced in three ponds although the ponds re- mained full and were uncontaminated by other fishes. Eleven small fish were found when the fourth pond was drained, and these fish were believed to have been RB F, hybrids although they were not positively identified as such (Childers & Bennett 1961:6). The water in this pond contained a high and constant clay turbidity that re- duced the transparency of the water and caused the parent fish to be ex- tremely pale in body color. The nor- mally scarlet portions of the opercle tabs of the red-ear males appeared as a faint rose color. The R x B cross has been attempted three times in Indiana without obtaining offspring (Krumholz 1950:113). The B X R cross was set up in four ponds. Three of these (not included in Table 11) were improper because of contamination by bluegill females in one and complete loss of water in the other two during the late summer. The fourth test appeared to have been valid but no hvbrids were produced. Smitherman & Hester ( 1962:.335, 3.37) attempted R x B and B x R crosses by stocking single pairs in plastic pools 9 feet in diameter and 2.5 feet deep. Each cross was attempted in two pools, and hybridization failed to occur in all four trials. In contrast to these results, the B X R cross has been producti\e of hybrids six times in Indiana ( Krum- holz 1950:113). Adults for the R x W cross were stocked in two ponds. One pond be- came contaminated with male and female green sunfish, and when the 182 Illinois Natural History Survey Vol. 29, Art. 3 Table 1 1.—Results of isolation experiments. Adult males of one species and females of another were isolated in each pond in May or June, and ponds were censused in August or September, 1957-1963. P, Cross' September, 1967 Childers: Hybridization of Sunfishes 183 In each of the 32 isolation experi- ments males constructed nests and re- mained in spawning condition for ex- tended periods throughout the summer. The failure of certain Pj crosses to hybridize was believed to result from incompatibilities in the reproductive behavior patterns of the various species and not because males and females failed to be in spawning condition at the same time. Fish hybridization might result from sperm driftage or interspecific matings. Sperm driftage is an important cause of hybridization among certain species of fishes, particularly minnows and dart- ers, which live in flowing water habitats and simultaneously spawn in close proximity to one another ( Hubbs 1955: 10, 16). Spemi driftage may also ac- count for some hybridization between pond- or lake-dwelling centrarchids; however, since average functional life spans of sunfish spermatozoans are so brief (Tables 2-4) and since there is such good synchronization in the re- lease of sperm and eggs by a spawning pair, most hybrid sunfish are probably the result of interspecific pair formation. The four experimental species are sexually dimorphic, closely allied, sym- patric species. Signals that ai-e in some way involved in reproductive isolation of such species are likely to be highly divergent ( Marler 1957:35) and may involve specific differences in shape, color, special movements, sounds, scents, etc. (Tinbergen 1951:56). The precise signals which are operative in conspecific pair formation of the four experimental species are not known; however, specific differences in color of opercle tabs, eyes, cheeks, and pelvic fins of nest-guarding males ( Tabic 1 ) may he important in controlling the be- havior of ripe females. When a female ready to spawn approaches a nest- guarding male, she usually stops some distance from the nest and the male exhibits a courtship display (Miller 1963:118). Species recognition appar- ently occurs during this short time, and the female flees or remains in the vicin- ity of the nest and accepts the advances of the male. Since in one isolation experiment there was an indication that the scarlet portions of the opercular tabs of male red-ear sunfish might possibly prevent hybridization between male red-ears and female bluegills, an experiment was conducted during 1964 to test this hypothesis. Two small earthen ponds (25 feet wide, 45 feet long, with a maximum depth of 4 feet) were each stocked during July with three ripe adult male red-ear sunfish and three adult female bluegiUs. The oper- cular tabs were clipped from all males stocked in one pond, and the tabs were left intact on the males stocked in the other pond. The ponds were drained during early October, and several thousand small (0.5-1.0 inch in total length) hybrid fry were collected from the pond contain- ing red-ear males whose opercular tabs had been removed. No small fish were found in the control pond. An examin- ation of the clipped males revealed that the blue portion of the opercular tabs had regenerated to almost normal size but the scarlet portions had not regen- erated. One tab on each of these males had a small, narrow, yellowish-orange margin. One such test cannot, of course, be considered conclusive proof that spe- cific differences in the color of the opercular tabs of male red-ears are highly functional in preventing their hybridization with female bluegills; however, additional investigation of the importance of color as a reproductive isolating mechanism in the sunfishes might prove rewarding. According to Hubbs (1957:17), fish hybridization is controlled to a large extent by environmental factors. Sunfish hybrids appear to he more common in ponds which are choked with aquatic vegetation or have high turbidities than in clear-water ponds which have ex- 184 Illinois Natural History Survey Vol. 29, Alt. 3 tensive spawning areas free from veg- etation. In weed-choked ponds or ponds with high turbidities the range of visibiHty must be short, and under these conditions ripe females might occasionally spawn with males without observing preliminary courtship dis- plays believed to be important in con- specific pair formation. HYBRIDS REARED IN PONDS Large numbers of each of the 10 viable F, hybrid types were stocked in one or more ponds. Most of these hy- brids were produced in the laboratory by combining sex products stripped from ripe adults of the four parent species. However, a few were produced by isolating males of one species and females of another species in ponds which contained no other fish. The laboratory hybrids were stocked on the day they became free-swimming; the pond-produced hybrids were seined and stocked in other ponds when they had attained a length of about 1 inch. The Fi hybrids were reared to ma- turity in their respective ponds and the sex ratio, fecundity, and degree of heterosis of each Fi hybrid population were studied. Sex Ratios After the Fi hybrids had grown to sexual maturity, fish were collected from each population and sexed, mostly by dissection. However, some were sexed by stripping eggs or milt from ripe individuals, and fewer still were sexed by reference to color, size of abdomen, and comparative size and shape of anus and urogenital openings. The sex of a mature sunfish is not difficult to determine by observation during the spawning period. Male sun- fish are usually more vividly colored than females. The abdominal region of females becomes greatly distended with eggs shortly before spawning occurs. The diameter of the urogenital opening in male sunfish is usually less than one- half the diameter of the anus, but in females it is equal to or larger than the diameter of the anus. In male sunfish the urogenital opening fonns a cup- shaped depression and in females it forms a small papilla. If diere was any doubt as to the sex of a particular individual, that individ- ual was dissected. The accuracy of determining sex by observation was checked several times by dissecting all fish in a particular sample in which the fish had been previously sexed by observation. No errors were revealed. The sex ratios, expressed as the per- centage of males, for each of the 10 different kinds of viable F, hybrids pro- duced from the four experimental spe- cies are reported in Table 12. Sex ratios determined for population of the six intrageneric Lcpowis hybrids re- ported by Childers & Bennett (1961:7) are included in Table 12. Table 12.—Sex ratios of Fi hybrid sun- fishes expressed as the percentage of males in individual populations. F, Hybrid Populcllions" RED-EAR SUNFISH BLUEGILL GREEN SUNFISH ID Fi HYBRID Fi HYBRID ID GREEN SUNFISH RW F, HYBRID BW F, HYBRID WG F, HYBRID GB F, HYBRID September, 1967 Childehs: Hybridization of Sunfishes 185 Of the 10 kinds of viable Fi hybrids, seven were predominately males ( RB, BR, and BG were 97 percent males; WG were 84 percent males; and RG, GB, and BW were approximately 70 percent males), two were approxi- mately 50 percent males (GR and RW), and one was predominately fe- male (GW was 16 percent males). Ricker (1948:93-94) determined the sex of 428 BR Fi hybrids in Indiana and fonnd them to be 97.7 percent males. Sex determination in sunfishes is very poorly understood. Bluegills, green sun- fish, and their Fi hybrids apparently have 24 pairs of chromosomes, and the sex chromosomes are indistinguishable from the autosomes (Bright 1937:36). Bright ( 1937:26 ) also reported that the chromosomes are so similar in shape and size that he was unable to detect specific differences. Roberts (1964:402) found that red-ear, bluegill, and war- mouth sunfishes each have 24 joairs of chromosomes; green sunfish from Wake County, N.C., had 24 pairs; but green sunfish from Leetown, W.Va., had only 23 pairs. The unbalanced phenotypic tertiary' sex ratios of the Fi hybrid sunfish could result from unbalance primary genetic sex ratios, specific differences in the strength of sex-determining factors, an overriding of the genetic sex by envi- ronmental factors, or differential mor- tality of the sexes. Since the WG Fi hybrids were 84 percent males and the reciprocal cross hybrids were 16 percent males, it is possible that the strength of sex- determining factors of warmouths are 5.25 times more powerful than those of green sunfish. Specific differences in the strength of sex-determining factors can- not alone explain the sex ratios of the remaining eight kinds of viable hybrids. *In this paper the terms primary, secondary, and tertiary sex ratios refer to sex ratios at the time of fertilization, lime cf hatching, and time of sexual ma- turity, respectively. Genetic sex refers to the t\pe of sex chromosomes an individnal receives from its par- ents, and phenotypic sex refers to whether its gonads are testes or ovaries. since none of these were predominantly females. RB and BG F, hybrids were both 97 percent males. If differential mortality were the cause of these unbalanced sex ratios, much of the mortality would have had to occur after the swim-up fry stage, since in the stripping experiments total mortality between fertilization and the swim-up fry stages was only 14 per- cent for the RB and 27 percent for the BG F, hybrids. It is not known which sex is the heterozygous condition for the sex chro- mosomes of the four experimental spe- cies; however, Haldane (1922:108) formulated a rule u'hich furnishes a clue. When in tlie Fi offspring of a cross between two animal species or races, one sex is absent, rare, or sterile, that sex is always the het- erozygous sex. Using Haldane's rule, Krumholz (1950:114), in a study concerning BR Fi hybrids, pointed out that the males of both bluegills and red-ear sunfish are probably homozygous for sex and the females heterozygous. The appli- cation of Haldane's rule to all possible Fi hybrids produced from red-ear sunfish, bluegills, and green sunfish in- dicates that the female is the heter- ozygous sex in these three species. Hybridization of male warmouths with females of the three Lepomis species resulted in partial or complete lethals, suggesting that in the warmouth the male is the heterogametic sex. Fecundities The reproductive success of each of the 10 kinds of viable Fi hybrids was investigated in one or more ponds. The occurrence and abundance of Fo hybrids were detemiined by seining, trapping, .shocking, poisoning, or draining the ponds after the Fi hybrids were 1 or more years of age. Eighteen separate populations were studied. The results of these studies are presented in Table 13. Of the 10 kinds of \ iable F, hybrids. 186 Illinois Natural History Survey Vol. 29, Art. 3 Table 13.- September, 1967 Childers: Hybridization of Sunfishes 187 hybrids to bass predation. Both possible causes may be important. Hale (1956: 105) found that green sunfish with forebrain lesions exliibited a marked reduction in their ability to learn to negotiate a maze. It would be inter- esting to know if the forebrains of the F2 hybrids are structurally or func- tionally different from those of Fi hy- brids or their parent species. WG Fo hybrids, stocked in a pond which contained no other fishes, pro- duced a large F?, generation. GW Fo hybrids, which were stocked in a pond containing no other fishes, also pro- duced a large F3 population. Backcrosses, outcrosses, a four-species cross, and a three-species cross involv- ing F] hybrids are listed in Table 14. The BW X B backcross was made by stocking adult male B\\' F, hybrids and adult female bluegills in a pond which contained no other fishes. The other 12 crosses listed in Table 14 were made by stripping gametes from ripe adults and rearing the young to the free-swimming fry stage in the labora- tory. R X RW, W X RW, B X RW, G x RW. R X GB, and RB x W young were killed after they developed into free-swim- ming fry because of the lack of ponds in which they could be stocked. All six kinds of fry appeared to be normal and probably would have developed into Table 14.—Successful backcrosses. out- crosses, four-way cross, and another cross in- volving Fi hybrid sunfishes.* Four-Species Three-Species Backcrosses Outcrosses Cross Cross 3 " 9 5 " 9 6 " 9 i " 9 R .\ RW R X GB RB X C\V BW x GW G X GW R X BW W X RW R .\ GW BW X B B X RG B X RW G X RW RB X W "R =: red-ear sunfish, B = Iilm-vlill, G=: green sun- fish, W = wamiouth. adults. Free-swimming fry of the re- maining six crosses in the laboratory were stocked in ponds and did develop into adult fishes. BW x B, G x GW, and B x RG populations produced large niunbers of young. Hubbs & Hubbs (1933:631-6,36) re- ported that in Michigan Fi hybrids of bluegills, green sunfish, longear sunfish, pumpkinseeds, and orangespotted sun- fish were unable to reproduce because males were sterile and ova stripped from the few adult females used in the experiments appeared distinctly abnonnal. This study, often cited in the literature, has resulted in a rather widespread belief that all male hybrid sunfish are sterile. Results of my experi- ments conclusively estabUsh that a number of diS^erent kinds of hybrid sunfishes produced in Illinois are not sterile, are fully capable of producing abundant Fo and F3 generations, and can be successfully backcrossed to parent species and even outcrossed to nonparental species. Hybrid Vigor Heterosis has been defined (Man- well, Baker, & Childers 1963:103) as that condition where, with respect to one or more particular characteristics, the values for most, if not all, of die individual hybrids fall significantly outside the range formed from the means for both parent populations. In cases of positive heterosis—hybrid vigor—the hybrid shows a faster growth rate than either of the parents, or it possesses some other char- acteristic, often an economically significant one, at a "better" level than the parents do. Rate of Growth.—The growth of the different kinds of hybrids stocked in ponds was recorded for each of the populations mentioned in Tables 12 and 13. Since various numbers (200- 10,000 per surface acre) of liybrids were stocked and since the ponds dif- fered in size, depth, shape of basin, and fertility, no valid comparisons can be made between the growth rates of t'ne hybrids or between those of the hybrids and their parent species. In general the rates of growth of these hybrids were inversely proportional to 188 Illinois Natural History Survey Vol. 29, Art. 3 their population densities. The most rapid growth during the 1st year of hfe occurred in a 1-acre pond stocked on May 24, 1957, with approximately 1,000 free-swimming, laboratory-pro- duced, 1-day-old RW Fi hybrid fry. Appro.ximately 41^ months later, Octo- ber 8, 66 hybrids from this population averaged 145 mm (5.7 inches) in total length and 69 grams (0.15 pound). The slowest growth occurred in a 0. 1-acre pond stocked on May 27, 1957, with approximately 1,000 (10,000 per surface acre) 1-day-old, free-swimming, laboratory-produced GR Fi hybrid fry. On August 30, 106 hybrids from this population averaged only 94 mm (3.7 inches) total length. The fish in this sample were not weighed; however, based on the weights of fish of similar lengths from later samples, the cal- culated average weight of fish from the August 30th collection was about 15 grams ( 0.03 pound ) . The largest hybrid captured to date (May, 1965) was a 6-year-old GB Fi hybrid whose total lengtli was 310 mm and weight was 965 grams (Frontis- piece ) . Hubbs & Hubbs (1931:291, 296-297) during 1929 and 1930 studied the growth of pumpkinseed, green sunfish, and the naturally produced hybrid of these two species in Wiard's pond near Ypsilanti, Mich. Average lengths of both 1- and 2-year-old hybrids in this pond were greater than those of either parent species. The effect on growth of the relative abundance of the three kinds of fishes in this pond (pumpkin- seeds, green sunfish, and their hybrids ) was not considered. Based on the total number of individuals reported, pump- kinseeds were approximately 10 times more abundant than green sunfish and green sunfish twice as abundant as hybrids. Intraspecific competition is usually keener than interspecific com- petition because individuals of the same species are more nearly equal in their structural, fimctional, and be- havioral adaptations (Kendeigh 1961: 183). Consequently, the greater growth of the hybrid sunfish in Wiard's pond may have been the result of less intra- specific competition among the hybrids than among the parent species and not because the hybrids exhibited hybrid vigor. In an attempt to determine whether certain Fi hybrid sunfishes actually grow faster than their parent species, two experiments were conducted in which equal numbers of uniformly sized Fi hybrids and parent species were stocked in ponds which contained no otiier fishes ( Childers & Bennett 1961:11-13). In the first experiment, 171 EG Fi hybrids and 171 green sun- fish averaging about 25 mm (1.0 inch) and 19 mm (0.75 inch), respectively, were stocked on July 10, 1958, in an 0.8-acre gravel pit pond. Ten months later (during 4 months of which the \\'aters were wann enough for fish growth) fish were removed from the pond by trapping and rotenone poison- ing. In the second experiment 200 GR Fi hybrids, 200 green sunfish, and 200 red- ear sunfish averaging 117 mm (4.6 inches), 107 mm (4.2 inches), and 89 mm (3.5 inches), respectively, were stocked in a 1.1 -acre farm pond during early August, 1958. Fish were removed from this pond April 20 through May 25, 1959, by hook-and-line fishing, trapping, and rotenone poisoning. In both experiments the average increase in total length of the hybrids was not significantly different from the increases of the parental species. The population densities of the fishes in both ponds were much lower than would be found in most normal nat- ural populations. In both experiments intraspecific and interspecific competi- tion was undoubtedly quite light; consequently, the question of whether certain Fi hybrid sunfishes are superior to their parent species in rate of growth cannot be answered until high density September, 1967 CniLUERS; Hybridization of Sunfishes 189 populations containing equal numbers of equal-sized hybrids and parent spe- cies are studied. Electrophoretic Patterns ok Hem- oglobins.—Manwell, Baker, & Childers (1963:118-119) determined that red- ear sunfish, bluegills, green sunfish, and warmouths each have a hemoglobin pattern in vertical starch gel electro- phoresis that is unique. Almost every one of the Fi hybrids of these species yields a hemoglobin pattern that is identical with that obtained by simply mixing hemoglobins of the two parental species; however, from 25 to 40 per- cent of the hemoglobin from BW, G\V, and ^VG F] hybrids has electrophoretic properties different from the hemoglo- bins of the parental species. Oxygen equilibria for the hemoglobins from these three hybrids show greater heme- heme interactions than those for hemo- globin from any of their parental spe- cies. As a result of this greater heme- heme interaction, hemoglobins from these three hybrids have better blood gas transport properties than those of their parental species, and in this re- spect each of these three hybrids is believed to exhibit hybrid vigor. Vulnerability to Hook-and-Lixe Capture.—Although no controlled ex- periment has tested whether Fj hybrid sunfishes are more vulnerable to anghng than their parental species, certain Fi hybrids are so easily caught that at several locations sport fisher- men have almost completely eliminated substantial hybrid sunfish populations in a few days of angling. For example, on May 30, 1958, the Illinois Department of Conservation opened the lake at Lincoln Trail State Park near Marshall to public fishing. From May 30 through September 29, fishermen caught and removed ap- proximately 10,800 naturally produced BG or GB Fi hybrids of which 50 per- cent (5,400) were caught during the 1st day of fishing, and 82 percent (8,600) were removed during the first 3 days. This hybrid sunfish population was almost completely eliminated during the 1st week of fishing. Ridge Lake, an 18-acre lake in Fox Ridge State Park near Charleston, 111., was drained during the fall of 1959 and fish were moved to other waters. In the spring of 1960, the lake was restocked with 4,503 BW Fi hybrids, 299 largemouth bass, 41 channel catfish, and 585 lake chubsuckers. An addi- tional 448 R\V Fi hybrids were stocked in May, 1961. A limited number of fishermen was permitted to fish during June, July, and August of 1960, 1961, and 1962. During 1960, fishermen totaled 1,583 man- hours of fishing and caught 3,772 of these hybrids, of which 64 were re- moved and 3,708 returned to the lake. In 1961 fishermen totaled 2,830 man- hours of fishing and caught 4,890 hybrids, of which 194 BW and 6 RW hybrids were remo\'ed. The remaining 4,690 hybrids were returned to the lake. The next season ( 1962 ) fishermen were permitted to remove all the hy- brids they caught, and during 2,817 man-hours of fishing they removed 1,075 BW and 134 RW Fi hybrids. Of these 1,209 hybrids, 65 percent were caught during the first 5 days of fishing, 81 percent during the first 10 days, and 88 percent during the first 15 days. The lake was drained during March, 1963, and was found to contain 8 BW and 64 RW Fi hybrids. HYBRID SUNFISHES FOR SPORT FISHING Overpopulation of sunfish is the single greatest problem encountered in the management of Illinois lakes and ponds containing largemouth bass and one or more of the Lepomis species. The Lepomis species have such high reproductive capacities and survival capabilities that they cominonK' be- come so abundant that they are unable to grow to sizes large enough to be of value to fishermen. Because certain 190 Illinois Natural Histoky Survey Vol. 29, Art. 3 kinds of Fj liybrid sunfishes appear to be unable to produce sizable F^ pop- ulations in ponds containing large- mouth bass, a number of experiments are now in progress to test the useful- ness of hybrids in combination with largemouths. Preliminary results indi- cate that several types of hybrids in combination with bass furnish fishing superior to that furnished by bass in combination with the hybrids' parent species. SUMMARY 1.—Red-ear sunfish, Lepomis micwlo- phtis (Gimther); bluegill, L. macrochi- nis Rafinesque; green sunfish, L. cij- anellus Rafinesque; and warmouth, Chaenobryttus giilostis (Cuvier) are present in a number of east-central Illinois lakes and ponds and are known to hybridize occasionally. During 1957 through 1964, the spawning seasons of these species were observed to extend from mid-VIay to August or September. 2.—Red-ear sunfish, bluegills, and green sunfish usually nested in colonies, and mixed colonies containing two and, less frequently, all three of these spe- cies were not uncommon. Warmouths tended to be more solitary in their nest site selections. 3.—Results of laboratory experiments indicate that average functional life spans of bluegill, green sunfish, and warmouth gametes are approximately 1 hour for ova and 1 minute for sper- matozoa. The brief functional life spans of spermatozoa are undoubtedly impor- tant in reducing hybridization caused by sperm driftage. 4.—Gametes stripped from the four species were paired in 16 different com- binations to produce zygotes repre- senting 12 kinds of Fi hybrids and the four parental species. W S x B 9 ^ and W $ X R 5 crosses were 100 percent lethal and the W $ x G 2 cross was partially lethal. Based on the percent- ^R = red-ear sunfish, B = bluegill, G = green sun- fish, W = warmouth. ages of zygotes that hatched and developed into normal-appearing fry, the viability of each of the remaining nine kinds of hybrids was not signif- icantly different from that of its ma- ternal parent. B ,^ R 9 and G ^ R 9 hybrid fry were significantly longer bodied than pure red-ear fry of com- parable ages. The other eight kinds of hybrid fry were not significantly dif- ferent in length from their respective maternal parent fry of comparable ages. 5.—Alpha temperature thresholds of development for red-ear sunfish, blue- gills, green sunfish, and nine kinds of hybrids were not significantly different from one another. The mean alpha threshold for all 12 kinds of fishes was 18.3° C.(64.9° F.). Approximately 280 developmental units ( degree-hours of effective temperature) centigrade scale or 500 units Fahrenheit scale were necessary for 50-percent hatching. 6.—Adult males of one species and adult females of another species were isolated in ponds to determine which of the 12 possible crosses may occur in nature. Thirty-four such experiments were conducted. Only Rrf x G9, G S .X B 5 , and W i x G 9 crosses (two, two, and one experiment, respec- tively) produced large Fi hybrid pop- ulations. Female bluegills successfully spawned with red-ear males whose opercular tabs had been removed (one experiment). Spawning did not occur in ponds containing normal red- ear males and female bluegills (five experiments). Results of the remaining 23 experiments were either negative or inconclusive. 7.—Large numbers of each of the 10 kinds of viable Fi hybrids were stocked in ponds, and after they grew to maturity, the sex ratio and fecundity exhibited by each population were investigated. R(iB9, B(jR9, and B $ G9 were 97 percent males; Wc$G9 was 84 percent males; R(5G9,Gc?B9, and B ' in hybrid animals. Journal of Genetics 12(2) : 101-109. Hale, E. B. 1956. Social facilitation and forcbrain function in maze performance of green sunfish, Lepomis ctianelltis. Phvsio- logical Zoology 29(2) :93- 107. Heiser, Charles B., Jr. 1949. Natural hy- bridization with particular reference to in- trogrcssion. Botanical Review 15(10):645- 687. Hubbs, Carl L. 1920. Notes on hybrid sun- fishes. Aquatic Life 5(9) : 101-103. . 1926. A check-list of the fishes of tlie Great Lakes and tributary waters, witli nomenclatorial notes and analytical keys. University of Michigan Museum of Zoology Miscellaneous Publication 15. 85 p. 1955. Hybridization between fish species in nature. Systematic Zoology 4(l):l-20. , and Laura C. Hubbs. 1931. In- creased growth in hybrid simfishes. Mich- igan Academv of Science, Arts and Letters Papers for 1930, 13:291-301. , and . 1932. Experimental verification of natural hybridization between distinct genera of sunfishes. Michigan Academy of Science, Arts and Letters Pa- pers for 1931, 15:427-4.37. and . 1933. The increased growth, predominant nialencss, and appar- ent infertility of hybrid sunfishes. Mich- igan Academv of Science, Arts and Letters Papers for 19.32, 17:613-641. -, and A. I. Ortenburger. 1929. Fur- ther notes on the fishes of Oklahoma with description of new species of Cyprinidae, p. 17-43. In University of Oklahoma Bul- 192 September, 1967 Childeks: Hybridization of Sunfishes 193 letin, new series 434. (Also listed as Uni- versity of Oklahoma Biological Survey Pub- lications 1(2): 17-43. ) Hunter, John R. 1963. The reproducti\e behavior of the green sunfish, Lepomis cy- unellus. Zoologica 48(1): 13-24. Kendeigh, S. Charles. 1961. Animal ecology. Prentice-Hall, Inc., Englewood Cliffs, New Jersey. 468 p. Krumholz, Louis A. 1950. Further obser\'a- tions on the use of hybrid sunfish in stock- ing small ponds. American Fisheries So- ciety Transactions for 1949, 79:112-124. Larimore, R. Weldon. 1957. Ecological life history of the warmouth ( Centrarchidae ) . Illinois Natural History Survey Bulletin 27(l):l-83. , and Philip W. Smith. 1963. The fishes of Champaign County, Illinois, as af- fected by 60 years of stream changes. Illi- nois Natural History Survey Bulletin 28:(2):299-382. Lopinot, Al. 1961. The red-ear sunfish. Illinois Wildlife 17(l):3-4. Manwell, Clyde, C. M. Ann Baker, and Wil- liam Childers. 1963. The genetics of hemoglobin in hybrids—I. A molecular basis for hybrid vigor. Comparative Bio- chemistry and Physiology 10:10.3-120. Marler, P. 1957. Specific distinctiveness in the communication signals of birds. Be- haviour ll(l):13-39. McAtee, W. L., and A. C. Weed. 1915. First list of the fishes of the vicinity of Plum- mers Island, Maryland. Biological Society of Washington Proceedings 28:1-14. Miller, Helen Carter. 1963. The behavior of the pumpkinseed sunfish, Lepomis gib- hosus ( Linneaus ) , with notes on the be- havior of other species of Lepomis and the pigmy sunfish, Elassoma cvergladei. Be- haviour 22 (l-2):88-151. Miller, Robert Rush. 1958. Origin and af- finities of the freshwater fish fauna of west- em North America, p. 187-222. In Carl L. Hubbs, editor. Zoogeography. American Association for the Advancement of Science Publication 51, Washington, D. C. 509 p. O'Donnell, D. John. 1935. Annotated list of tlic fishes of Illinois. Illinois Natural History Survey Bulletin 20(5) :473-491. Radcliffe, Lewis. 1914. A hybrid centrar- chid. Copeia (7):26-28. Raney, Edward C. 1940. Reproductive ac- tivities of a hybrid minnov\', Notropis cor- nutiis X Notropis ruhellus. Zoologica 25 (24):361-367. Ricker, William E. 1948. Hybrid sunfi.sh for stocking small ponds. American Fish- eries Society Transactions for 1945, 75:84- 96. Roberts, Franklin L. 1964. A chromosome study of twenty species of Centrarchidae. Journal of Moqihology 115(3) :401-417. Ross, Herbert H. 19.58. Evidence suggest- ing a hybrid origin for certain leafhopper species. Evolution 12(3) :337-.346. Schuchert, Charles, and Carl O. Dunbar. 1941. A te.vtbook of geology. Part II — historical geology. Fourth ed. John Wiley & Sons, Inc., New York. 544 p. Shelford, Victor E. 1927. An experimental investigation of the relations of the codling moth to weather and climate. Illinois Nat- ural History Survey Bulletin 16(5):311- 440. Smitherman, R. Oneal, and F. Eugene Hester. 1962. Artificial propagation of sunfishes, with meristic comparisons of three species of Lepomis and five of their hybrids. Amer- ican Fisheries Society Transactions 91(4): 333-341. Slastenenko, E. P. 1957. A list of natural fish hybrids of the world. Publication of the Hydrobiological Research Institute, Faculty of Science, University of Istanbul. Series B. 4(2-3): 76-97. Stebbins, G. Ledyard. 1959. The role of hybridization in evolution. American Phil- osophical Society Proceedings 103:231-251. Swingle, H. S. 1949. Some recent develop- ments in pond management. North Ameri- can Wildlife Conference Transactions 14: 295-^12. . 1956. A repressive factor control- ling reproduction in fishes. Eighth Pacific Science Congress Proceedings 3A:865-871. Tinbergen, N. 1951. The study of instinct. O.xford University Press, London. 228 p. Trautman, Milton B. 1957. The fishes of Ohio. The Ohio State University Press, Columbus. 683 p. Listed below are some recent papers on hybrid sunfishes. These papers have been pub- lished since the completion of the manuscript of this bulletin. Birdsong, Ray S., and Ralph W. Verger. 1967. A natural population of hybrid simfishes: Lepomis macrochirus x Chaenobryttus gu- losus. Copeia (1):62-71. Childers, William F., and George W. Bennett. 1967. Hook-and-line yield of largcmouth bass and redear x green siuifisli jiybrids in a one-acre pond. Progressive Fish-Culturist 29(l):27-35. Clark, Francis W., and Miles H. A. Kcenley- side. 1967. Reproductive isolation between the sunfish Lepomis p.i}>bosiis and L. macro- ehirus. Journal of the I'isheries Research Board of Canada 24(3) :495-514. West, Jerry L., and F. Eugene Hester. 1966. Intergeneric hybridization of ccntrarchids. Tran.sactions of the American Fisheries So- ciety 95(3):280-288. APPENDIX Table A1.—Partial chemical analysis of water taken August 2, 1963, from the well on Parkhill's Lake Park Subdivision Number Two, approximately 3 miles south of Champaign, III. The water was aerated and filtered through activated charcoal for I week prior to the analysis. The water had no measurable color or odor. The pH was 8.8. The analysis was made by the Illinois Water Survey. Chemical Composition Parts Per Million Iron (total) Calcium Magnesium Chloride Phenotlialine alkalinit>' ( as CaCO= ) Metliyl orange alkalinity ( as CaCOs) Hardness (as CaCO.i) Total dissolved minerals Trace 24 21 3 12 204 148 239 Table A2.—Experiment of warmouths.* Wl : duration of fertility of activated gametes from one pair Age of Eggs in Minutes Age of Sperm in Minutes Number of Eggs Percent of Eggs that Hatched Percent of Eggs that Developed itito Fri/t 0.75 3.15 5.55 8.45 10.35 0.25 2.75 5.25 7.75 10.25 Aging of Sperm and Eggs 93 211 102 149 86 47 11 46 10 'The temperature at time of fertilization was 27.8° C.(82.0" F.). During the entire experiment, 132 hours, the temperature ranged from 26.1^ to 27.8" C.(79.0'' to 82.0° F.) and averaged 27.2^ €.(80.9° F.). tFr>' were free swimming. Only nonual fry are included in this figure. Table A3.—Experiment W2: duration of fertility of activated gametes from one pair of warmouths.'" Age of Efigs in Minutes Age of Sperm in Minutes Number of Eggs Percent of Eggs that Hatched Percent of Eggs that Developed into Fryt 0.75 September, 1967 Childers: Hybridization of Sunfishes 195 Table A4.—Experiment W3 : duration of fertility of activated eggs from a single female warmouth. Sperm used in this experiment were from a single male.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Developed Minutes Minutes Eggs that Hatched into Fryt Aging of Eggs 0.50 39 100 95 30.00 79 92 90 60.00 91 72 70 120.00 81 49 48 180.00 89 7 2 ®The temperature at time of fertilization was 27.2° C.(81.0' F. ). During the entire experiment, 100 hours, the temperature ranged from 26.1° to 28.3° C.(79.0° to 83.0° F. ) and averaged 27.3° C.(81.1° F.). tFry able to swim for short periods but not completely free swimming. Only normal fry are included in this figure. Table A5.—Experiment W4: duration of fertility of activated eggs from a single female warmouth. Sperm used In this experiment were from a single male.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Deccloped Minutes Minutes Eggs that Hatched into Frc/t Aging of Eggs 0.50 89 89 73 30.00 82 16 16 60.00 81 44 41 120.00 69 14 6 180.00 89 ••The temperature at time of fertilization was 27.5°C.(81.5° F.). During the entire experiment, 125 hours, the temperature ranged from 26.4° to 27.8° C.(79..5° to 82.0° F. ) and averaged 27.0° C.(80.6° F.). fFry were free swimming. Only normal fry are included in this figure. 196 Illinois Natural History Survey Vol. 29, Art. 3 Table A6.—Experiment B1 : duration of fertility of activated gametes from one pair of bluegills.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Developed Minutes Minutes Eggs that Hatched into Fryt Aging of Sperm and Eggs 48 50 33 40 45 8 38 18 18 63 2 2 42 27 1.00 September, 1967 Childers: Hybridization of Sunfishes 197 Table A8.—Experiment B3 : duration of fertility of activated eggs from one female blue- gill. Sperm used In this experiment were from a single male.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Developed Minutes Minutes Eggs that Hatched into Fryi Aging of Eggs 96 96 96 71 82 82 59 51 51 93 16 16 44 20 20 •The temperature at time of fertilization was 26.9° C.(80.5' F.). During the entire experiment, 98 hours, the temperature ranged from 26.1° to 28.3° C.(79.0° to 83.0° F. ) and averaged 27.3° 0.(81.1° F.). tFry able to swim for short periods but not completely free swimming. Only normal fr>- are included in this figure. Table A9.—Experiment B4: duration of fertility of activated eggs from a single female bluegill. Sperm used in this experiment were from a single male.* 0.50 198 Illinois Natural History Survey Vol. 29, Art. 3 Table AlO.—Experiment Gl : duration of fertility of activated gametes from one pair of green sunfish.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Developed Minutes Minutes Eggs that Hatched into Fryj Aging of Sperm and Eggs 1.00 0.25 40 52 50 1.90 1.25 32 6 6 2.80 2.25 34 1 1 3.70 3.25 31 4.60 4.25 43 5.50 5.25 38 5 5 Aging of Eggs 33 91 88 57 24 23 74 8 7 49 4 2 27 7 4 "The temperature at time of fertilization was 23.6° C.(74.5° F. ) • During the entire experiment, 185 hours, the temperature ranged from 22.8' to 25.0° C.(73.0° to 77.0" F.) and averaged 24.2 C.(75.6° F.). tFry able to svvim for short periods but not completely free swimming. Only normal try are included in this figure. Table Al 1.—Experiment G2; duration of fertility of activated gametes from one pair of green sunfish. =•' Percent of Eggs Age of Eggs hi Age of Sperm in Nuinhcr of Percent of Eggs that Developed Minutes Minvtes Eggs that Hatched into Fri/t 0.50 September, 1967 Childers: Hybridization of Sunfishes 199 Table A12.—Experiment G3 ; duration of fertility of activated gametes from one pair of green sunfish.* Percent of Eggs Age of Eggs in Age of Sperm in Number of Percent of Eggs that Developed Minutes Minutes Eggs that Hatched into Fry{ Aging of Spfrm and Eggs 1.00 200 Illinois Natural History Survey Vol. 29, Art. 3 >4- O oo o m cU 3r- September, 1967 Childers: Hybridization of Sunfishes 201 3 202 Illinois Natural History Survey Vol. 29, Art. 3 3 O o— j: . on CO o - N > §-"2 £2 On. BO >-o = u Too (U •4- O o »0 •• a.m X c£ <^- (U September, 1967 Childers: Hybridization of Sunfishes 203 3 OX T , 00 CCtS Qi '~^— ' 00 CO Z. 00 3V0 5-0 ^-0 •s™ 204 Illinois Natural History Survey Vol. 29, Art. 3 3 O cf: CO 03 o Cod t/) S. c o.° N > J3 "D -I •D C C S O LL 00 >o = d Q) — o c^ crs - ™ N > N _ 'oflOO o > 9; ^1 CO JU si I S U t- c c "^ cwQ be ^ 0) 206 Illinois Natural History Survey \'ol. 29, Alt. 3 3 O jrt LiJ -D>J3 -D > C C (/I /> QJ . O ~. Mll >- N o •Sin ^ 1^ c — 5(J tiJJ(N 4- DO c ra (U E (J W 0) 3 Q. O E^ z 3 < *: •e E ™ a. I- o T3 « W5Q S- otq O o l^ September, 1967 Childers: Hybridization of Sunfishes 207 o H- O 00 0),— -DVD N T) > •- > (U ^ '. 189, 191 ('I'litrarchidac classification, 160 evolution, 159 geographic range, 160-161 introgressivc hybridization, 159 Chaenohryttus gulosus (see Warmouth) Chromosomes (number), 185 Deformities, 172-173, 176-178, 200-205 Developmental units (U values), 179-181, 190 Egg(s) average functional life, 167-170, 190 development into normal-appearing fry, 168-169, 171-179, 190, 194-210 mean number per sample, 171 ovulation, 183 physiological state, 168-169 size, 180 Evolution Centrarchidae, 159 Lepomini, 159, 161-162 Experimental hybridization isolation experiments, 181-184 stripping experiments, 170-181 Experimental species (see also under individ- ual species), 162, 190 F, hybrids (.sec under Hybrids, F, genera- tions ) F. hybrids (.sec under Hyl)rids, Fz genera- tions ) Fa hybrids (see under Hybrids, Fa genera- tions ) Fecundity, 185-187, 190-191 Fossils black crappie, 161 warmouth, 161 Four-species cross, 187 Fry (body length), 171-175, 179, 190, 200- 210 Fry (free-swimming stage), 167, 171, 184, 201-202 Fry (.swim-up stage), 171-176, 178-179, 200, 203-210 G Gametes (sec aho eggs and sperm under in- dividual species) average functional life, 167-170, 190 physiological state, 168-169 stripping, 167, 170 (^.cnetic sex, 185 Geographic range Centrarchidae, 160-161 experimental species, 162 Green i Blucgill ? hybrids (see under Hy- brids, Fi, Fi generations ) 211 212 Illinois Natural History Survey Vol. 29. Art. 3 Green S Red-ear 9 hybrids {see tinder Hy- brids, Fi, Fj generations) Green S Warmouth 9 hybrids (see under Hybrids, Fi, Fj, Fa generations) Green sunfish alpha temperature threshold of develop- ment, 179-181, 190 care of young, 167 chromosomes (number), 185 developmental units (U values), 179-181, 190 eggs (.see cd.so Eggs), 167-171, 174, 176- 177, 180, 190, 198-199, 206-208 fry (bodv length), 171, 174, 179, 190, 206- 208 fry (free-swimming stage), 167 fry (swim-up stage), 171, 174, 176, 179, 206-208 genetic sex, 185 geographic range, 162 growth rate, 174, 179, 188, 191, 206-208 habitat selection, 162-163 hatching time, 174, 177-179, 190, 206-208 hemoglobin, 189, 191 hybrids (natural), 161 incubation temperature, 168, 171, 174, 179, 198-199. 206-208 morphological characteristics, 162, 164-165, 183 mortality (see also viability), 170-171, 181, 185 nest location, 166-167 reproductive isolating factors, 170, 183-184 spawning behavior, 167, 18.3-184 .spawning time, 163, 166 sperm (average functional life), 167-170, 190 viability (see aha mortality), 170-171, 174, 176-177, 198-199, 206-208 Growth rate, 172-175, 179, 187-191, 200-210 H Habitat selection, 162-163 Hatching time, 171-175, 177-179, 190, 200- 210 Hemoglobin, 189, 191 Heterosis (hybrid vigor), 187-189, 191 Hybridization experimental, 170-184 introgressive, 159 natural, 159, 161-162, 182, 190 reproductive i.solating factors, 170, 183-184 Hybrid(s) backcrossed generations, 186-187 definition, 159 four-species crosses, 187 natural, 1.59, 161-162, 182, 190 outcrossed generations, 187 three-species crosses, 187 Hylirid(s), F, generations alpha temperature threshold of develop- ment, 179-181, 190 Bluegiil i Green 9 F, hybrids, 161, 174, 176-177, 179-181, 184-186, 188-191, 206- 208 Bluegiil c5 Red-ear 9 F, hybrids, 161, 172, 176-177, 179-182, 184-186, 189-191, 200- 202 Bluegiil $ M'armouth 9 F, hybrids, 161, 175-177, 179, 181-182, 184-186, 189-191, 209-210 catchability, 189, 191 chromo.somes (number), 185 definition, 170 deformities, 172-173, 176-178, 200-205 developmental units (U values), 179-181, 190 fecundity, 185-187, 190-191 fry (body length), 171-175, 179, 190, 200- 210 fry ( free-swimming stage ) , 177, 179, 184 fry (.swim-up stage), 171-176, 178-179, 200, 203-210 Green S Bluegiil 9 Fi hybrids, frontis- piece, 161, 173, 176-177, 179-182, 184- 186, 188-191, 203-205 Green £ Red-ear 9 F, hybrids, 161, 172, 176-177, 179-182, 184-186. 188-191, 200- 202 Green i Warmoutli 9 Fi hybrids, 161, 175-177, 179, 181-182, 184-186, 189-191, 209-210 growth rate, 172-175, 179, 187-189, 191, 200-210 hatching time, 172-175, 177-179, 190, 200- 210 hemoglobin, 189, 191 heterosis (hybrid vigor), 187-189, 191 incubation temperature, 171-175, 177-179, 200-210 isolation experiments, 181-184 mortahty (sec also viability), 171, 176, 181, 190 Red-ear i Bluegiil 9 F, hybrids, 161, 173, 176-177, 179-186, 189-191, 20.3-205 , Red-ear i Green 9 F, hybrids, 161, 174, ' 176-177, 179-182, 184-186, 189-191, 206- 208 Red-ear i Warmouth 9 Fi hybrids, 161, 17.5-179, 181-182, 184-186, 188-191, 209- 210 sex ratio, 184-185, 190-191 sport fishing, 189-191 stripping experiments, 170-181, 200-210 temperature (incubation), 171-175, 177- 180, 200-210 viabilitv (see also mortality), 170-178, 190, 200-210 Warmouth c5 Bluegiil 9 F, hvbrids, 161, 173, 176-182, 190, 20.3-205 Warmouth i Green 9 F, hybrids, 161, 174, 176-177, 179-182, 184-186, 189-191, 206- 208 September, 1967 Childers: Hybridization of Sunfishes 213 Warmouth i Red-ear 9 F, hybrids, 161, 172, 176-182, 190, 200-202 Hybrid(s), F= fienerations, 170, 185-187, 190- 191 Bluejiill S Green 9 F= hybrids, 186 Bluesill (5 Red-ear 9 F. hybrids, 186 Bluegill i Warmouth 9 F= hybrids, 186, 191 Green S BhiegiU 9 F. hybrids, 186, 191 Green c5 Red-ear 9 F. hybrids, 186, 191 Green £ Warmouth 9 F. hybrids, 186-187, 191 Red-ear £ Bhiegill 9 F. hybrids, 186 Red-ear £ Green 9 F« hybrids, 186, 191 Red-ear £ Warmouth 9 F. liybrids, 186, 191 Warmouth £ Green 9 F^ hybrids, 186-187, 191 Hybrid(s), Fa generations, 187 Green £ Warmouth 9 Fu hybrids, 187, 191 Warmouth £ Green 9 F, hybrids, 187, 191 I Incubation temperature, 168, 171-175, 177- 180, 194-210 Introgressive hybridization, 159 Isolation experiments, 181-184 Lepomini (tribe) classification, 160 evolution, 159, 161-162 hybridization (experimental), 170-184 hybridization (natural), 159, 161-162, 182 Lcpomis cijaneUus (sec Green sunfish) Lcponiis macrocliirus (see Bluegill) Lepomis microlophus (see Red-ear sunfish) M Morphological characteristics, 162, 164-165, 183 Mortahty (see also Viability), 170-171, 176, 181, 185, 190-191 N Natural hybrids, 159, 161-162, 182, 190 Natural hybridization, 159, 161-162, 182, 190 Nest location(s), 166-167 Red-ear £ \\'armouth 9 hybrids (see under Hybrids, F,, Fj generations ) Red-ear sunfish alpha temperature threshold of develop- ment, 179-181, 190 care of young, 167 chromosomes (number), 185 developmental units (U values), 179-181, 190 eggs (see also Eggs), 167, 170-172, 176- 177, 180, 190, 200-202 frv (body length), 171-172, 179, 190, 200- 202 frv (free-swimming stage), 167, 171-172, 'l77, 179, 201-202 fry (swim-up stage), 171-172, 176, 179, '200 genetic sex, 185 geographic range, 162 growth rate, 172, 179, 188, 190-191, 200- 202 habitat selection, 163 hatching time, 172, 177-179, 190, 200-202 hemoglobin, 189, 191 hybrids (natural), 161 incubation temperature, 171-172, 177, 179, 200-202 morphological characteristics, 162, 164-165, 183 mortality (see also viability), 171, 181, 185 nest location, 166-167 reproductive isolating factors, 170, 183-184 spawning behavior, 167, 183-184 spawning time, 163, 166 sperm ( average functional life ) , 170 viability (see also mortality), 170-172, 176- 177, 190, 200-202 Reproductive isolating factors, 170, 183-184 Sacramento i^erch, 160 Sex ratio, 184-185, 190-191 Spawning behavior, 167, 183-184 Spawning time, 163, 166 Sperm (average functional life), 167-170, 190 Sperm driftage, 167, 170, 190 Sport fishing (hybrids), 189-191 Stripping experiments, 170-181 Outcrossed generations, 187 Parental species (see also under individmd species), 162, 190 Red-ear £ Bluegill 9 liybrids (see under Hybrids, Fi, Fj generations ) Red-ear £ Green 9 hybrids (see under Hy- brids, Fi, Fa generations) Temperature (incubation), 168, 171-175, 177- 180, 194-210 'I'hree-species crosses, 187 u U values (developmental units), 179-181, 190 Viability (see also Mortality), 170-178, 190, 194-210 214 Illinois Natural History Survey Vol. 29, Art. 3 w Warmoutli alpha temperahire threshold of develop- ment, 179-181 care of young, 167 chromosomes (number), 185 developmental units (U values), 179-181, 190 eggs (see also Eggs), 167-171, 175-177, 179-180, 194-195, 209-210 fossils, 161 fry (body length), 171, 175, 179, 190, 209- 210 fry (free-swimming stage), 167 fry (swim-up stage), 171, 175-176, 179, 209-210 genetic sex, 185 geographic range, 162 growth rate, 175, 179, 190, 209-210 habitat selection, 162 hatching time, 175, 177-179, 190, 209-210 hemoglobin, 189, 191 hybrids (natural), 161, 190 incubation temperature, 168, 171, 175, 179, 194-195, 209-210 morphological characteristics, 162, 164-165, 183 mortality (see also viability), 170-171, 181, 185, 190 nest location, 166-167 reproductive isolating factors, 170, 183-184 spawning behavior, 167, 183-184 spawning time, 163, 166 sperm (average functional life), 167-168, 170, 190 viability (see also mortality), 170-171, 175- 177, 190, 194-195, 209-210 Warmouth $ Bluegill 9 hybrids (see under Hybrids, Fi generations) Warmouth (J Green 2 hybrids (see under Hybrids, F,, F2, Fs generations) Warmouth t? Red-ear 2 hybrids (see under Hybrids, Fi generations) i Some Publicahons of the ILLINOIS NATURAL HISTORY SURVEY BULLETIN Volume 27, Article 5.—Hook-and-Line Catch in Fertilized and Unfertilized Ponds. By Donald F. Hansen, George W. Bennett, Robert J. Webb, and John M. Lewis. Au- gust, 1960. 46 p., frontis., 11 fig., bibhogr. Volume 27, Article 6.—Sex Ratios and Age Ratios in North American Ducks. By Frank C. Bellrose, Thomas G. Scott, Arthur S. Hawkins, and Jessop B. Low. August, 1961. 84 p., 2 frontis., 23 fig., bibliogr. Volume 28, Article 2.—The Fishes of Cham- paign County, Illinois, as Aflfected by 60 Years of Stream Changes. By R. Weldon Larimore and Philip W. Smith. March, 1963. 84 p., frontis., 70 fig., bibliogr., in- dex. Volume 28, Article 3.—A Comparative Study of Bird Populations in lUinois, 1906-1909 and 1956-1958. By Richard R. Graber and Jean W. Graber. October, 1963. 146 p., 4 frontis., 32 fig., bibhogr., index. Volume 29, Article 1.—A Biological Investi- gation of the Fishes of Lake Chautauqua, Illinois. By WiUiam C. Starrett and Arnold W. Fritz. March, 1965. 104 p., frontis., 40 fig., bibhogr., index. Volvune 29, Article 2.—Stocking and Sport Fishing at Lake Glendale (lUinois). By Donald F. Hansen. July, 1966. 54 p., frontis., 9 fig., bibliogr., index. CIRCULAR 39.—How to Collect and Preserve Insects. By H. H. Ross. November, 1966. (Eighth printing.) 71 p., frontis., 79 fig. 46.—Illinois Trees: Their Diseases. By J. Cedric Carter. June, 1964. ( Third printing, with alterations. ) 96 p., frontis., 89 fig. 47.—Illinois Trees and Shrubs: Their Insect Enemies. By L. L. Enghsh. October, 1965. (Third printing, with revisions.) 92 p., frontis., 59 fig., index. 48.—Diseases of Wheat, Oats, Barley, and Rye. By G. H. Boewe. June, 1960. 159 p., frontis., 56 fig. 49.—The Dunesland Heritage of Illinois. By Herbert H. Ross. (In cooperation with Illi- nois Department of Conservation. ) August, 1963. 28 p., fronUs., 16 fig., bibliogr. 50.—The Wetwood Disease of Elm. By J. Cedric Carter. May, 1964. 20 p., 19 fig. 51.—Uhnois Trees: Selection, Planting, and Care. By J. Cedric Carter. August, 1966. 123 p., frontis., 108 fig. 52.—Fertilizing and Watering Trees. By Dan Neely and E. B. HimeUck. September, 1966. 20 p., 9 fig., bibliogr. BIOLOGICAL NOTES 43.—Hot-Water and Chemical Treatment of Illinois-Grown Gladiolus Cormels. By J. L. Forsberg. March, 1961. 12 p., 8 fig., bib- hogr. 44.—The Filmy Fern in Illinois. By Robert A. Evers. April, 1961. 15 p., 13 fig., bibUogr. 48.—Systemic Insecticide Control of Some Pests of Trees and Shrubs—^A Preliminaiy Report. By L. L. Enghsh and Walter Hait- stim. August, 1962. 12 p., 9 fig., bibhogr. 50.—Some Unusual Natural Areas in Illinois and a Few of Their Plants. By Robert A. Evers. July, 1963. 32 p., 43 fig., bibhogr. 52.—^A Synopsis of the Mosquitoes of Illinois (Diptera, Cuhcidae). By Herbert H. Ross and William R. Horsfall. March, 1965. 50 p., 231 fig., bibhogr., index. 53.—Experimental Field Studies on Shade Tree Fertilization. By E. B. Himehck, Dan Neely, and Webster R. Crowley, Jr. J»me, 1965. 12 p., 8 fig., bibhogr. 54.—^A Preliminary Annotated List of the Lampreys and Fishes of Illinois. By Phihp W. Smith. June, 1965. 12 p.. 3 fig., bib- liogr. 55.—^A Guide to Age Determination of Bob- white Quail Embryos. By John Rosebeny and Willard D. Khmstra. July, 1965. 4 p., 2 fig., bibhogr. 57.—Man's Effect on the Fish and Wildlife of the Illinois River. By Harlow B. Mills, Wilham C. Starrett, and Frank C. Belhose. June, 1966. 24 p., 16 fig., bibliogr. 58.—The Life History of the Slou^ Darter, Etheostoma gracile (Pisces, Percidae). By Marvin E. Braasch and Phihp W. Smith. June, 1967. 12 p., 8 fig., bibliogr. MANUAL 4.—Fieldbook of lUinois Mammals. By Don- ald F. Hoffmeister and Carl O. Mohr. June, 1957. 233 p., color frontis., 119 fig., glos- sary, bibhogr., index. List of available publications mailed on request No charge is made for pubhcations of the Ilunois Naivral History Survey. A single copy of most pubhcations will be sent free to anyone requesting it mitil the supply becomes low. Costly pubhcations, more than one copy of a pubhcation, and pubhcations in short supply are subjects for special correspondence. Such correspondence should identify the writer and explain the use to be made of the pubhcation or pubhcations. Address orders and correspondence to the Chief, Illinois Natural History Survey, Noturol Resources Building, Urbano, Illinois 61801 i