01_device Focal benthic mollusks 1 Focal benthic mollusks (Mollusca: Bivalvia and Gastropoda) of selected sites in Tubbataha Reef National Marine Park, Palawan, Philippines Roger G. Dolorosa* and Sabine Schoppe Western Philippine University-Puerto Princesa Campus, Santa Monica Heights, 5300 Puerto Princesa City, Palawan, Philippines Tel. No.: (048) 434-398; TelFax: (048) 433-4367; E-mail: rogerdolorosa@yahoo.com Date received: March 2, 2006; Date accepted: July 6, 2006 ABSTRACT Science Diliman (July-December 2005) 17:2, 1-10 The study was conducted at Tubbataha Reef National Marine Park from May 6-11, 2005. Seven pre- established stations with survey sites at 5 and 10 m depth and one intertidal area were assessed using 150 m permanent belt transects. Focal benthic mollusks found one meter to the left and right of transects were identified and counted. A total of 19 species belonging to eight families were recorded, of which 15 species are univalves. In the intertidal area a total of 12 species were noted, 13 species at the shallow (5 m) and five species at deeper (10 m) areas. Species belonging to the family Tridacnidae and Trochidae were the most abundant. Among the subtidal stations, the highest number of individuals was noted at a shallow reef flat (station VI). In terms of density, the intertidal area had the highest (213,310 ind. km-2) followed by the shallow (72,870 ind. km-2) and the deep with 5,720 ind. km-2. The densities of Tridacna crocea (133,330 ind. km-2) and Hippopus hippopus (3,330 ind. km-2) at the intertidal area were found to be higher than in most other survey sites in Palawan but previous density records at the park indicate a stiff decline. On the contrary, the first record on the density of T. squamosa (950 ind. km-2) at the park is much lower compared to that from other parts of Palawan. Large and commercially valuable gastropods like, Trochus niloticus, Tectus maculatus and T. pyramis that are rarely encountered at the intertidal areas were abundant at the TRNMP. Other important species like Tridacna gigas, Charonia tritonis and Cassis cornuta were not encountered at the study sites. To fully assess the abundance of focal mollusks, permanent transects should be established in the same seven sites but in shallow reef flat of about 2 m deep, in the lagoon and in the intertidal of North and South Islets where species composition, density and growth could be monitored on an annual basis which could be used to evaluate the management effectiveness at the TRNMP. Key words: bivalves, focal species, gastropods, Palawan, Philippines, Tubbataha Reef *Corresponding author Dolorosa and Schoppe 2 Coral reefs are the most biologically diverse of shallow water marine ecosystems but are being degraded worldwide by human activities and climate warming (Roberts et al., 2002). They are critically important for the ecosystem goods and services they provide to maritime tropical and subtropical nations (Souter & Linden, 2000). Yet reefs are in serious decline; an estimated 30% are already severely damaged, and close to 60% may be lost by 2030 (Wilkinson, 2000). There are no pristine reefs left (Jackson et al., 2001). Local successes at protecting coral reefs over the past 30 years have failed to reverse regional scale declines, and global management of reefs must undergo a radical change in emphasis and implementation if it is to make a real difference (Hughes et al., 2003). In the 1980s, the previously pristine and unique Tubbataha Reef of Palawan started to show evidence of serious destruction and over-exploitation that has continued even after its declaration as a Natural Marine Park in 1988 (Arquiza & White, 1999). With the creation of the Tubbataha Management Foundation in 1990, the declaration as World Heritage Site in 1993, and finally the establishment of the Protected Area Management Board in 1999, conditions did improve under the impact of resource management strategies with national and international support. The current Tubbataha Reef National Marine Park (TRNMP) Management has the goal to preserve the globally significant biological diversity and ecological processes of Tubbataha and to manage them and the surrounding areas on a sustainable basis. While regular monitoring of coral reef conditions and associated fish fauna, has been conducted for the past eight years (Sabater et al., 2004), the assessment of focal species was only in November 2003 during the TRNMP Management Effectiveness Workshop recognized as a gap. Focal species are organisms of ecological importance and/or human value that are of priority interest for management through the marine protected area. Aside from top predators, benthic mollusks were identified as important indicators of habitat health and proper resource management and protection. Since species abundance of focal species is one of the most widely used biological ‘success’ measures of management effectiveness (Pomeroy et al., 2003) the present study intended for INTRODUCTION Fig. 1. Map showing Tubbataha Reef National Marine Park. TRNMP is part of the municipality of Cagayancillo, an oceanic island in the Philippines (inset). (Source: Sabater et al., 2004). Tubbataha Reef National Marine Park Cagayancillo Puerto Princesa Province of Palawan, Philippines SULU SEA Focal benthic mollusks 3 Fig. 2. Location of the seven monitoring stations (triangles) in Tubbataha Reef National Marine Park. (Source, Sabater et al., 2004). I II III IV V VI VII Dolorosa and Schoppe 4 the first time to determine the species composition and density of focal benthic mollusk species in TRNMP. MATERIALS AND METHODS Tubbataha Reef is located in the Sulu Sea, 150 km southeast of Puerto Princesa City, Palawan. It is under the political boundaries of the municipality of Cagayancillo, located about 80 km northeast of Tubbataha (Fig. 1). Seven sites were surveyed within the reef area. Geographic locations of the sites are provided in Sabater et al. (2004). Four sites (1-4) are in the north atoll, and three sites (5-7) in the south atoll (Fig. 2). Site selection around the atoll is based on the various coral reef structures found representative of the monsoonal exposure effects. The reef structure ranges from monospecific stands of Acropora brueggemanni and sand-reef patch areas located at the sheltered reef areas to spur-and-groove habitats with mixed coral assemblage dominated by robust corals located at the exposed areas (Sabater et al., 2004). Each site has two permanent transects of 150 m length; one at about 5 m and one at about 10 m depth. These transects have been established since 1997 for the regular monitoring of benthic communities (especially corals) and reef-associated fishes. Shallow transects are typically along slightly sloping parts of the reef characterized by branching corals while deep transects are located along steep slopes (walls). In the present study, focal bivalve and gastropod species were assessed alongside these transects. Transect width was limited to 1-m belts right and left of each transect. All except two transects were assessed during day time. The number of species abundance of each focal species such as flagship, indicator (e.g. triton shell), keystone, target (commercially valuable) and threatened species (according to IUCN 2004) were assessed. In addition to the shallow and deep transects, the intertidal area near Stn. V at the South Islet was assessed. The intertidal was characterized by few live corals at the seaward margin followed by massive coral rocks and rubble intercepted by sandy-muddy substratum. A 150 m transect line was laid during low tide from the exposed reef margin towards the light house. The shells were identified using the following references: Carpenter & Niem (1998), Knop (1996), Allen & Steen (1994), Sabelli (1991), Garcia (1986), Springsteen & Leobrera (1986) and Oliver (1975). Diversity and abundance data were analyzed per site, with intertidal transects separated from shallow and deep transects. Density per square meter was obtained by dividing the number of individuals recorded with the area (300 m2) surveyed per transect, then it was extrapolated into number of individuals per square kilometers (ind. km-2). RESULTS Species Composition A total of 19 species belonging to eight families were recorded, of which 15 species are univalves. Of these, Tridacnidae and Conidae had the highest number of species comprising 21 % for each family. This was followed by Trochidae and Cypraeacide (both with 16 %). Lowest percent share belong to families Fasciolariidae, Mitridae and Turbinidae (Fig. 3). Density A total of 12 focal species belonging to six families were recorded at the intertidal area at the South Islet. Bivalves were only represented by the family Tridacnidae. Among all species Tridacna crocea was the most abundant and dense (133,330 ind. km-2) followed by Conus lividus (20,000 ind. km-2) and Trochus niloticus (166,670 ind. km-2). The calculated density of focal mollusks within that intertidal area was 213,310 ind. km-2 (Table 1). At the shallow area, a total of 13 focal species under eight families were recorded. Gastropods dominated with seven families and 10 species. In terms of abundance, T. crocea ranked first with an average of 30,480 ind. km-2, followed by Tectus pyramis (15,710 ind. km-2), and T. maxima (12,860 ind. km-2) (Table 1). Tridacna squamosa was so rare (950 ind. km-2) that it was only encountered in stations IV and VI. The calculated density of all focal mollusks in shallow areas was 72,870 ind. km-2. Highest abundance was recorded at Stn. VI which is a reef flat dominated by massive Focal benthic mollusks 5 Tridacnidae 21% Trochidae 16% Turbinidae 5% Strombidae 11% Cypraeacidae 16% Mitridae 5% Fasciolariidae 5% Conidae 21% Fig. 3. Percent species composition of focal benthic mollusks at TRNMP. Family Species Intertidal* Shallow (5m) Deep (10m) Tridacnidae Hippopus hippopus 33,330 Tridacna crocea 133,330 30,480 Tridacna maxima 12,860 950 Tridacna squamosa 950 Total no. of bivalve species 2 3 1 Trochidae Tectus pyramis 3,330 15,710 1,910 Trochus maculatus 3,810 1,430 Trochus niloticus 16,670 3,330 Turbinidae Turbo chrysostomus 3,330 1,430 Strombidae Lambis chiragra chiragra 480 Lambis sp. (juvenile) 3,330 Cypraeacidae Cypraea annulus 13,330 Cypraea moneta 6,670 Cypraea tigris 1,430 950 Fasciolariidae Pleuroploca trapezium 480 Conidae Conus lividus 20,000 Conus miles cf 3,330 950 Conus sp. 3,330 480 Conus virgo 3,330 480 Mitridae Mitra papalis 480 Total no. of gastropod species 10 10 4 Total density 213,310 72,870 5,720 *one transect only Table 1. Density (ind. km-2) of focal benthic mollusks at TRNMP. Dolorosa and Schoppe 6 corals. Other stations were covered with branching corals or were located at the drop off so that mollusk abundance was relatively lower compared to that in Stn. VI and the intertidal area. Broken shells of Trochus and Turbo were noted in many stations indicating the presence of mollusk predators. Large (<60 cm long) holothurians (Bohadschia argus, Pearsonothuria graeffei, Holothuria sp., Thelenota ananas, Stichopus horrens, S. chloronotus) were common along the shallow areas in Stn. III. Very few focal mollusks were noted in the deep transects. They were represented by five species under four families. In terms of abundance, T. pyramis had a calculated density of 1,910 ind. km-2, followed by T. maculatus (1,430 ind. km-2). Tridacna maxima and Cypraea tigris both had a density of 950 ind. km-2 (Table 1). The density of all focal mollusks at 10 m depth was only 5720 ind. km-2. Empty and or broken shells of Trochus and Turbo were noted in some stations. Generally, mollusks in TRNMP were more abundant in shallow waters, and had the highest density along the intertidal transect (Fig. 4). DISCUSSION Among the 19 focal species, only the giant clams are listed under the IUCN Red List of Threatened Species (IUCN, 2004). Tridacna squamosa, T. maxima and H. hippopus are under Lower Risk/Conservation Dependent (LR/cd ver 2.3 1994) while T. crocea falls under the Lower Risk/Least Concern (LR/lc ver 2.3 1994) category. However, under the Fisheries Administrative Order No. 208 series of 2001, all seven species of giant clams are listed as endangered. Trochus niloticus is also listed as threatened (BFAR, 2006). For a World Heritage Site like TRNMP, the recorded number of threatened and endangered mollusks is quite low. The observed pattern of increasing density of focal mollusks from subtidal to intertidal sites is substrate- and depth-related. Deep transects were either along steep reef walls or on reef slopes dominated by branching corals. In most of the sites surveyed, the reef starts sloping at about 3 m, hence most shallow transects were on sloping areas, too often covered with branching coral formations. Only shallow reef platforms dominated by massive corals (such as Stn. VI,) as well 0 50000 100000 150000 200000 250000 I II III IV V VI VII S. Islet Stations In di vi du al s/ sq . k m Shallow Deep Intertidal Fig. 4. Density (ind. km-2) of focal mollusks in the different stations. Focal benthic mollusks 7 as intertidal areas with coral rocks and partly sand- muddy substrate (such as the one in South Islet) provide the habitat preferred by the mollusks under study. This was especially obvious for T. crocea which needs hard substrate to bore into and sunlight for photosynthesis. The niche of T. crocea, living embedded in massive corals, had been observed by various authors (see Alcala, 1986; Juinio et al., 1988; Bolen, 2005), and explains its high density in the intertidal and shallow reef flat areas in this study. Yamaguchi (1996) also found T. crocea and T. maxima to be common in coral rocks around seagrass beds in the North Reef of Tubbataha. In terms of T. crocea density, TRNMP ranks second after survey sites in Northern Palawan (Alcala, 1986), followed by Snake Island, Honda Bay (Bolen, 2005), Binaluan Bay, Taytay (Sayson, 2003) and Roxas (Condesa, 2005) (Table 2). Density patterns among those sites are not expected to be related to resource management and protection of giant clams since T. crocea is little if at all exploited. High density of T. crocea seems solely substratum related, reflecting the availability of coral heads into which the species burrows. In the intertidal areas of Roxas surveyed by Condesa (2005) e.g. such coral heads were almost completely absent, resulting in only 7,000 and 15,000 ind. km-2 (Table 2). Although density of tridacnids, especially T. crocea in TRNMP was found to be higher than in most other survey sites in Palawan, previous density records of T. crocea and T. maxima from TRNMP in 1993-1995 indicate a stiff decline (Calumpong & Cadiz, 1993; Ozoa, 1995). Tridacna crocea for example decreased from 2,200,000 ind. km-2 in 1993 (Calumpong & Cadiz, 1993) to 1,000,000 in 1995 (Ozoa, 1995), and to 133,330 in 2005 (Table 2). The much lower density at intertidal area at South Islet, however, is not enough to conclude that there is a continuous decline of tridacnids, considering that the present data were only taken from a single transect. Hence, permanent mollusk transects should be established in the same seven sites but in shallow reef flats of about 2 m deep. In addition, permanent transects should be established in the lagoon and in the intertidal of North and South Islets where species composition, density and growth could be monitored along these transects on an annual basis. Unlike T. crocea which was still relatively abundant, the other giant clam species, especially non-boring and heavily exploited giant clam were rather rare. Very low numbers of T. squamosa and H. hippopus were noted during the present study, as well as earlier by Yamaguchi (1996). Although Yamaguchi (1996) reports dead shells of H. hippopus in seagrass beds, he believes that this was not necessarily attributed to harvesting by fishermen. In Papua New Guinea, however, fishermen, as well as poachers, collect giant clams by extracting the meat leaving the shell behind (Kinch, 2002). White et al. (2003) affirms that large scale extraction of precious and common shells and giant clams has been done in the past in the Tubbataha Reef. And still, recently, hundreds of giant clams were confiscated from Chinese fishermen who poached in TRNMP. Records by Ticke (2002) state 200 confiscated T. gigas while Benavent-Villena & Pido (2004) are referring to 30 sacks of dried giant clams confiscated that time. Although no T. gigas was encountered during the present study, Tubbataha park rangers mentioned that there are still some T. gigas in the park. The current status of the giant clams in TRNMP might lead to similar conditions as in the Mu Ko Surin Marine National Park in Thailand where T. crocea is abundant, T. maxima is rare, T. squamosa is nearing extinction, and T. gigas and H. hippopus are considered extinct (Talaythai 2001). According to Kinch (2002) giant clams are highly vulnerable to stock depletion, for the stocks will become non-sustaining when densities fall below the critical mass. Aside from that, the low settlement, survival and growth of T. squamosa on live coral substrate (Calumpong et al., 2003) would take hundreds of years for the re-establishment of a stock particularly in isolated areas (Munro, 1993 in Kinch, 2002). These may be the reasons that despite protecting TRNMP for more than a decade, populations of commercially valuable giant clam species at the seven established stations remain low. This condition requires the establishment of specific sites for non-boring tridacnid species to ensure fertilization and larval recruitment. The stability or an increase in the population of the focal benthic mollusks especially the following giant clam species: T. gigas, T. squamosa and H. hippopus over the next few years would indicate success of the conservation measures employed by the TRNMP. Dolorosa and Schoppe 8 Other large and commercially valuable species such as the gastropods, Trochus niloticus, Tectus maculatus and T. pyramis which are nowadays rarely encountered in intertidal areas were abundant at the TRNMP. Intertidal areas near coastal residents of Palawan are usually overexploited as density data indicate. Condesa (2005) for example recorded a mean density of only 1,000 ind. km-2 of T. niloticus in Green Island, Roxas. Still lower is the density of the species in the Cartier Reef, Australia where it only ranged from 133-333 ind. km-2 (Smith et al., 2002). It is expected that Tubbataha’s previously overexploited gastropods stocks have recovered under the past years of protection. Since gastropods mate, fertilization and therewith recruitment and increase of stocks are easier achieved compared to broadcast spawning in bivalve species, whose gametes are easily dispersed in water by currents. The very low number of other commercially important gastropods like Lambis spp. and the absence of Charonia tritonis tritonis and Cassis cornuta both in shallow and deeper areas may again be attributed to the substrata of the survey sites. Cassis cornuta for example is common on sandy substrate between coral formations (Carpenter & Niem 1998) which was rarely encountered along the surveyed transects. Over exploitation in previous years might also have led to the absence of the species in the survey sites, which were easily accessible to poachers before. Park Rangers claimed that C. cornuta and C. tritonis are still present in the lagoon, an area that was always difficult to access by poachers. Therefore, it is recommend that in order to better assess the species composition and density of focal species in the TRNMP, more survey transects per site need to be established at the intertidal and shallow reef flats. ACKNOWLEDGMENTS This study was funded by Conservation International Palawan as part of the biophysical monitoring efforts of Tubbataha Management Office and Tubbataha Protected Area Management Board. Thanks are also due to Tubbataha Management Office and the crew of the M/bca Minerva of the WWF and to Marla Chassels of Conservation International. The comments and suggestions of the two reviewers are greatly acknowledged. Table 2. Density (ind. km-2) of commercially exploited benthic mollusks at TRNMP in comparison with other studies. Palawan, Philippines Tridacna crocea 15,000 7,000 120,000 55,833 18,069 328,619 1,400,000 2,200,000 1,000,000 133,330 30,480 T. maxima 100,000 25,509 2,667 500,000 200,000 333,300 12,860 T. squamosa 20,000 3,050 1,240 2,714 950 T. gigas Hippopus hippopus 300 1,110 111,100 3,330 All giant clam spp. 15,000 7,000 240,300 59,993 44,818 344,000 1,900,000 2,400,000 1,444,400 136,660 44,290 Author & Location Condesa (2005) Bolen (2005) Sayson (2003) Alcala (1986) Calumpong and Cadiz (1993) Ozoa (1995) This study (2005) Green Island, Roxas, Inter- tidal Cara- may, Roxas, Inter- tidal Snake Island, PPC, 1-5m Binaluan Bay, Taytay, 1.5-3.5m Cagayan Island, 0.5-5m Northern Palawan, 0.5-5m TRNMP North Islet, 6-10 m TRNMP South Islet, Intertidal TRNMP Intertidal TRNMP Intertidal TRNMP Reef slope, 5mSpecies Focal benthic mollusks 9 REFERENCES Alcala, A.C., 1986. Distribution and abundance of giant clams (Family Tridacnidae) in the South-Central Philippines. Silliman Journal, 33(1-4): 1-9. 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