ACTA BOT. CROAT. 82 (2), 2023 83 Acta Bot. Croat. 82 (2), 83–86, 2023 CODEN: ABCRA 25 DOI: 10.37427/botcro-2023-012 ISSN 0365-0588 eISSN 1847-8476 First record of the dinoflagellate Tripos rotundatus in the Adriatic Sea Nika Pasković*, Iris Dupčić Radić University of Dubrovnik, Institute for Marine and Coastal Research, Kneza Damjana Jude 12, P.O. Box 83, HR-20000 Dubrovnik, Croatia Abstract – This report presents the first record of Tripos rotundatus (Jørgensen) Gómez in the Adriatic Sea. The spe- cies was found in a net sample in the 50 – 100 m depth layer, taken on July 2021 off the southern coast of the Adriatic Sea. The species T. rotundatus was probably previously misidentified as the morphologically similar species T. digitatus (Schütt) Gómez. Keywords: biodiversity, NE Mediterranean, phytoplankton, southern Adriatic, taxonomy Introduction Dinoflagellates are an important group of protists with a remarkable diversity of life forms (i.e. free-living, para- sites, and mutualistic symbionts), habitats (plankton and benthos), and nutrition modes (heterotrophic, chloroplast- containing) (Gómez 2012). In the Mediterranean Sea, 673 taxa have been identified, while 322 taxa have been reported for the Adriatic (Gómez 2003). In general, dinoflagellates are organisms dominantly found in oligotrophic waters (Gómez 2003). Among dinoflagellates, the genus Tripos Bory includes the greatest number of taxa (~800) (Gómez 2021) and is globally widespread in marine waters. Previously, species of the genus Tripos were known as the marine section of the genus Ceratium Schrank, until morphological and molecu- lar data supported the separation of marine and freshwater species of Ceratium at the genus level and restricted Ceratium to freshwater species (Gómez et al. 2010). Marine species are consistently large and robust, often with horns (Gómez 2021). The aim of this study is to report the first record of Tripos rotundatus (Jørgensen) Gómez for the Adriatic Sea. Materials and methods Sampling was conducted on July 17, 2021 at the Lokrum coastal station, near Dubrovnik (southern Adriatic Sea, 42°37’21” N, 18°06’05”E) (Fig. 1). Vertical profiles of tem- perature and salinity were measured from the surface to the bottom (90 m depth) at each meter using a multiparametric conductivity-temperature-depth (CTD) probe, and density (sigma-t) was calculated from these data. Water samples were collected using 5-L Niskin bottles for dissolved oxy- gen, nutrients, chlorophyll a (Chl-a), and phytoplankton on the surface and at depths of 5, 10, 20, 50, 75, and 100 m. Net samples were collected using a Nansen net with 53 μm mesh and 200 μm mesh in two layers: 0–50 m and 50–100 m. Dissolved oxygen was determined by Winkler titration, and oxygen saturation (O2/O2') was calculated from the solubil- ity of oxygen in seawater as a function of corresponding temperature and salinity (Weiss 1970, UNESCO 1987). Nu- trient samples were analysed in the laboratory using a spec- trophotometer according to Strickland and Parsons (1972). To estimate Chl-a, 1 L subsamples were filtered through Whatman GF/F glass microfiber filters and were analysed fluorimetrically (Holm-Hansen et al. 1965). The trophic in- dex (TRIX) was calculated to classify the trophic status of the coastal marine area (Vollenweider et al. 1998). Phyto- plankton samples were preserved in neutralized formalde- hyde (2.5% final concentration) and observed with an Olympus IX-71 inverted microscope according to the Utermöhl method (Utermöhl 1958). For a detailed descrip- tion of the method used to analyse nutrients, Chl-a, and phytoplankton (for details see Jasprica et al. 2022). The nomenclature of taxa follows Guiry and Guiry (2023). * Corresponding author e-mail: nika.paskovic@unidu.hr PASKOVIĆ N., DUPČIĆ RADIĆ I. 84 ACTA BOT. CROAT. 82 (2), 2023 Results Vertical thermal stratification of the water column was found during sampling date. The water column in the up- per layer (50 – 0 m) was stratified, with a temperature range (15.93 – 26.25 °C), salinity (36.56 – 38.93) and density (24.36 – 28.78) (Tab. 1). O2/O2’ ranged from 0.83 to 1.06 and was lower (0.83 - 0.86) in the bottom layer. TIN was calculated as the sum of nitrate (NO3), nitrite (NO2), and ammonium (NH4). NH4 accounted for the high- est proportion of TIN in the entire water column, 59.5%, and NO3 for the lowest, 16.3%. PO4 ranged from 0.01 to 0.066 µM, with the highest value measured at 10 m depth. SiO4 ranged from 1.23 to 4.84 µM, with higher values in the upper 50 m. The highest Chl-a (0.32 mg m–3) and phytoplankton abundance (2.1 × 105 cells L–1) were found at a depth of 75 m. TRIX ranged from 1.1 to 2.8, classifying the station as oligotrophic. Altogether, 48 phytoplankton taxa were identified in seven samples. Thirty-five taxa were dinoflagellates, 10 diatoms and three coccolithophorids. Among larger phytoplankton cells (> 20 µm cell long) Thalassionema nitzschioides ( Grunow) Mereschkowsky, Oxytoxum sphaeroideum Stein, Oxytoxum variabile J. Schiller, Oxytoxum caudatum Schiller were the most abundant (> 945 cell L–1). Nanophytoflagel- lates (2–20 µm cell long) dominated (96.3%) in total phyto- plankton abundance. Tripos rotundatus was found in the net sample in the 50–100 m depth layer (Fig. 2). Discussion Water column stratification is a common occurrence during summer, as is the occurrence of a thermocline and of water column stability (Ninčević Gladan et al. 2015). The low nutrient and Chl-a, and in general the low trophic status, indicated a summer situation common in the oligotrophic coastal area. Moreover, the oligotrophy of the Lokrum station is confirmed by the abundance of phyto- plankton dominated by nanophytoplankton, typical of the spring and summer period in the coastal southern Adriatic Sea (Caroppo et al. 1999), and by the highest abundance of dinoflagellates, typical organisms of oligotrophic waters (Gómez 2003). As T. rotundatus was pooled as T. digitatus (Schütt) Gómez, and most of the records are not illustrated, we have Tab. 1. Physico-chemical parameters, chlorophyll a and total phytoplankton abundance in two layers (0–50, 50–100 m) of the water column on the Lokrum station in the southern Adriatic Sea, 17th July 2021 (n = 7). Parameters Layer (m) 0 – 50 50 – 100 Temperature (°C) 15.93 – 26.25 15.43 – 15.95 Salinity 36.56 – 38.93 38.93 – 38.96 Density (kg m–3) 24.36 – 28.78 28.79 – 28.93 Oxygen saturation (O2/O2’) 0.95 – 1.06 0.83 – 0.86 Phosphate (µM) 0.01 – 0.06 0.04 – 0.06 Total inorganic nitrogen (µM) 0.23 – 0.33 0.29 – 0.60 Nitrate (µM) 0.01 – 0.08 0.06 – 0.09 Nitrite (µM) 0.008 – 0.03 0.008 – 0.32 Ammonium (µM) 0.21 – 0.23 0.19 – 0.22 Silicate (µM) 1.49 – 4.84 1.23 – 4.06 Chlorophyll a (mg m–3) 0.06 – 0.16 0.17 – 0.32 Total phytoplankton abundance (cells L–1) 9.1´104 – 1.4´105 7.7´104 – 2.1´105 Fig. 1. Position of the coastal Lokrum station in the southern Adriatic Sea, where the dinoflagellate species Tripos rotundatus (Jørgensen) Gómez was found, 17th July 2021 (derived and adapted from Google earth). TRIPOS ROTUNDATUS IN THE ADRIATIC SEA ACTA BOT. CROAT. 82 (2), 2023 85 little information on the distribution of the species. Tripos rotundatus was previously reported as T. tasmaniae (E.J.F.Wood) F.Gómez in Australian Pacific waters (Wood 1963). Tripos rotundatus also had a different taxonomic status in the literature (Guiry and Guiry 2023). Originally, T. rotundatus was named Ceratium digitatum Schütt var. rotundatum Jørgensen (Jørgensen 1920). The species was found west of the island of Rhodes in the Aegean Sea in Au- gust 1910. Our data coincided with Jørgensen’s findings, re- garding the sampling season (in summer) and the position of the station (coastal). Tripos digitatus, a morphologically similar species, was found in the Mediterranean Sea (Gómez 2003) and in the northern Adriatic Sea (Revelante 1985). The differences in the morphological characteristics of these two taxa have been recently highlighted (Gómez 2021). There are differences in the epitheca and hypotheca. The epitheca of T. digitatus is strongly directed dorsally, the left antapical horn is directed anteriorly, and the apex has a short projection. In T. rotundatus, the epitheca is less bent towards the dorsal side, the short projection on the apex is missing, and the left antapical horn is directed laterally (Gómez 2021). The morphological descrip- tion of the species T. rotundatus follows the morphology of the taxa recorded at the Lokrum station. In our case, the cell of T. rotundatus was 110 μm long and 30 μm wide. This finding contributes to a better understanding of the diversity of dinoflagellates in the Adriatic Sea. 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