Diversity and density of sea urchins populations in rocky shores off Nilwella in Southern province of Sri Lanka
M.A. GAYASHAN and S. JAYAKODY
Sea urchins (or regular echinoids) are one of the most common macro-grazers and keystone species in rocky shore systems. However, their ecology is little explored in Sri Lankan waters. Therefore, a study was conducted to investigate the spatio-temporal variations in the diversity and density of sea urchins off Nilwella in the Southern coast of Sri Lanka. Duwagahawella harbour area and Hirikatiya bay area were selected as disturbed and less disturbed areas respectively considering the presence and absence of a harbour and other anthropogenic impacts. Random permanent strip transects of varying lengths (11-20m×0.5m) were used to estimate the number of sea urchins for 5 months with 20 repeated weekly samplings. Sea urchin density and diversity were estimated for two sites and means were compared. According to the results, a total of 8 sea urchin species were detected from the two areas. Stomopneustes variolaris was the dominant species in both areas. The mean density of Stomopneustes variolaris was significantly higher in disturbed area compared to the less disturbed area (P<0.05). However compared to disturbed area, densities of Tripneustes gratilla, Diadema setosum, Diadema savignyi, Echinostrephus molaris and Echinometra mathaei were higher (P<0.05) in less disturbed area. Throughout the sampling, Shannon Weinner index of diversity was higher in less disturbed area (P<0.05). Results highlighted the dominance of S. variolaris particularly in disturbed area. Further studies are necessary to establish the competitiveness of S. variolaris and if it out-competes the other associated sea urchin species.