The nearshore prosobranch gastropod epifauna of Signy Island, South Orkney Islands
Id30/2/2727The neakshore prosobranch gastropod epifauna of Signy Island, South Orkney Islands. Picken,-.G.B.The prosobranch gastropod epifauna associated with a mature algal community on a rocky substrate in the depth range 2-12 m was studied at Signy Island, South Orkney Islands, Antarctica (60°S.43°W). Quantitative samples were collected by divers at monthly intervals from April 1975 to March 1977, using an air-lift suction sampler. Thirty-one prosobranch species were found in a total sample of about 138,000 animals; 7 species were undescribed, and 10 were new to the South Orkney Islands. Small, cryptic "Rissoids" such as Eatoniella kerguelenensis, E.caliginosa and Ovirissoa adarensis were numerically dominant, but the limpet Nacelle (Patinigera) concinna was dominant in terms of biomass. Mean prosobranch density was 4,591 individuals 1r2. and mean biomass 17.1 g dry tissue m2. Diversity was high throughout most of the depth range and the assemblage was both complex and stable. Ten species developed non-pelagically by means of large yolky eggs laid on algal fronds or stones in the sub-littoral. Spawning periods were prolonged and largely aseasonal, and development times were long. Recruitment of juveniles was also aseasonal, though a peak of recruitment was observed in one species during the Austral spring. Laevilacunaria antarctica shows several characteristics of 'K-selection'; it develops non-pelagically, grows slowly and lives for more than one year. Other weed-dwelling prosobranchs at Signy Island which develop non-pelagically probably also tend towards 'K-selection'. The limpet Nacella develops pelagically after adults congregate briefly in summer to form spawning "stacks" of 3-6 individuals. Nonetheless, the species shows several 'K-selection' traits, particularly very slow growth and deferred maturity, and tends towards 'K-selection'.Collections from other sites to 80 m depth off Signy Island yielded 20 more species; 13 of these were undescribed and 5 were new to the South Orkney Islands. The study has shown that the prosobranch fauna of the South Orkney Islands was incompletely known, and that a greater understanding of the distribution, endemism and origins of the Antarctic prosobranch fauna will not be possible until more key sites are investigated in greater detail.