Effect of sperm concentration and sperm ageing on fertilisation success inthe Antarctic soft-shelled clam Laternula elliptica and the Antarctic limpet Nacella concinna

Sperm concentration and sperm ageing effects on fertilisation success were evaluated in the laboratory in the free-spawning Antarctic soft-shelled cla m Laternula elliptica and Antarctic limpet Nacella concinna. Fertilisation success was highly dependent on sperm concentration. Highest levels of fert ilisation success were consistently replicated at similar to 10(7) sperm ml (-1) for L. elliptica, and similar to 10(6) to 10(8) sperm ml(-1) for N. co ncinna. However, both species exhibited extremely low fertilisation rates a t concentrations similar to 10(6) sperm ml(-1). At sperm concentrations sim ilar to 10(6) sperm ml(-1) N, concinna displayed a rapid increase in abnorm ally developing larvae which, along with only a small decline in total fert ilisation success above 10(8) sperm ml(-1), was taken to indicate polysperm y. A small increase in abnormal development followed by a rapid decline in fertilisation success at high sperm concentrations (> 10(7) sperm ml(-1)) f or L, elliptica was attributed to oxygen depletion. Using the optimum sperm concentration found for fertilisation success, spermatozoa were capable of fertilising fresh ova for > 90 h in L. elliptica, and similar to 65 h in N . concinna. The sperm concentrations required for fertilisation success and sperm longevities reported here are at least an order of magnitude greater than those reported for nearshore temperate molluscs. Our data strongly su ggest that the specific reproductive behaviour and timing of spawning activ ities displayed by these Antarctic molluscs is vital to enhance fertilisati on success in the polar marine environment.