Effect of sperm concentration and sperm ageing on fertilisation success in the 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 clam Laternula elliptica and Antarctic limpet Nacella concinna. Fertilisation success was highly dependent on sperm concentration. Highest levels of fertilisation success were consistently replicated at ~107 sperm ml-1 for L. elliptica, and ~106 to 108 sperm ml-1 for N. concinna. However, both species exhibited extremely low fertilisation rates at concentrations <106 sperm ml-1. At sperm concentrations >106 sperm ml-1 N. concinna displayed a rapid increase in abnormally developing larvae which, along with only a small decline in total fertilisation success above 108 sperm ml-1, was taken to indicate polyspermy. A small increase in abnormal development followed by a rapid decline in fertilisation success at high sperm concentrations (>107 sperm ml-1) for 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 ~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 suggest that the specific reproductive behaviour and timing of spawning activities displayed by these Antarctic molluscs is vital to enhance fertilisation success in the polar marine environment.