THE EFFECTS OF SPERM CONCENTRATION, SPERM-EGG RATIO, AND GAMETE AGE ON FERTILIZATION SUCCESS IN CROWN-OF-THORNS STARFISH (ACANTHASTER PLANCI) IN THE LABORATORY

Laboratory experiments varying gamete concentrations and gamete age de monstrated significant reductions in fertilization success of the star fish Acanthaster planci (L.) with decreasing sperm concentration and i ncreasing age of both eggs and sperm. The effect of aging in sperm was faster than that of eggs, and the speed of sperm aging increased with increasing dilution of sperm. Fertilization success was high over a w ide range of sperm: egg ratios but declined rapidly at ratios less tha n 50, particularly at low sperm concentrations. A. planci gametes aged more slowly, and the loss of fertilizing capacity of sperm with dilut ion (the respiratory dilution effect) was far less, than in sea urchin s. These characteristics provide a mechanism for enhanced fertilizatio n success at given sperm concentrations and at greater distances and t imes from the point of gamete release, and may explain the higher fert ilization rates achieved over longer distances in the wild by A. planc i relative to sea urchins. Gametes would remain competent for longer p eriods at more dilute concentrations and so better achieve long-distan ce fertilization. Gametes obtained at the end of the breeding season w ere qualitatively different from those obtained early in the breeding season and showed reduced fertilization success for a given combinatio n of variables, and different fertilization dynamics.