SMALL-SCALE DISPERSION OF EGGS AND SPERM OF THE CROWN-OF-THORNS STARFISH (ACANTHASTER PLANCI) IN A SHALLOW CORAL-REEF HABITAT

The dispersal of eggs and sperm of crown-of-thorns starfish, Acanthast er planci (L.), was measured in the held using an array of collectors up to 10 m downstream of a spawning starfish. Hydrodynamic measurement s, gamete dispersal numerical models, and the gamete cloud dispersal m easurements for the first time quantified the relationship between hyd rodynamic conditions and the dispersion of eggs and sperm in the field . In general, gamete concentrations fell rapidly and logarithmically w ith distance from the spawning starfish; egg concentrations at 3 m wer e 1% of those near the starfish. Simplified dispersal models showed a good correspondence with these field data, and confirmed the observati on that eggs rose higher in the water column and spread more laterally at low current speeds over the short spatial scales being considered. Fertilization rates, predicted from laboratory measurements of fertil ization success and the gamete concentrations measured in the held, we re estimated to be 90-100% within 1 m and 70-100% at 10 m. These resul ts are explained by high success rates of fertilization (fertilizing c apacity) at the measured dilutions, and were similar to fertilization rates previously measured by others for crown-of-thorns starfish in th e field. Although the eggs were observed to spread upwards into the wa ter column due to turbulence, laboratory measurements of sinking rates showed eggs to be very slightly negatively buoyant (median fall veloc ity of 0.072 mm.s(-1)), whereas sperm were neutrally buoyant. A signif icant fraction of eggs also entered the seabed near the starfish; the proportion decreased with increasing current strength. This process ma y provide a mechanism for enhanced fertilization of these gametes and/ or a mechanism for self-recruitment to a given reef population.