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, Acanthaster planci (L.), was measured in the held using an array of collectors up to 10 m downstream of a spawning starfish. Hydrodynamic measurements, gamete dispersal numerical models, and the gamete cloud dispersal measurements for the first time quantified the relationship between hydrodynamic conditions and the dispersion of eggs and sperm in the field. In general, gamete concentrations fell rapidly and logarithmically with distance from the spawning starfish; egg concentrations at 3 m were 1% of those near the starfish. Simplified dispersal models showed a good correspondence with these field data, and confirmed the observation 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 fertilization success and the gamete concentrations measured in the held, were estimated to be 90-100% within 1 m and 70-100% at 10 m. These results are explained by high success rates of fertilization (fertilizing capacity) at the measured dilutions, and were similar to fertilization rates previously measured by others for crown-of-thorns starfish in the field. Although the eggs were observed to spread upwards into the water column due to turbulence, laboratory measurements of sinking rates showed eggs to be very slightly negatively buoyant (median fall velocity of 0.072 mm.s(-1)), whereas sperm were neutrally buoyant. A significant fraction of eggs also entered the seabed near the starfish; the proportion decreased with increasing current strength. This process may provide a mechanism for enhanced fertilization of these gametes and/or a mechanism for self-recruitment to a given reef population.