SPERM DIFFUSION-MODELS AND IN-SITU CONFIRMATION OF LONG-DISTANCE FERTILIZATION IN THE FREE-SPAWNING ASTEROID ACANTHASTER PLANCI

This study was undertaken to compare fertilization rates of the sea st ar Acanthaster planci that were predicted using sperm diffusion models with those that were determined under natural conditions in the field . During experimentally induced spawnings, measured fertilization rate s for broadcast eggs were high. More than 70% of the eggs were fertili zed at distances as great as 8 m downstream from a single spawning mal e starfish, and more than 20% were fertilized at separations of more t han 60 m. Fertilization was still measurable, at 5.8%, 100 m downstrea m. Lateral diffusion of sperm away from the axis of now produced ferti lization rates of 13.8% at 8 m normal to the now and 32 m downstream. The large volumes of sperm released by male A. planci are the primary cause of high rates of fertilization for eggs derived from widely spac ed individuals. Models of sperm diffusion using high sperm release rat es such as those found in this starfish accurately confirmed the ferti lization rates measured in situ for two populations of A. planci with widely differing rates of sperm release. We observed some changes in s tarfish density and degree of aggregation in the study population for spawning periods during two spawning seasons, though these were not st riking. High levels of aggregation may not be necessary for fertilizat ion success in this starfish, due to the potential for long-distance f ertilization and the probability that, for any spawning starfish, the total number of zygotes formed will be greater at some distance from t he point of spawning. Although fertilization rates in areas distant fr om the sperm source were relatively low, the total area for potential gamete encounters is much greater and may make a large contribution to net fertilization. We predict that other behaviors, such as migration to shallow water, commonly associated with spawning in A. planci and other marine invertebrates will have measurable impacts on fertilizati on success. The potential for high levels of fertilization in A. planc i was realized during natural spawnings. Fertilization rates as high a s 99% were recorded when levels of spawning synchrony were high.