Effects of Spatial Distribution and Reproductive Biology on in situ Fertilization Rates of a Broadcast-Spawning Invertebrate.

In situ fertilization was examined in the gorgonian Pseudoplexaura porosa during 1994 and 1995 spawning events in the San Blas Islands, Panama, to assess spatial and temporal variation in fertilization success and to determine whether in situ fertilization was sperm limited. Fertilization rates did not differ significantly between years (60% vs. 55%), but monthly means were significantly different, ranging from 22% to 66%. Fertilization rate varied among days, ranging from 0 to 85%; 80% of this variability was explained by daily variation in the number of colonies that spawned. A weighted average of in situ fertilization rates suggests that 67% or more of spawned eggs are fertilized in nature. Sperm limitation did not occur on the nights when most of the colonies synchronously spawned and when most of the eggs were released. Eggs collected downstream of the population often had higher fertilization rates than eggs collected either adjacent to their source colony or eggs collected in the middle of the population, which indicates that in dense populations, eggs may have multiple opportunities to be fertilized. Traits such as highly synchronous spawning, high fecundity, large egg size, large polyps, and large colonies directly and indirectly enhance P. porosa gamete production and fertilization. These life-history traits reduce the effects of gamete dilution during spawning events and thus decrease the importance of sperm limitation in the population dynamics of P. porosa.