An in situ measurement of sperm dispersal in a colonial marine hydroid

Because gamete dispersal in the water column is an essential feature of sexual reproduction in most sessile invertebrates, an analysis of gamete dispersal patterns in the field is vital to an understanding of gene flow in these organisms. This paper presents the results of an in situ measurement of sperm dispersal in the colonial hydroid Hydractinia echinata conducted in calm water. The environmental cue mediating gamete release (exposure to light) was manipulated to temporally isolate experimental animals from the natural population. As assayed by the development of eggs collected at various distances from a male colony, the fertilization rate was very high within 3 m but dropped off rapidly with no fertilization observed beyond 7 m. The short sperm dispersal distances reported here, combined with a general trend toward larval dispersal distances on a similar spatial scale, suggest that gene flow in colonial invertebrates occurs on a relatively small spatial scale.