Spatial distribution of ascidian sperm: two-dimensional patterns and short vs. time-integrated assays

Abstract

Understanding sperm dispersal patterns in free-spawning marine invertebrates is fun- damental to assessing the likelihood of successful fertilization under a range of biological and physi- cal conditions. Field estimates of sperm dispersal in egg broadcasters have relied on sperm assays that sample short-term sperm availability, and tended to focus on simple patterns of availability with distance along a 1-dimensional transect at a single site. However, sperm are diluted in multiple dimensions, and the decrease in concentration with increasing distance from a sperm source in any single dimension is dependent on the rate of dilution in other dimensions. We used short-term sperm availability assays to evaluate the 2-dimensional distribution of sperm downstream of 2 different pop- ulations of the egg-brooding colonial ascidian Botryllus schlosseri. Female colonies were exposed to water samples collected at various distances and 2 depths. Because many egg brooders that feed on phytoplankton appear to collect sperm throughout an extended period of time, short-term assays of sperm availability may underestimate absolute fertilization. Consequently, at one site we also com- pared fertilization in our short-term assays with time-integrated fertilization levels obtained by deploying colonies in the sampling locations for the entire period of egg viability. Our results indicate that vertical patterns of sperm availability with horizontal distance can vary between sites, suggest- ing that sperm transport patterns will be site-specific and dependent on local flow, and that vertical mixing may cause a more rapid decline in sperm concentration with increasing distance. Our com- parison of instantaneous vs. time-integrated assays suggests that short-term assays may vastly under- estimate actual field fertilization levels in some free-spawners.