RIFFELL, J.A.*; ZIMMER, R.K.: Egg limitation may regulate fertilization success in a free-spawning marine invertebrate

Sperm limitation has often been invoked as a major determinant of reproductive success for organisms that spawn their gametes into the ocean environment. However, the extent to which sperm limitation mediates fertilization success in the field requires knowledge of sperm-egg encounter rates, as well as physiochemical properties of male and female gametes. In the laboratory, we investigated the influence of gamete age, sperm-egg contact time, and concentration on fertilization in the red abalone, Haliotis rufescens. Our results indicate that eggs, not sperm, are the limiting factor controlling reproductive success. Sperm remained viable for up to 8 h, but egg viability decreased exponentially after the first thirty minutes. Whereas egg suspensions were negatively buoyant and rapidly sank, sperm were neutrally buoyant, and thus in gamete plumes transport farther distances. The viscosities of egg and sperm suspensions both exhibited shear-thinning behavior (decrease in viscosity with an increase in shear). However, the viscosity of sperm solutions quickly decay to levels approaching that of sea water at lower concentrations (< 10⁷ sperm-ml^-1). In contrast, egg concentrations were still highly viscous at relatively low concentrations (10² egg-ml^-1) and high shear rates (> 60 s^-1). Thus, the physical properties of sperm and eggs may profoundly influence the transport and mixing of free-spawned gametes. These data suggest that egg limitation determines the rate of gamete contact, and hence fertilization success, in the red abalone.