Biological Consequences of Topography on Wave-swept Rocky Shores: I. Enhancement of External Fertilization.

Surge channels on wave-swept rocky shores are characterized by the violent hydrodynamic mixing that accompanies broken waves. It has been suggested that this mixing rapidly dilutes gametes shed into the surf zone, thereby severely reducing the fraction of eggs that can be fertilized externally. Although surge channels are well mixed within themselves, field experiments show that the exchange of water between these small embayments and the adjacent mainstream is surprisingly slow. Thus, surge channels may act as "containment vessels," limiting the rate at which gametes are diluted, and thereby enhancing the efficacy of external fertilization. Indeed, a mathematical model of fertilization in surge channels suggests that given a sufficient population of adult males within a surge channel, 80-100% of eggs may be fertilized. This result must be tempered, however, by the possibility that the small-scale shears induced by turbulence interfere with fertilization.