Many coastal marine invertebrates reproduce via external fertilization. Spawned gametes are exposed to the turbulence generated as waves break on the shore. This turbulence not only rapidly dilutes the gametes, but also mechanically damages them, and makes it difficult for the egg and sperm to come into contact. Eggs and sperm from 16 species of externally fertilizing invertebrates were exposed to environmentally relevant shear stresses in a Couette cell. This device consists of two concentric rotating cylinders that subject fluid between the cylinders to well-characterized shear stresses that recreate some aspects of surf zone turbulence. While all species tested (ALLOCENTROTUS FRAGILIS, COLOBOCENTROTUS ATRATUS, DENDRASTER EXCENTRICUS, ECHINOMETRA MATHEI, E. OBLONGA, LYTECHINUS PICTUS, STRONGYLOCENTROTUS PURPURATUS, TRIPNEUSTES GRATILLA, ASTERINA MINIATA, PISASTER OCHRACEOUS, CALLOISTOMA LIGATUM, HALIOTIS RUFESCENS, MYTILUS CALIFORNIANUS, M. GALLOPROVINCIALIS, ASCIDIA CERATODES, and URECHIS CAUPO) showed a decrease in fertilization success when exposed to shear stress, the magnitude of the decrease, and the shear stress at which the effect was apparent, varied among species. The gametes of some species were more susceptible to shear-induced damage than were others. The development of some species was delayed, or was abnormal, when fertilization occurred under moderate or high shear stresses. Many of the effects of shear stress are correlated with the wave exposure of the source population's habitat; others are probably related to a complex suite of structural features of the gametes.

The mechanisms by which shear stress affects fertilization and development were examined by performing fertilization experiments in a Couette cell under different velocity gradients and viscosities. Experiments with the purple sea urchin S. PURPURATUS showed that while shear stress (the product of the velocity gradient and the viscosity) governs fertilization success, velocity gradient is the primary factor affecting developmental success. Different suites of mechanisms fit these two patterns, and are thus implicated in either fertilization or development.

Field experiments with S. PURPURATUS indicate that surge channels, which are long, narrow topographical features of some coast lines, could serve as reproductive refuges for free-spawning invertebrates. Water containment and rapid mixing within the surge channel enable the eggs and sperm to come into contact, even when released meters apart. This increases fertilization success and is likely to influence the population structure.