Interspecific variation in egg size of marine invertebrates has been previously explained by a trade-off between gamete quality and quantity: the production of many small eggs with high mortality or fewer large eggs that develop quickly and experience reduced planktonic mortality. This theory assumes 100% fertilization of eggs and predicts that either strategy results in a similar number of settling offspring per unit of energy invested in reproduction. Empirical support for the theory has been equivocal. Here I offer an alternative hypothesis: larger eggs present a larger target for sperm and thus are fertilized at a higher rate. This theory suggests a trade-off between the production of many small eggs with a low probability of fertilization or fewer large eggs with a higher probability of fertilization. This hypothesis is tested with three congeneric sea urchins, Strongylocentrotus purpuratus, Strongylocentrotus franciscanus, and Strongylocentrotus droebachiensis, with a fivefold difference in egg volume. Species with larger eggs are fertilized at a higher rate and, if one assumes an equal allocation of resources, produce at least as many zygotes as species with smaller, more numerous eggs. This alternate hypothesis can explain continuous variation in egg size between species and provides a strong link between larval and adult life histories.

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Gamete recognition proteins can evolve at astonishing rates and lie at the heart of reproductive isolation and speciation in diverse taxa. However, the source of selection driving this evolution remains unknown. We report on how the sperm bindin genotype influences reproductive success under natural conditions. An interaction between genotype frequency and spawning density determines how sperm bindin genotype influences reproductive success. Common genotypes are selected under sperm-limited conditions, whereas rare genotypes are selected under conditions of intense sperm competition and sexual conflict. Variation in the evolutionary rates of bindin may reflect historic differences in sperm availability.

Gamete production and fertilization influence zygote production. While gamete production is correlated positively with body size, individual fertilization success may be a function of population density. Usually it is assumed that when high population density leads to reduced body size and gamete production, per capita zygote production is diminished. This field study of the sea urchin Diadema antillarum Philippi provides a test of this assumption. Three experiments were conducted to determine the effect of male spawning on fertilization success. In the first experiment, unfertilized eggs were placed in Nitex bags at three distances up and downstream from a spawning male. In the second experiment, unfertilized eggs were released and captured at three distances downstream from a sperm source. In both experiments, fertilization decreased with distance from the sperm source. The final experiment tested the influences of male size and population density on fertilization success; the effect of density was significant, but size was not. A simple model estimates the average number of zygotes produced by females of average size under different density regimes: at high population density, increased fertilization success can compensate for decreased gamete production. Small individuals at high population density may have similar per capita zygote production as large individuals at low population density. Thus, estimates of reproductive output based on body size or gamete production alone can be misleading.

Many organisms reproduce by releasing gametes into the environment. However, very little is known about what proportion of released eggs become fertilized. We examined the influence of spawning group size, degree of aggregation, position within an aggregation, and water flow, on in situ fertilization in the sea urchin Strongylocentrotus franciscanus. This study was conducted at a depth of 9 m on the west coast of Vancouver Island, British Columbia, Canada. Males were simulated by syringes filled with sperm; females were simulated by sperm—permeable containers filled with eggs. Individuals were placed 0.5 or 2.0 m apart within a 2 x 2 or 4 x 4 (group size of 4 or 16 individuals) experimental array. The results indicate that group size, degree of aggregation, position within a spawning group, and water flow all affect fertilization success. Fertilization success. Fertilization success ranged from 0 to 82%. Increases in group size and aggregation, decreases in flow velocity, and central and downstream positions within an aggregation all lead to increase in fertilization success. Thus, individual reproductive performance is dependent on, and highly sensitive to, population parameters and environmental conditions.