Influence of Body Size and Population Density on Fertilization Success and Reproductive Output in a Free-Spawning Invertebrate.

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.