Fertilization success in free-spawning marine invertebrates: Why egg size isn't important

Recent studies have suggested that the probability of fertilization success in free-spawning marine invertebrates increases with larger egg sizes. The eggs of many species release compounds which activate sperm and induce swimming toward the egg. It is widely assumed that the adaptive significance of these responses lays in increasing fertilization success. Thus, the diffusion of these compounds into the water surrounding the egg would cause sperm to orient and swim toward an egg while still some distance away from it. This will increase the probability of fertilization, and hence increase the effective size of the egg. Consequently, the physical diameter of eggs may not be a good indication of the "effective" egg size.

The effective egg size of the solitary ascidian Ciona intestinalis was estimated from molecular diffusion models using experimentally derived data on the molecular weight of the chemoattractant, its release rate from the egg, and threshold chemoattractant concentrations required to elicit a sperm response. Four non-turbulent diffusion scenarios were estimated. In three of these scenarios, effective egg size reached 1.0 mm (radius) from the physical edge of the egg between 5.4 and 35.6 minutes after egg release. In the remaining scenario, effective egg size was never found to reach this distance. Models that incorporated eddy (turbulent) diffusion indicated that the effective egg size never extended beyond the boundaries of molecular diffusion. The implications of these findings for existing hypotheses on the evolution of egg size will be discussed.

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