Fertilization success and the effects of sperm chemoattractants on effective egg size in marine invertebrates

The effects of sperm chemoattractants released from eggs of the solitary ascidian Ciona intestinalis were estimated using a molecular-diffusion model. The diffusion coefficient of the chemoattractant was estimated to be between 0.933x10(-5) cm(2) s(-1) and 0.124x10(-5) cm(2) s(-1); the release rate of the chemoattractant from the egg was estimated at 10.6 pg s(-1), and the chemoattractant threshold concentration required to elicit sperm motility was estimated at between 91.3 mg ml(-1) and 9.31 mg ml(-1) Our results show that sperm chemoattractants diffuse rapidly from the 170-mum-diameter egg to create an effective egg size (the distance at which free-swimming sperm detect, and are attracted to, the egg membrane) of 2 mm diameter. If even half of the sperm encountering this chemoattractant halo subsequently reach the egg membrane, fertilization success will increase more than 50-fold. This difference is substantial in comparison to the changes in fertilization success, which accrue from changes in physical egg size. We suggest that current theory linking the evolution of marine-invertebrate: egg size with fertilization success should be considered with caution until the effects of chemoattractant compounds on fertilization success are better understood.