Abstract
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 0.933×10 -5 cm 2 s -1 and 0.124×10 -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 - Our results show that sperm chemoattractants diffuse rapidly from the 170-μm-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.
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