ABSTRACT
Egg size correlates with fertilization rate and a range of life-history parameters. The sea urchins Strongylocentrotus droebachiensis and S. pallidus show spectacular intraspecific variation in egg size with latitude along the Norwegian coast, as well as differences in egg jelly-coat thickness that are inversely proportional to egg size. Recent studies suggest that ‘effective egg size’ (egg extracellular coat) rather than ovum size, determines ‘target size’ and thus affects fertilization rates in free-spawning marine organisms; a larger target increases fertilization at limiting sperm concentrations. Surprisingly, while mean egg surface area increases more than 2-fold along a south-north gradient in these species, the surface area of the jelly coat remains nearly constant. Results from fertilization experiments with large eggs (200 m, Spitzbergen) and small eggs (145 m, Bergen) indicate that ovum size, not effective egg size, predicts fertilization success in these species. Variation in ovum size had a stronger effect on fertilization success than predicted for, using a standard fertilization kinetics model. Sulfated fucans in the jelly layer are required to trigger the acrosome reaction in these species, but a proportionally thicker jelly coat may hinder rather than augment fertilization by acting as a physical barrier to sperm and may contribute to the lower fertilization rates observed in small eggs with thick jelly coats.
