Two closely related species in the camarodont sea urchin family Strongylocentrotidae, Strongylocentrotus pallidus and Strongylocentrotus droebachiensis, have circumarctic ranges with overlapping depth distributions and spawning seasons. Both species show genetic differences between Europe and the Pacific in the nuclear sperm bindin gene, and S. droebachiensis also differs in mitochondrial DNA. Gamete recognition systems of allopatric populations may or may not have drifted apart, but are unlikely to have diverged by reinforcing selection. Geographic divergence in gamete compatibility may illustrate mutational biases and variability and hence possible initial stages of reproductive isolation.

We carried out fertilisation experiments with both species' gametes from Norway and from the Eastern Pacific Ocean. Serially diluted sperm were added to egg suspensions in factorial designs, and fertilisation success was assessed by counting the proportion of cleaving embryos. To separate the specificity of the acrosome reaction from subsequent sperm–egg binding steps, a subsample of sperm was pre-activated by performing the last dilution step in diluted egg jelly from conspecific females. Fertilisation successes were compared by logistic regression ANCOVAs over five, 4-fold sperm dilutions for each cross. The graphs shown here illustrate fertilisation success for one sperm concentration only.