Fitting fertilisation kinetics models for free-spawning marine invertebrates

To determine how fertilisation varied with sperm concentration for two species of scallop, Chlamys (Equichlamys) bifrons (Lamarck) and C. asperrima (Lamarck), we performed a simple series of sperm dilution experiments, and measured egg size and sperm swimming speeds. C. bifrons eggs were much larger (average diam = 116.5 mum), and sperm swimming speeds faster (209.8 mum s(-1)), than C, asperrima (71.2 mum, 166.0 mum s(-1)). In both species, maximum fertilisation occurred at an ambient sperm concentration of around 100 sperm mul(-1) the maximum proportion of eggs fertilised was less than 0.70 in the C. bifrons experiments, but nearer 1.0 with C. asperrima. At high sperm concentrations (>100 sperm mul(-1)), fertilisation decreased (presumably due to polyspermy) with increasing sperm concentration, but decreased more rapidly in C. bifrons than C. asperrima. A polyspermy-adjusted fertilisation kinetics model could be fitted to the experimental data, but unique parameter estimates could not be determined.