Fitting fertilisation kinetics models for free-spawning marine invertebrates

To determine how fertilisation varied with sperm concentration for two spec ies of scallop, Chlamys (Equichlamys) bifrons (Lamarck) and C. asperrima (L amarck), we performed a simple series of sperm dilution experiments, and me asured 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, maxim um fertilisation occurred at an ambient sperm concentration of around 100 s perm mul(-1) the maximum proportion of eggs fertilised was less than 0.70 i n the C. bifrons experiments, but nearer 1.0 with C. asperrima. At high spe rm 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 fertilisati on kinetics model could be fitted to the experimental data, but unique para meter estimates could not be determined.