Synchronous spawning and reproductive incompatibility of two bivalve species: Paphies subtriangulata and Paphies australis

The bivalve species Paphies subtriangulata (Wood, 1828) and Paphies australis (Gmelin, 1791) are abundant in New Zealand waters and may occasionally occur in sympatry where their habitats overlap at harbor mouths. Spawning events involving simultaneous gamete release of both species were observed on several occasions in October 1993, suggesting the possibility of in situ hybridization. High in situ fertilization rates, ranging between 83.3% and 100% were obtained for P. australis during one of these October spawning events. Hybridization was examined in vitro through a series of cross-fertilization experiments and compared to intraspecific crosses and controls (oocytes of each species). The expected high fertilization rates for conspecific crosses were observed over all experiments, giving means of 90.8% SE ± 1.5% and 93% SE ± 0.6% for P. subtriangulata and P. australis, respectively. The interspecific crosses generally resulted in low levels of fertilization but, at high sperm concentrations, the P. australis sperm × P. subtriangulata egg cross showed a reasonably high level of fertilization (approx. 63%). At the same sperm concentration, the fertilization rate of crosses of P. australis egg × P. subtriangulata sperm did not differ significantly from sperm-free controls. We conclude that while the possibility of hybridization exists, the probability of significant levels of hybridization is minimal due to potential prezygotic reproductive incompatibility.