Asynchrony in settlement time between the closely related oysters Crassostrea angulata and C. gigas in Ria Formosa lagoon (Portugal)

The Portuguese oyster Crassostrea angulata and the Pacific oyster Crassostrea gigas are very closely related species originating from Asia that were introduced in Europe. The objectives of the present study were to determine if there is asynchrony in settlement time of C. angulata and C. gigas larvae in an area where both taxa coexist and if hybridization between the two species occurs throughout the breeding season. Oyster collectors were deployed every two weeks during one year in two different sites in Ria Formosa lagoon (Portugal) and the oysters that settled were genotyped with nuclear and mitochondrial markers. Settlement of Crassostrea sp. was observed from April to November 2004 (n = 694), with two distinct periods. In the first period (April), when 19% of the oysters settled, the haplotypes and alleles characteristic of C. gigas were mainly observed. In the following settlement period (July to November), the frequency of haplotypes and alleles of C. gigas decreased over time while those characteristics of C. angulata reached high frequencies by the end of the settlement period. Our results suggest that C. gigas was able to reproduce at lower temperatures than C. angulata in spring. However, the non-significant allelic disequilibrium observed during almost all settlement periods suggests that hybridization is widespread in time. The results obtained in the present study show that there are differences in the reproductive behavior between C. angulata and C. gigas that may limit gene flow between the two species.