Presence of free-living Pacific oyster Crassostrea gigas (Thunberg, 1793) at El Vizcaino Biosphere Reserve, Mexico, confirmed by morphological and molecular approaches.

The Pacific oyster (Crassostrea gigas) is an important commercial species introduced to Mexico in the 1970s for aquaculture production. A purportedly infertile triploid variety of this species was authorized for farming in the El Vizcaino Biosphere Reserve (EVBR), to generate economic benefits for local residents while at the same time avoiding offsite propagation. Nevertheless, the presence of oysters at Ojo de Liebre lagoon, a core area of the reserve, has been observed. To test for the occurrence of C. gigas beyond its rearing areas, 240 oysters were collected from 10 sites in the lagoon and analyzed on the basis of its morphology and with the mitochondrial cytochrome oxidase subunit I; these approaches positively identified 95 and 70 C. gigas specimens respectively, corroborating the existence of individuals living outside the farming areas in the core zone of the reserve. Histological examinations discovered oysters with gonads in different stages of maturity (including spawning), verifying that the free-ranging specimens were reproductively active. We propose three hypotheses of how colonization of C. gigas took place at EVBR: the recruitment of larvae from unknown nearby populations, the inadvertent introduction of diploid organisms in the aquaculture facilities, and the possibility that triploid specimens growing in the local hatcheries have produced viable diploid gametes. Although C. gigas should be considered as an introduced species at the reserve, its population size is small, the specimens mostly settled on artificial substrata and do not cover large areas on the bottom, and there is no evidence of ecological damage or substitution of resident species. We suggest that the Pacific oyster is in the first stages of colonization in the EVBR, and that collaborative work of reserve managers and producers have the possibility to address the invasion and minimize potential negative effects on the local ecosystems, as well as possible dispersal to other areas. [ABSTRACT FROM AUTHOR]