Mercenaria mercenaria introductions into Florida, USA, waters: duration, not size of introduction, influences genetic outcomes

Wild stocks of hard clams from the genus Mercenaria historically supported commercial fisheries, but, as these stocks have been depleted, market demand has been filled by clams produced in aquaculture operations. Along the east coast of the USA, M. mercenaria is indigenous and used for aquaculture. Indigenous Gulf of Mexico (GOM) clam populations consist of M. campechiensis, a species less suited to aquaculture because of its short shelf life. Aquaculture operations have introduced M. mercenaria into GOM waters, resulting in altered Mercenaria species composition to the detriment of M. campechiensis. Where M mercenaria have been introduced, the population now includes both M. mercenaria and M campechiensis, and hybrids Eire common. This is particularly evident where commercial aquaculture is ongoing and introductions are, therefore, continuous. Where short-term introductions have occurred for research purposes, M. mercenaria is restricted to oyster reefs. At those sites, and at sites where no documented introductions of M. mercenaria have occurred, hybrids are rare and M. campechiensis dominate the more common sand/seagrass habitats. As the 2 species continue to interbreed, pure M campechiensis will become rarer and hybrids more common, potentially resulting in reduced fitness of M campechiensis from GOM waters. The trend towards hybrids may be ameliorated by an increased intensity of gonadal neoplasia in the hybrid clams. These results indicate that M mercenaria introductions into the GOM, even on the relatively small scale of scientific research efforts, have a substantial and long-lasting influence on the genotype composition of hard clam populations in GOM waters.