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

Abstract

Wild stocks of hard clams from the genus Mercenariahistorically 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. mercenariais 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. mercenariainto GOM waters, resulting in altered Mercenariaspecies composition to the detriment of M. campechiensis. Where M. mercenariahave been introduced, the population now includes both M. mercenaria and M. campechiensis, and hybrids are 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. mercenariais restricted to oyster reefs. At those sites, and at sites where no documented introductions of M. mercenaria have occurred, hybrids are rare and M. campechiensisdominate 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. campechiensisfrom 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. mercenariaintroductions 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. © Inter-Research 2009.