DNA barcoding reveals Brachidontes (Bivalvia: Mytilidae) from two ecologically distinct intertidal habitats on Long Key, Florida Keys, are cryptic species, not ecotypes

The nominal morphospecies Brachidontes exustus (Linnaeus, 1758) represents a cryptic species complex with multiple genetic disjunctions resulting in regionally dominant, but range-restricted, species throughout the western Atlantic, Caribbean, and Gulf of Mexico. In the Florida Keys, four species were previously identified using molecular techniques. Specimens were collected in January 2005 from two distinct habitats, a seawall and a mangrove, on Long Key, Florida Keys. The locations are separated by <5 km. Eight specimens from the mangrove and four from the seawall were sequenced for the mitochondrial COI gene. Two seawall specimens were sequenced at the internal transcribed spacer 2 (ITS-2), in the nuclear ribosomal gene cluster, after the COI sequences appeared to be from the male mitochondrial line. The COI and ITS-2 sequences indicate that the two locations on Long Key, Florida Keys, have different single-species populations. The four seawall specimens were the Antillean species while the eight mangrove specimens were the Gulf species. Given that these mussels broadcast spawn, with subsequent planktotrophic larval development, the sites likely share a common pool of potential larval recruits. Single-species populations at each location are suggestive of habitat partitioning, ecological filters, or differential recruitment.