Habitat partitioning and phenotypic plasticity in the intertidal bivalve Brachidontes exustus cryptic species complex

The bivalve Brachidontes exustus is a common but under-studied member of intertidal hard-substrate communities along the southeast coast of the United States. Recently, this morphospecies was shown to be a cryptic species complex with regionally dominant, but range restricted species throughout its Western Atlantic, Caribbean and Gulf of Mexico range. In the Florida Keys, four cryptic species have been identified from nuclear and mitochondrial gene trees; two sister clades, each containing one common and one rare species. An investigation of the distribution of sister clades from a suite of habitats and locations throughout the Florida Keys archipelago was done by PCR of clade-specific fragment length differences of the Internal Transcribed Spacer-2 (ITS-2) ribosomal gene region. Populations at some locations were a mix of the two clades while other locations were represented by a single clade. Most striking, discrete intertidal habitats located only kilometers apart had different single-clade populations. As a broadcast spawning genus with planktonic development, larvae of all clades are likely supplied to all potential settlement locations. The observed distribution is evidence for clade-specific exclusion mechanisms acting in single-clade locations but absent in locations of coexistence. B. exustus, like other intertidal mussels, had been assumed to have highly plastic morphology dependent on environmental cues. A discriminant function was constructed that separates sister clades with 97% confidence, regardless of habitat, suggesting consistent clade-specific morphologies, even in locations of coexistence. The B. exustus species complex appears to be another example of genetic differentiation underlying assumed phenotypic plasticity in a marine invertebrate.