Insight into the population structure of hardhead silverside, (Teleostei: Atherinidae), in Belize and the Florida Keys using .

Little is known about the natural history, biology, and population genetic structure of the Hardhead Silverside, a small schooling fish found around islands throughout the Caribbean. Our field observations of in the cays of Belize and the Florida Keys found that populations tend to be in close association with the shoreline in mangrove habitats. Due to this potential island-based population structuring, represents an ideal system to examine questions about gene flow and isolation by distance at different geographic scales. For this study, the mitochondrial gene was amplified from 394 individuals collected from seven different Belizean Cays ( = 175) and eight different Floridian Keys ( = 219). Results show surprisingly high haplotype diversity both within and between island-groups, as well as a high prevalence of unique haplotypes within each island population. The results are consistent with models that require gene flow among populations as well as in situ evolution of rare haplotypes. There was no evidence for an isolation by distance model. The gene tree consists of two well-supported monophyletic groups: a Belizean-type clade and a Floridian-type clade, indicating potential species-level differentiation.