The cryptic population biology of Chthamalus fragilis Darwin, 1854 (Cirripedia, Thoracica) on the Atlantic coast of North America

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Better understanding of the mechanisms by which novel species arrive in marine communities is not only important for documenting such arrivals but can also lead to a better awareness of the abiotic and biotic limits on species distributions. Here we integrate spatial survey data, metapopulation hindcast modeling, and new genetic data from allozymes and mitochondrial loci to identify how cryptic diversity within the barnacle species Chthamalus fragilisDarwin, 1854 responds to the coastal environment to establish contemporary patterns of diversity and abundance. This is of interest because C. fragilis has long been recognized as novel in the southern New England intertidal community, having arrived in the late 1800s as the likely result of either human introduction or increased global temperature. Our results demonstrate an elevated and somewhat distinct genetic diversity for this species in southern New England relative to the rest of its distributional range. Our models explain this best as resulting from a “natural” expansion of the barnacle’s range late in the 19th century, with genetic connectivity between its northern and southern populations now hampered by a recruitment gap along the Delmarva Peninsula.