Deep-sea octocorals and antipatharians show no evidence of seamount-scale endemism in the NW Atlantic

Seamounts are undersea mountains commonly characterized by accelerated currents, exposed hard-substrates, and relatively high biomass and biodiversity. Hydrographic features associated with seamounts have led authors to hypothesize that benthic invertebrate populations from geographically separated searnounts (and the continental slope) may experience varying degrees of genetic isolation, resulting in high levels of endemism. While this hypothesis has been tested for multiple taxonomic groups in the Pacific, it has rarely been addressed in the Atlantic. We tested the null hypothesis that the geographic ranges of corals from NW Atlantic seamounts are restricted to individual seamounts. We examined 188 octocoral and 50 antipatharian colonies (representing 6 and 2 genera, respectively) from 14 seamounts, spanning 1700 km, and the adjacent continental margin and estimated their genetic variation using mitochondrial loci (msh1 for all octocorals, as well as an intergenic region for isidids, and 3 multi-gene spanning segments for antipatharians). Well-sampled haplotypes were not geographically isolated on individual seamounts, thus refuting the hypothesis of local endemism of coral fauna on the New England and Corner Seamounts. The narrow geographic distribution of rare haplotypes is most likely due to undersampling rather than endemism. Our results do not preclude that cryptic variation and endemism not revealed by mitochondrial DNA may become evident should more variable markers be developed.