Phylogeography of a pan-Atlantic abyssal protobranch bivalve: implications for evolution in the Deep Atlantic

The deep sea is a vast and essentially continuous environment with few obvious barriers to gene flow. How populations diverge and new species form in this
remote ecosystem is poorly understood. Phylogeographic analyses have begun to provide some insight into evolutionary processes at bathyal depths (< 3000 m), but much
less is known about evolution in the more extensive abyssal regions (>3000 m). Here we quantify geographic and bathymetric patterns of genetic variation (16S rRNA
mitochondrial gene) in the protobranch bivalve Ledella ultima, which is one of the most abundant abyssal protobranchs in the Atlantic with a broad bathymetric and
geographic distribution. We found virtually no genetic divergence within basins and only modest divergence among eight Atlantic basins. Levels of population divergence
among basins were related to geographic distance and were greater in the South Atlantic than in the North Atlantic. Ocean-wide patterns of genetic variation indicate
basin wide divergence that exceeds what others have found for abyssal organisms, but considerably less than bathyal protobranchs across similar geographic scales.
Populations on either side of the Mid-Atlantic Ridge in the North Atlantic differed suggesting the Ridge might impede gene flow at abyssal depths. Our results indicate
that abyssal populations might be quite large (cosmopolitan), exhibit only modest genetic structure and probably provide little potential for the formation of new
species.