ZARDUS, J.D.*; ETTER, R.J.; CHASE, M.R.; REX., M.A.; BOYLE, E.E.; The Citadel; Univ. Massachusetts, Boston; Univ. Massachusetts, Boston; Univ. Massachusetts, Boston; Univ. Massachusetts, Boston: Genetic Divergence with Depth and Distance in a Deep-Sea Bivalve, Deminucula atacellana

Evolutionary origins are obscure for the diverse and highly endemic fauna of deep-sea soft-sediment environments. Current knowledge of geographic patterns of genetic variation in the deep sea, information fundamental to inferring population differentiation and speciation, is limited in its scope and scale. Utilizing museum-based collections, we characterized genetic variation in Deminucula atacellana, a protobranch bivalve widespread at bathyal and abyssal depths throughout the Atlantic Ocean. Samples were obtained from 18 localities across three hydrographic basins: North American, West European and Argentine. For 130 individuals a hypervariable region of mitochondrial 16S rDNA was amplified by PCR and direct sequenced. Twenty-one haplotypes were identified that, with several important exceptions, were unique to each basin. Overall gene diversity was high, population structure was pronounced, and highly significant geographic associations were detected with nested clade analysis. Four major clades were identified corresponding with geography and depth. Significant genetic variation occurred from basin to basin but the greatest levels of divergence were found among populations at different depths within the same basin. Large genetic divergence was not accompanied by morphological differentiation. Isolation by distance probably explains inter-basin or horizontal variation whereas depth-related or vertical differences may reflect historical patterns of colonization or selective environmental gradients driving speciation.