BATHYMETRIC PATTERNS OF GENETIC-VARIATION IN A DEEP-SEA PROTOBRANCH BIVALVE, DEMINUCULA-ATACELLANA

The origin of the deep-sea benthic fauna is poorly under-stood and rep resents an enormous gap in our understanding of basic evolutionary phe nomena. One obstacle to studying evolutionary patterns in the deep sea has been the technical difficulty of measuring genetic variation in s pecies that are typically minute, rare, and must be recovered from ext reme depths. We used molecular genetic techniques to quantify variatio n in the 16S rRNA mitochondrial gene within and among populations of t he common protobranch bivalve Demimucula atacellana (Schenck, 1939). W e analyzed 89 individuals from nine samples collected in the 1960s alo ng a depth gradient from 1100 to 3800 m in the western North Atlantic. Genetic variability within populations :is much lower than between po pulations, and peak haplotype numbers occur near the center of its dep th distribution. Continental slope (<2500m) and rise ( > 2500 m) popul ations were genetically distinct despite the lack of any obvious topog raphic or oceanographic features that would impede gene flow. These fi ndings indicate that the deep-sea macrofauna can have strong populatio n structure over small(134 km) spatial scales, similar to that observe d in shallow-water and terrestrial organisms. This surprisingly high b iodiversity at the genetic level affords the potential for adaptation and evolutionary diversification, the ultimate historical causes of hi gh species diversity in the deep-sea benthos.