GENETIC POPULATION-STRUCTURE AND GENE FLOW AMONG DEEP-SEA AMPHIPODS, ABYSSORCHOMENE SPP, FROM 6 CALIFORNIA CONTINENTAL BORDERLAND BASINS

Studies of geographic population variation needed to estimate gene flow are lacking in deep-sea biology. Using allozyme electrophoresis, I have studied population-level geographic variation among scavenging lysianassoid amphipod populations (Abyssorchomene spp.) inhabiting deep-water basins of the Southern California Continental Borderland. Samples were collected from November 1987 to November 1990, using baited traps, from six basins whose bottom depths ranged from ca. 1000 to 21 00 m. Five basins (San Diego Trough, Santa Catalina, San Nicolas, Santa Cruz, Tanner Basins) could be grouped together as ''shallow-sill'' basins, with physical conditions distinctly different from a single ''deep-sill'' basin (San Clemente Basin). Amphipods tentatively identified as Abyssorchomene sp. 1 collected from the shallow-sill basins were morphologically discriminated from those collected in the San Clemente Basin, which were identified as Abyssorchomene sp. 2. Results from eight enzyme loci revealed significant genetic differentiation [Nei's genetic distance (D) > 0.155] of deep-sill basin-dwelling Abyssorchomene sp. 2 vs Abyssorchomene sp. 1 from the shallow-sill basins and low levels of gene flow (migration rate, M < 1). Comparisons of benthic fauna suggest the presence of an abyssal-related assemblage in the deep-sill basin isolated from the northern shallow-sill basins. Genetic distances among the five shallow-sill basin populations of Abyssorchomene sp. 1 were very low (D < 0.003). Estimates of gene flow among these populations were very high (M > almost-equal-to 24 to 170) and suggested weak isolation by distance.