GENETIC DIFFERENTIATION BETWEEN SPATIALLY-DISJUNCT POPULATIONS OF THE DEEP-SEA, HYDROTHERMAL VENT-ENDEMIC AMPHIPOD VENTIELLA-SULFURIS

Species endemic to deep-sea hydrothermal vent ecosystems have disjunct distributions imposed by the island-like arrangement of their specialized habitats. Using allozyme electrophoresis, we examined genetic population structure of the hydrothermal vent amphipod Ventiella sulfuris Barnard and Ingram, 1990. Samples from five sites along the East Pacific Rise (EPR) and two along the Galapagos Rift were collected in 1990 and 1988, respectively. Variability, based on 12 enzyme loci, was relatively high [P95BAR (proportion of polymorphic loci whose most common allele not greater than 0.95 in frequency) = 41.6%; HBAR (mean heterozygosity) = 0.158] compared with shallow-water marine and freshwater amphipods, and similar to the deep-sea lysianassid Eurythenes gryllus. Genetic divergence among populations spread along a contiguous rift axis (i.e., EPR) was low [Nei's genetic distance (D) ranged from <0.001 to 0.018]. Genetic structure analysis suggests that along a contiguous ridge axis migration occurs in a stepping stone manner and is unconstrained by distances as great as 1200 km (migration rate, MBAR, ranged from 1.9 to 67.8 ind. generation-1). However, genetic divergence between populations on disjunct ridge axes was extremely high (D ranged from 0.438 to 0.476). Most of the variance in gene frequencies was due to the differences between the major subpopulations inhabiting the two distinct ridge axes, EPR and Galapagos Rift. Apparently, very little migration and gene flow occur between these major subpopulations (M much less than 1). This level of genetic divergence may be sufficient to justify separation of EPR and Galapagos Rift populations at the species level. Further analyses of morphological characters is required before taxonomic status can be assigned.