EVOLUTIONARY RELATIONSHIPS AMONG DEEP-SEA MYTILIDS (BIVALVIA, MYTILIDAE) FROM HYDROTHERMAL VENTS AND COLD-WATER METHANE SULFIDE SEEPS

A protein electrophoretic survey of mytilids inhabiting deep-sea hydro thermal vents and cold-water methane/sulfide seeps revealed electromor ph patterns diagnostic of 10 distinct species. From hydrothermal vents located at sites on the Galapagos Rift, the Mid-Atlantic Ridge, and t he Mariana Back Are Basin, we detected four species of mytilids. Six a dditional species were detected from three cold-water seep sides in th e Gulf of Mexico. The patchy distribution and temporal stability of se eps may provide a greater opportunity for mytilid diversification and persistence than vent sites Nei's genetic distances (D) between specie s were relatively large (range: 0.528 to infinity) both within and amo ng habitat types. This pronounced degree of genetic differentiation su ggests a relatively ancient common ancestor for the group. Phylogeneti c trees were generated using distance Wagner and parsimony analyses of allozyme and morphological characters. The tree topologies obtained f rom both methods support: (1) the hypothesis that a seep ancestor gave rise to the deep-sea hydrothermal vent mytilids, (2) a historical pro gression from shallow-water to deep-water habitats, and (3) a co-evolu tionary progression from external to internal localization of bacteria l symbionts. Whether the seep mytilid taxa constitute paraphyletic or polyphyletic groups remains un resolved. Our phylogenetic hypotheses a lso provide a benchmark for the phylogeny of mytilid bacterial symbion ts.