Distinct patterns of genetic differentiation among annelids of eastern Pacific hydrothermal vents

Population genetic and phylogenetic analyses of mitochondrial COI from five deep-sea hydrothermal vent annelids provided insights into their dispersal modes and barriers to gene flow. These polychaetes inhabit vent fields located along the East Pacific Rise (EPR) and Galapagos Rift (GAR), where hundreds to thousands of kilometers can separate island-like populations. Long-distance dispersal occurs via larval stages, but larval life histories differ among these taxa. Mitochondrial gene flow between populations of Riftia pachyptila, a siboglinid worm with neutrally buoyant lecithothrophic larvae, is diminished across the Easter Microplate region, which lies at the boundary of Indo-Pacific and Antarctic deep-sea provinces. Populations of the siboglinid Tevnia jerichonana are similarly subdivided. Oasisia alvinae is not found on the southern EPR, but northern EPR populations of this siboglinid are subdivided across the Rivera Fracture Zone. Mitochondrial gene flow of Alvinella pompejana, an alvinellid with large negatively buoyant lecithotrophic eggs and arrested embryonic development, is unimpeded across the Easter Microplate region. Gene flow in the polynoid Branchipolynoe symmytilida also is unimpeded across the Easter Microplate region. However, A. pompejana populations are subdivided across the equator, whereas B. symmitilida populations are subdivided between the EPR and GAR axes. The present findings are compared with similar evidence from codistributed species of annelids, molluscs and crustaceans to identify potential dispersal filters in these eastern Pacific ridge systems.     

Tubeworm succession at hydrothermal vents: use of biogenic cues to reduce habitat selection error?

Species colonizing new deep-sea hydrothermal vents along the East Pacific Rise show a distinct successional sequence: pioneer assemblages dominated by the vestimentiferan tubeworm Tevnia jerichonana being subsequently invaded by another vestimentiferan Riftia pachyptila, and eventually the mussel Bathymodiolus thermophilus. Using a manipulative approach modified from shallow-water ecological studies, we test three alternative hypotheses to explain the initial colonization by T. jerichonana and its subsequent replacement by R. pachyptila. We show that R. pachyptila and another vestimentiferan, Oasisia alvinae, colonized new surfaces only if the surfaces also were colonized by T. jerichonana. This pattern does not appear to be due to restricted habitat tolerances or inferior dispersal capabilities of R. pachyptila and O. alvinae, and we argue the alternative explanation that T. jerichonana facilitates the settlement of the other two species and is eventually outcompeted by R. pachyptila. Unlike the classic model of community succession, in which facilitating species promote their own demise by modifying the environment to make it more hospitable for competitors, we suggest that T. jerichonana may produce a chemical substance that induces settlement of these competitors. This process of selecting habitat based on biogenic cues may be especially adaptive and widespread among later-successional species that occupy a physically variable and unpredictable environment. In these cases, the presence of weedy species implies some integrated period of environmental suitability, whereas an instantaneous assessment of physical habitat conditions, such as water temperature for vent tubeworms, provides a poorer predictor of long-term habitat suitability. Received: 13 July 1999 / Accepted: 2 November 1999