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 R ise 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 Ba thymodiolus thermophilus. Using a manipulative approach modified from shall ow-water ecological studies, we test three alternative hypotheses to explai n 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 colo nized by T. jerichonana. This pattern does not appear to be due to restrict ed habitat tolerances or inferior dispersal capabilities of R. pachyptila a nd O. alvinae, and we argue the alternative explanation that T. jerichonana facilitates the settlement of the other two species and is eventually outc ompeted by R. pachyptila. Unlike the classic model of community succession, in which facilitating species promote their own demise by modifying the en vironment to make it more hospitable for competitors, we suggest that ir: j erichonana may produce a chemical substance that induces settlement of thes e competitors. This process of selecting habitat based on biogenic cues may be especially adaptive and widespread among later-successional species tha t occupy a physically variable and unpredictable environment. In these case s, the presence of weedy species implies some integrated period of environm ental 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.