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Janet R. Voight
Department of Zoology, Field Museum of Natural History, Roosevelt
Rd at Lake Shore Dr., Chicago, IL 60605. voight@finah.org
Large mobile predators are members of all abyssal hydrothermal vents species assemblages that have been studied to date. Whether these taxa are restricted to vent habitats is difficult to assess, as a taxon can be considered as habitat-endemic only after it has been demonstrated to be absent from other habitats. Given our meager knowledge of abyssal non-vent taxa, current assessments of vent specificiy are suspect. Biological criteria were defined to estimate the extent to which members of predatory taxa are restricted to vents. These criteria include the comparative distribution of a taxon between vent and non-vent habitats, its distribution within vent fields, whether animals of all sizes occur at vents, evidence of success£ul reproduction at vents, and how the predators exploit vent fauna. These criteria are designed to reflect habitat use in post-settlement individuals and their degree of reliance on vent habitats.
Collections and observations made at hydrothermal vent fields on Juan de Fuca Ridge in the North Pacific using the ROV ROPOS. Within-taxon evaluations based on these criteria suggest that the galatheid crab, Munidopsis alvisca. is potentially restricted to vents or other chemosyntbetic habitats. Other predators documented at vents, including the octopodid Graneledone pacifica, the zoarcid fish, Pachycara gymninium and the spider crab, a Macroregonia machrochira are likely to be opportunistic in exploiting the vent habitats.
Although opportunistic predators are common at North Pacific hydrothermal vents, apparent vent-endemic predators dominate East Pacific Rise vents. As specialist predators are predicted to be rare in ephemeral habitats such as hydrothermal vents, the contrast suggests that the vent systems differ fundamentally. Individual vents on Juan de Fuca Ridge are larger and perhaps more temporally stable than are those at the East Pacific Rise; these large habitats would be predicted to be more likely to sustain lineages of vent specialist predators. The smaller and more open disturbed vents on the East Pacific Rise, however, are more dense. Large, vagile predators may move among these small, transient but relatively close vents to experience the habitat as a mosaic. Because North Pacific vents are more isolated, predators in these areas may perceive the vents as individual islands. The low habitat predictability in this area may preclude the evolution and/or maintenance of vent specialist predatory lineages.
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