A Biogeographical Perspective of the Deep-Sea Hydrothermal Vent Fauna

Abstract

ABSTRACT Biogeography seeks to distinguish patterns in the distribution of species and to determine causal processes. Hydrothermal vent habitats have several properties that invite biogeographic studies: constrained to active deep-sea ocean ridges, known in most oceans and anticipated in the rest, patchy in distribution, extreme conditions and a limited group of inhabitants. Biologists have studied 30 vent sites mostly in the Pacific and Atlantic. Currently, 443 invertebrate species are known to generic level although many more are under study. Additionally, 32 octopus and fish species are observed in and around vents. The faunas of other sulphide-rich deep-water habitats such as margin cold seeps and organic masses (wood, carcasses) do not show great affinity at the species level to the vent fauna but the higher taxonomic affiliations suggest close evolutionary ties for many groups. Many studies address the formation of regional faunas using the first- known sites on the Galapagos Rift and northern East Pacific Rise as the major sites of comparison. Physical disjunction of ridge crests is a likely factor in promoting the extensive provinciality that currently exists. Nonetheless, faunas on two sides of the Pacific and in the Atlantic have closer relations to each other than to the nearby “normal” deep-sea fauna. At the individual ridge scale, extensive gene flow among separated populations occurs in many species and serves to maintain the regional species pool. However, major discontinuities between major ridges reduce or eliminate gene flow; vicariant processes appear to be important. The role of differing rates of spreading in different provinces and the concomitant effects on vent habitats and faunas need further investigation. Over 75% of vent species occur at only one site and none occur at all sites. Examination of the reproductive characters of some widespread species reveals no special dispersal strategy nor does reproductive strategy predict the extent of distribution. Vestimentiferan tubeworm species are highly endemic and found only at Pacific vents; their limited spread may be a result of recent entry into the habitat. Alvinellid polychaete cladogenetic pattern does not match geographic regions, indicating independent penetration by numerous lineages. Endemicity among vent gastropods is high with over 60% of genera limited to this habitat; many affiliations with other sulphide-rich habitats can be identified. Among the vent gastropods there are some lineages that may have entered the vent habitat in the Mesozoic. Hydrothermal vents provide a good testing ground for processes that control patterns in diversity. Ecological and historical controls at both local and regional scales can be discerned with further study.