The ecology and biogeography of meiofauna at deep-sea hydrothermal vents have historically received less attention than those of mega- and macrofauna. This study examines the composition of major meiofaunal taxa in beds of the mussel Bathymodiolus thermophilus at hydrothermal vents on the northern and southern East Pacific Rise (EPR) and presents the first comparison of species assemblages of the dominant taxon, the nematodes, among sites spanning 27 degrees of latitude. Meiofaunal samples were collected by submersible from three mussel beds at 9°N on the EPR and four mussel beds between 17 and 18°S in 1999. Estimated ages of the mussel beds at the time of sampling range from 4 to >20 years, enabling investigation of the influence of mussel bed age on meiofaunal assemblages. Overall, the meiofauna of the mussel beds was dominated by nematodes, with copepods constituting the second most abundant meiofaunal group. There was variation in the ratio of nematodes to copepods between sites, however, with copepods more abundant than nematodes in the youngest mussel beds. Apart from polychaete larvae, other meiofaunal groups were generally present at very low abundance (<1%) in the samples and restricted in diversity to gastropod larvae, acari, foraminifera, ostracoda and turbellaria. Seventeen nematode species from 14 genera and 11 families were found in the samples, with no evidence of endemicity to hydrothermal vents at the generic level. Four genera present were not previously recorded at hydrothermal vents. Nematode species richness, species:genus ratios and abundances were low compared with other deep-sea habitats, though the ecological relevance of comparisons with soft-sediment benthos is discussed. Nematode assemblages exhibited high dominance by a few species, with one species of Thalassomonhystera most abundant at five of the seven vent sites. Multivariate analysis of nematode assemblages reveals similarities among sites that do not match geographical proximity. The youngest mussel beds were most similar to each other and exhibited lower species richness than other sites, consistent with colonization of mussel bed habitat by nematodes over time. Similarity in the composition of nematode assemblages among sites separated by ~3000 km indicates that they lie within a single biogeographic province, consistent with that proposed for mussel bed macrofauna. At a generic level, samples exhibited some overlap with nematode assemblages at vents elsewhere on the EPR, on the Mid Atlantic Ridge and in the North Fiji Basin.
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