Patterns of nematode diversity at hydrothermal vents on the East Pacific Rise

The diversity and abundance of meiofauna in quasi-quantitative samples collected from Bathymodiolus mussel beds in a deep-sea hydrothermal vent field on the East Pacific Rise (EPR) was studied with particular reference to nematodes. Copepods, polychaetes and nematodes were found to be the most abundant meiofaunal taxa at all sites processed so far. Species richness (S = 4 - 6), Shannon-Wiener diversity indices (H'(log e) = 0.9 - 1.5) and Pielou's eveness indices (J' = 0.5 - 0.8) were similar at all sites. Six different species of nematode were identified, both sites exhibiting a dominance of Monhystrid species. Cluster analysis of Bray-Curtis similarities using group-average linkage on standardised data shows that samples from Train Station and East Wall separate out at around the 45{%} similarity level. This separation results from the different abundances of nematode species present at both sites and the exclusive presence of Thalassomonhystera sp. B at Train Station and Anticoma sp. A at East Wall. A pot sample from a mussel bed at "Oasis" at 17 25' S on the EPR exhibits approximately 30{%} similarity with the NEPR samples as a result of higher abundance and species richness of nematodes. To date, meiofauna have seldom been included in ecological studies at vents and it is therefore not clear to whether patterns of faunal zonation and biogeography similar to those known in megafauna are also present in this size class. Future work may reveal possible large-scale variation in nematode species richness.