HYDROTHERMAL VENT MEIOBENTHOS ASSOCIATED WITH MUSSEL AGGREGATIONS FROM EAST PACIFIC RISE AND MID-ATLANTIC RIDGE
Julia Zekely1, Cindy Lee van Dover2,
Monika Bright1
1Marine Biology, IECB, University of Vienna, 1090 Vienna, Austria
2Biology Department, College of William & Mary, Williamsburg, Virginia 23187, USA
e-mail: j.zekely@gmx.at
Deep-sea hydrothermal vents occur along the mid-ocean ridges around the globe. Quantitative ecological studies on vent macrofauna are rare, while community analyses on vent meiobenthos are all but lacking. The central objective of this study was to identify and quantify for the first time the entire metazoan meiobenthic community associated with mussel aggregations of Bathymodiolus thermophilus from the East Pacific Rise (EPR) 11°N, and of B. puteoserpentis from the Mid-Atlantic Ridge (MAR), 23°N. Bathymodiolus aggregations are known from many deep-sea hydrothermal vents and cold seeps. They create a unique habitat and refuge for meiobenthic animals hidden among shells in which few precipitates are collected. Using a quantitative sampling method abundances, biomass, sex ratio, species richness, diversity, eveness, and trophic structure from three samples each site were studied.
Related to a standardized volume of 10 ml sediment, the overall density of meiobenthos at MAR (1764.5 ± 341.5 individuals) was similar to that at EPR (1951.1 ± 216.7), while biomass was nearly twice as large at EPR than at MAR. Approximately 4000 individuals were identified to species level for community structure analysis. Meiobenthic community was composed of nematodes, copepods, ostracods, and mites with a total of 24 species at EPR and 15 species at MAR. At MAR, nematodes (62.9%) dominated, followed by copepods (34.9%), while at EPR, copepods (85%) were more abundant than nematodes (6.4%). Mites and ostracods were scarcely present. The trophic structure of hydrothermal vent meiobenthos was similar at both sites and dominated by primary consumers (>80%), followed by parasites. Predatory meiofauna were absent.
Different diversity indices pointed to a higher meiobenthic biodiversity at EPR (e.g. Shannon´s Hlog2 2.3) mussel beds than at MAR (Shannon´s Hlog2 3.3). Evenness was lower at MAR (0.6) than at EPR (0.8), indicating a large number of individuals found in a low number of species. Two reasons could account for differences in diversity. Studies from intertidal mussel beds showed that diversity is correlated with age of mussel aggregations (Tsuchiya & Nishihira 1985, 1986). Similar trends were found in our study as mean mussel size was larger and points to older mussels at EPR compared to smaller and younger mussels at MAR. Slow spreading center such as MAR are characterized by fewer more isolated vents than fast spreading centers such as EPR with more vents at closer distance to each other. In general, also macrobenthos associated with mussels is more diverse at EPR than at MAR.
Biodiversity of vent meiobenthos is similar or even higher than diversity of vent macrobenthos. This underlines the importance of including meiobenthos into estimating the biodiversity of an ecosystem.