The hidden community – hydrothermal vent meiobenthos associated with Bathymodiolus aggregations from Mid-Atlantic Ridge and East Pacific Rise

Zekely J. ¹, Van Dover C.L. ², Bright M.¹

¹Institute of Ecology and Conservation Biology, Althanstr. 14, A- 1090 Vienna, Austria

²Biology Department, College of William & Mary, Williamsburg, Virginia 23187, USA

Bathymodiolus mussel aggregations are known from many deep-sea hydrothermal vents and cold seeps. They create a unique habitat and refugee for smaller meiobenthic animals hidden among shells in which few precipitates are collected.

The central objective of this study was to identify and quantify the metazoan meiobenthic community associated with 6 mussel aggregations of Bathymodiolus thermophilus from the East Pacific Rise (EPR) 11°N, Buckfield and of B. puteoserpentis from the Mid-Atlantic Ridge (MAR), 23°N Snake Pit, so that the species diversity, abundance, biomass, and distribution are documented and can be compared. The overall density of meiobenthos at MAR (1780.8 ± 421.3; n = 3) was similar to that of EPR (2073.2 ± 303.7; n = 3) when related to a standardized volume of 10 ml of wet sediment. The dominance of the two most abundant taxa, nematodes and copepods, was remarkably different between the sites. At the MAR mussel bed, nematodes dominated (60.4 ± 19.4% of the total number of individuals) and copepods comprised 35.2 ± 3.9% of the total meiofauna; at the EPR, copepods dominated (85.7 ± 2.0% of the total meiofaunal community), followed by nematodes at 6.6 ± 2.0%. The remaining 5 - to -10% of the individuals were juvenile polychaetes, crustaceans, and gastropods. A total of 4000 individuals were identified. 25 species (12 families and 20 genera) were found at EPR and 14 species (9 families, 10 genera) at MAR. The nematode Thalassomonhystera with one species at MAR and EPR each and the copepod Aphotopontius also with one species each dominated. EPR meiobenthos had a higher diversity (Shannon´s H´= 3,88) as MAR (H´= 2,26). Trophic diversity of nematodes was low at both sites and lacked predators and omnivors.

This is the first time that the entire metazoan meiobenthos was identified from a specific moderate diffuse flow vent habitat inhabited by mussel aggregations. The overall low diversity but relatively high abundance of individuals follows the general trend known from vent macrobenthos. Similar community structures are also known from shallow water extreme habitats, for example the intertidal. The mussel aggregation community is dominated by meiobenthic species and underlines the importance of meiobenthos in estimating the biodiversity of an ecosystem. While many copepods have been described from vents, contributing to 10% of total vent fauna, taxonomic work on nematodes is urgently needed since not a single species has been described.