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The deep-sea free-living nematode Halomonhystera hermesi, dominant in the sulphidic sediments of the Håkon Mosby mud volcano (1280 m, Barent sea slope), is part of the mainly estuarine Halomonhystera disjuncta species complex consisting of five cryptic species (GD1–GD5). Cryptic species have a very similar morphology raising questions on their specific environmental differences. This study analyzed total fatty acid (FA) compositions of H. hermesi and GD1, one of H. hermesi’s closest relatives. Additionally, we experimentally investigated the effect of a temperature reduction, salinity increase and sulphide concentrations on GD1’s FA composition. Because nematodes are expected to have low amounts of storage FA, total FA compositions most likely reflect FA contents of cellular membranes. The deep-sea nematode H. hermesi had significantly lower saturation levels and increased highly unsaturated fatty acid (HUFAs) proportions due to the presence of docosahexanoic acid (DHA—22:6ω3) and higher eicosapentaenoic acid (EPA—20:5ω3) proportions. HUFAs were absent in H. hermesi’s food source indicating the ability and need for this nematode to synthesize HUFAs in a deep-sea environment. Our experimental data revealed that only a decrease in temperature resulted in lower saturated fatty acids proportions, indicating that the FA content of H. hermesi is most likely a response to temperature but not to sulphide concentrations or salinity differences. In experimental nematodes, EPA proportions were low and DHA was absent indicating that other factors than temperature, salinity and sulphides mediate the presence of these HUFAs in H. hermesi.

Closely related intertidal and deep-sea Halomonhystera species have distinct fatty acid compositions