Seasonal variation in the biochemical composition of deep-sea nematodes: bioenergetic and methodological considerations

Deep-sea nematode assemblages from one station (950 m depth) in the Cretan Sea (Eastern Mediterranean) were studied for seasonal variations in the gross biochemical composition. Seasonal changes in nematode composition were related to qualitative and quantitative changes in the sedimentary organic detritus and particle fluxes. Additional coastal sediment samples were collected to evaluate the effects of preservative (4 % formalin) on biochemical determinations and to compare nematode body composition in coastal and deep-sea assemblages. Protein represented the major biochemical class of organic compounds in nematodes, accounting for 43.7 and 48.5 % of organic matter (on average 24 and 29% of dry weight [DW]) in coastal and deep-sea nematodes, respectively. Carbohydrate levels of nematodes (accounting on average for 8 and 26 % of DW and for 16 and 40 % of organic matter, respectively, in coastal and deep-sea nematodes) appeared to be rather high when compared to literature values dealing with other taxa. Carbohydrates were the dominant biochemical class both in the sediments and in particle fluxes. Body composition of nematodes revealed rather high Lipid levels (on average 11. and 17 % of DW and 16 and 35 % of organic matter, respectively in coastal and deep-sea nematodes). Such discrepancy appears to indicate a different bioenergetic strategy by deep-sea organisms that develop efficient energy storage systems to be able to cope with long periods of very low food availability. Deep-sea nematode body weight decreased between February-March and September 1995 concomitantly with the decrease in particle flux. Coastal nematodes displayed higher individual biomass than the deep-sea ones (0.44 +/- 0.06 vs 0.37 +/- 0.06 mu g DW ind.(-1), respectively), but were characterised by lower caloric values (5.57 vs 6.68 cal mg(-1) DW). The caloric value of deep-sea nematodes was close to that of high-energy zooplankton, indicating that nematodes might represent an important high-quality food source for higher trophic levels. The reduced food inputs (especially carbohydrates and proteins) observed from February to September 1995 determined a parallel decrease in the concentration of sedimentary labile compounds and induced a clear decrease of the nematode energy content. However, no equally large seasonal changes in the total energy content (expressed as J m(-2)) of the nematode assemblages were observed. We hypothesise that deep-sea nematodes might respond to diminished food inputs by changing their bioenergetic strategies to better exploit the available resources.