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 gro ss biochemical composition. Seasonal changes in nematode composition were r elated to qualitative and quantitative changes in the sedimentary organic d etritus and particle fluxes. Additional coastal sediment samples were colle cted 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 organi c 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 nematode s, respectively. Carbohydrate levels of nematodes (accounting on average fo r 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 compo sition of nematodes revealed rather high Lipid levels (on average 11. and 1 7 % of DW and 16 and 35 % of organic matter, respectively in coastal and de ep-sea nematodes). Such discrepancy appears to indicate a different bioener getic 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 Septemb er 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 de ep-sea nematodes was close to that of high-energy zooplankton, indicating t hat nematodes might represent an important high-quality food source for hig her trophic levels. The reduced food inputs (especially carbohydrates and p roteins) observed from February to September 1995 determined a parallel dec rease in the concentration of sedimentary labile compounds and induced a cl ear decrease of the nematode energy content. However, no equally large seas onal changes in the total energy content (expressed as J m(-2)) of the nema tode assemblages were observed. We hypothesise that deep-sea nematodes migh t respond to diminished food inputs by changing their bioenergetic strategi es to better exploit the available resources.