Size spectra, body width and morphotypes of intertidal nematodes: an ecological interpretation

Nematode species from three intertidal assemblages (St Lawrence Estuary, Qu ebec, Canada) were studied in order to form an ecological interpretation of three allometric attributes: body width, size spectra, and morphotypes. Th e three assemblages were characterized by a very similar sediment grain med ian (Md) but different silt-clay proportions: Al (upper-tidal level; Md=122 mu m; silt=34.8%), A3 (mid-tidal level; Md=182 mu m; silt=12.8%), and A5 ( lower-tidal level; Md=122 mu m; silt=6.8%). Silt-clay proportions were an i nfluential factor in determining the mean nematode body width, used as a mo rphological discriminant between burrowing and interstitial organisms. A pl ot of the number of species vs the body width-classes showed two peaks: bet ween 19.3 and 22.6 mu m (interstitial), and between 32.0 and 45.5 mu m (bur rowers). As for the size spectra, in sandy sediments the mean nematode indi vidual biomass was smaller than in muddy sediments. As a consequence, the e stimated mean individual respiration rate was greater in muddy (A1=2.26 nl O-2 h(-1)) than sandy sediments (A3=1.25 nl O-2 h(-1); A5=1.12 nl O-2 h(-1) ). In contrast, estimated metabolic ratios were lower in Al (2.78 nl O-2 h( -1) mu g(-1) dry weight, DW) than in A3 (2.95 nl O-2 h(-1) mu g(-1) DW) and A5 (3.01 nl O-2 h(-1) mu g(-1) DW) suggesting different productivity and/o r physiological adaptations to different lifestyles (burrowing vs interstit ial) between species inhabiting muddy or sandy sediments. Morphotypes (body width/body length ratio=w/l ratio) were found to be associated with feedin g groups. Small w/l ratios were typical of microvores, while greater ratios were typical of epigrowth feeders and predators. Ciliate-feeders, deposit- feeders and facultative predators had intermediate ratios. A morphotype foo d-related hypothesis is proposed: the species morphotype reflects the quali ty of exploited food; a small w/l ratio (i.e. long gut) would favour digest ive efficiency and would be an adaptation to low quality food (microvores); inversely a greater w/l ratio (i.e. short gut) would be an adaptation to h igh quality food (epigrowth-feeders and predators).