Journal of the Marine Biological Association of the UK

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

G.  Tita a1p1c1, M.  Vincx a2 and G.  Desrosiers a1
a1 Université du Québec à Rimouski, Institut des Sciences de la Mer (ISMER), Rimouski, Québec, G5L 3A1, Canada
a2 University of Gent, Department of Biology, Ledeganckstraat 35, B-9000 Gent, Belgium


Nematode species from three intertidal assemblages (St Lawrence Estuary, Quebec, Canada) were studied in order to form an ecological interpretation of three allometric attributes: body width, size spectra, and morphotypes. The three assemblages were characterized by a very similar sediment grain median (Md) but different silt–clay proportions: A1 (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 influential factor in determining the mean nematode body width, used as a morphological discriminant between burrowing and interstitial organisms. A plot of the number of species vs the body width-classes showed two peaks: between 19.3 and 22.6 [mu]m (interstitial), and between 32.0 and 45.5 [mu]m (burrowers). As for the size spectra, in sandy sediments the mean nematode individual biomass was smaller than in muddy sediments. As a consequence, the estimated mean individual respiration rate was greater in muddy (A1=2.26 nl O2 h−1) than sandy sediments (A3=1.25 nl O2 h−1; A5=1.12 nl O2 h−1). In contrast, estimated metabolic ratios were lower in A1 (2.78 nl O2 h−1 [mu]g−1 dry weight, DW) than in A3 (2.95 nl O2 h−1 [mu]g−1 DW) and A5 (3.01 nl O2 h−1 [mu]g−1 DW) suggesting different productivity and/or physiological adaptations to different lifestyles (burrowing vs interstitial) between species inhabiting muddy or sandy sediments. Morphotypes (body width/body length ratio=w/l ratio) were found to be associated with feeding 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 food-related hypothesis is proposed: the species morphotype reflects the quality of exploited food; a small w/l ratio (i.e. long gut) would favour digestive 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 high quality food (epigrowth-feeders and predators).

c1 ISMER–UQAR 310, allée des Ursulines Rimouski, Québec, G5L 3A1, Canada,
p1 ISMER–UQAR 310, allée des Ursulines Rimouski, Québec, G5L 3A1, Canada,