What is the relative importance of sediment granulometry and vertical gradients to nematode morphometrics?

Abstract

Nematode morphometrics were compared at 11 fine-sand and three muddy-sediment sites on the Louisiana, USA continental shelf to explore nematode–sediment relationships. Over 1800 nematodes were measured, and morphometric profiles through 4 cm were examined. The mean length of nematodes did not differ between muds and sands, although nematode diameter was greater in muds below 2 cm in sediment depth. Sand-dwelling nematodes, which slide through interstitial pore spaces, were longer relative to their diameter compared to nematodes that burrow through muddy sediments lacking interstices. Nematode diameter increased with both increasing particle size and particle heterogeneity among sandy stations. In muds, nematode length, diameter and biovolume increased with sediment depth, although length:width ratio did not vary with depth in the sediment. In sands, nematode length and length:width ratio increased with increasing sediment depth, but neither diameter nor biovolume varied with depth in the sediment. These results suggest that nematode body size and shape are related both to variation in sediment pore spaces and to vertical gradients within sediments. Within sands, body size variation attributable to granulometry was greater than variation associated with vertical gradients, while within muds variation in body size with sediment depth was greater than variation associated with particle size. Variation in body shape was similar along the vertical sediment profile and with variation in particle size in both sands and muds. We conclude that nematodes are excellent test organisms with which to test hypotheses regarding the confounded effects of sediment granulometry and chemical gradients in benthic ecosystems.