Factors affecting nematode biomass, length and width from the shelf to the deep sea

The decrease of nematode size with water depth is well documented in the literature. However, many nematode size data sets originate from bathymetric gradients, with strong bias towards deep-water, muddy sediments. This has narrowed our perception of the environmental factors that may influence nematode morphometry. Here we perform a morphometric analysis with data collected from a variety of sampling locations in the Indian Ocean and around Europe at a wider range of depths and sediment types. All nematode size descriptors decreased significantly with water depth, which explained more than 60% of total variation. This trend was most pronounced for mean nematode dry weight, which decreased by similar to 20% for every doubling in water depth. This coefficient of decrease was smaller than the described decline in food deposition with depth, as estimated from sediment community oxygen consumption rates (similar to 35%), but on the same order of magnitude as the decrease in nematode density. Order of magnitude estimates based on these trends suggest that nematodes contribute about 7.5% to benthic metabolism over the depth range. In contrast to nematode dry weight, the decrease in nematode length and width with water depth was less steep. However, nematode length was also affected by grain size, where shallow-water coarse sediments were inhabited by longer nematodes. Nematodes from the oligotrophic Aegean Sea were characterised by low length values and high width values, probably as an adaptation to sediments poor in organic matter. These observations suggest that local factors can also be very important for shaping the morphometric landscape of the nematode communities.