We studied the composition and variation of nematode body size as a function of nematode depth in the sediment and water depth at 18 stations, ranging from 137 to 4951 m depth, along the Galician margin (NE Atlantic). The analysis is based on more than 10000 individuals and demonstrates that nematodes have adapted mainly in response to the vertical changing conditions in the sediment. Nematode length and width are similar in the upper 0.5 cm of sediment at all stations, but differ greatly as their depth in the sediment increases. The observed trends are consistent with an adaptation to changing oxygen concentrations in the sediments. In lower slope and abyssal plain stations, oxygen decreases gradually until depletion at about 5 cm depth, and nematode body width decreases concurrently, resulting in higher oxygen absorption efficiency per unit of volume. On the shelf and in the canyons, sediments become anoxic at short distances below the sediment-water interface, and more than 50% of the nematodes are present in the anoxic part of the sediment. Here, mean nematode length increases as their depth in the sediment increases. This may result in increased mobility, which enables the organisms to migrate between anoxic patches of food and parts of the sediment where oxygen is available and/or the ability to bridge the gap between oxic and anoxic patches with their body. Order of magnitude calculation shows that in shelf sediments, the chemosynthetic bacterial production, which accompanies the oxidation of reduced substances, can not fulfill the feeding requirements of nematodes in the anoxic zone. The different size adaptations with sediment depth in shallow as compared to deep stations combined with deeper penetration of nematodes into the sediment at shallow water depth cause the observed pattern of miniaturization with increasing water depth. We conclude that nematode size distribution is determined by food availability. However, organic matter, as a food source, is not directly responsible, as a limiting resource, but indirectly because its respiration generates reduced conditions in the sediment. Within the smaller size range

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