Is the relationship between body length and body mass consistent across habitats? A case study on Niphargus (Crustacea: Amphipoda).

Body size is one of the main characteristics of any organism and influences various aspects of individual's biology. In animal ecology, it represents a key functional trait that can be quantified using different measures and is often used as a proxy for different organismal functions. The way we quantify body size is critical in any study using this measure alone or to scale other organismal traits. It is especially important in groups that act as model systems across different fields of biological research. One of such groups are amphipods, which are at focus in many ecological studies where appropriate quantification of body size is needed. Here, we explored the relationship between body length and body mass in the largest freshwater amphipod genus Niphargus, and evaluated whether the two measures lead to different conclusions in a putative ecological study of species coexistence. We selected 16 species inhabiting two different subterranean habitats, cave lakes and cave streams. The relationship between log-transformed body mass and body length was linear in all species, but body mass increased steeper among species from cave lakes than from cave streams, reflecting the stouter body shape of the former. In the simulated ecological study, the comparisons of the two measures showed that they may yield different results: in 10 % of cases, body length detected differences between species when body mass did not and vice versa (13 %). Usage of body length or body mass can thus lead to different conclusions. We recommend avoiding direct transformations between body length and body mass in ecological studies. Whenever needed, such transformations should be done with caution using habitat-specific body mass - body length ratios.