Habitat preferences, genetic isolation and climatic vulnerability of an endangered freshwater crayfish and a widespread freshwater crab in streams of northern Japan.

Stream habitat is distributed discontinuously, which restricts the dispersal of animals among streams. Freshwater decapod crustaceans have poor dispersal ability and thus may suffer high extinction risk in the face of habitat destruction and other forces. Here, the ecological characteristics of two freshwater decapods, the threatened crayfish Cambaroides japonicus (de Haan, 1841) and the widespread crab Geothelphusa dehaani (White, 1847), were compared. The objective of the study was to determine which characteristics are most likely to limit the distribution of C. japonicus.Environmental DNA sampling was conducted to detect C. japonicus and G. dehaani populations, and environmental characteristics were measured to identify suitable stream habitat. Phylogenetic divergence and genetic differentiation among populations were examined. Using species distribution modelling, the future distributions of both species under climate change were predicted.Stream habitats harbouring C. japonicus tended to be supplied with more beech leaves and fewer cedar leaves, whereas those of G. dehaani were relatively wide, indicating that C. japonicus favours upstream areas in natural broad‐leaved forest. The results also showed greater genetic divergence among populations of C. japonicus than those of G. dehaani. Modelling indicated that most areas within the current distribution of C. japonicus were predicted to be areas with low distribution probability under a future climate scenario.To protect C. japonicus, further loss of local populations should be prevented to allow for the maintenance of high genetic diversity among populations, which may provide the evolutionary capability of surviving under future climate conditions. To prevent further loss of C. japonicus populations, natural deciduous forests in mountainous areas need to be preserved. C. japonicus can be the indicator species within the freshwater environment in these forests. Conservation measures for C. japonicus should also be effective for the conservation of the other freshwater invertebrates in upstream areas. [ABSTRACT FROM AUTHOR]