Crustacea in standing waters are a diverse taxonomic assemblage with representatives in all available habitats from the benthic zone to the pelagial in larger water bodies. While most higher taxa are widespread and occasionally cosmopolitan, this is only partially true at the genus and species level. The crustacean fauna of geologically young lakes, or ponds, is characterized by widespread species that are not even necessarily restricted to lentic habitats. These species generally have good to excellent dispersal capabilities, especially those dwelling in ephemeral habitats. Small groups such as branchiopods and copepods dominate under these conditions among obligate still-water dwellers. In contrast, endemism and occasional striking adaptations are the hallmarks of crustacean species flocks, especially in the radiations of amphipods, decapods, and ostracods in the fewer than 10 ancient lakes worldwide. These radiations have arisen in situ through the diversification of unspecialized ancestors. All comparatively well-studied radiations for which molecular phylogenetic, taxonomic, and ecological data are available show particular adaptations of trophic morphology correlated to specific habitats. Prime examples are the species flocks of amphipods in Lake Baikal and of atyid shrimps in Lake Tanganyika and in two Indonesian lakes. These groups have most likely evolved through adaptive radiation. A major challenge for research on crustaceans in ancient lakes, and in standing waters generally outside Europe and North America, is the lack of basic data from species diversity to genetics for many, if not most, taxa. Getting a grip on species diversity, distributions, ecology, and, at a different level, genomics will be a research priority for coming decades.
VARIATION IN SPECIES DIVERSITY AND SHELL SHAPE IN HAWAIIAN LAND SNAILS: IN SITU SPECIATION AND ECOLOGICAL RELATIONSHIPS