The biology and ecology of lotic water mites (Hydrachnidia)

1. The Hydrachnidia (water mites, Hydracarina) are the most diversified gro up of the Acari in freshwaters and are abundant and speciose in lost habita ts. Lower-order streams may contain up to 50 species (including benthic and hyporheic forms) and small springs up to 20 crenobiont species. 2. Water mites are grouped into 8 superfamilies, 50 families, 300 genera co ntaining more than 5 000 species. Representatives of all superfamilies (abo ut 3 000 species worldwide) occur in lotic ecosystems, although most lotic species belong to the Hydryphantoidea, Lebertioidea and Hygrobatoidea. Iden tification of water mite families, genera and subgenera, throughout the wor ld, is possible using taxonomic publications. Keys to species level are als o available but mainly for local faunas. Descriptions of larvae and deutony mphs are rare. 3. The life cycle of the Hydrachnidia is unique among the Acari and is simi lar to that of holometabolous insects, with a heteromorphic parasitic/phore tic larva and two pupa-like resting stages. The larva parasitises mainly in sect hosts with apparently no strict host-specificity. Deutonymphs and adul ts are voracious predators feeding mainly on insect eggs, insect larvae and microcrustaceans. In some cases, water mite parasitism and predation may s ubstantially affect the structure of lotic communities. 4. Most species show a high degree of sabitat/microhabitat specialization. Temperature, current-speed, substratum type, physiographic and geomorpholog ical factors are the major determinants of species composition in water mit e communities. 5. The complex, fully aquatic, life cycle and multilevel biocoenotic intera ctions make water mites well suited for the detection of physical and chemi cal disturbances to lotic ecosystems. 6. Future research should address the distribution, biology, autecology, co mmunity dynamics and ecological interactions of lotic water mites.