Hydrachnellae, Hydrachnida, Hydrachnidia, water mites

[note: subkey to families and subfamilies attached]

The 'true' water mites, the Hydracarina, includes about 5000 mainly freshwater species in more than 40 families. Water mites are members of the Cohort Parasitengona from the Suborder Prostigmata (Order Acariformes). Parasitengones are characterized by a complex life cycle with parasitic larvae and predatory post-larval stages. Larval water mites parasitise all orders of aquatic insects with the exception of the Megaloptera, Neuroptera, and Ephemeroptera. Post-larval stages feed on small arthropods, including copepods and cladocerans, insect eggs, and sometimes rotifers and nematodes (Proctor 1997).

Water mites inhabit all types of aquatic habitats, from hot springs to treeholes. One family, the Pontarachnidae, has invaded nearshore marine waters. Some taxa are excellent swimmers while others crawl on or in the substrate. Many genera, particularly early derivative ones, are characterised by bright orange or red colouration. These mites also tend to be distasteful to fish (Kerfoot 1982).

Although earlier researchers considered the Hydracarina to be grossly polyphyletic, studies of larval morphology, genitalia, sperm ultrastructure, and the presence of unique trigger-hair/gland structures (glandularia) support the monophyly of nine superfamilies: Hydrachnoidea, Eylaoidea, Hydryphantoidea, Lebertioidea, Hygrobatoidea, Arrenuroidea, Sperchontoidea, Hydrovolzioidea, and Stygothrombidioidea (Harvey 1998; Walter & Proctor 1999).

In Australia there are representatives of all superfamilies with the exception of Sperchontoidea, Hydrovolzioidea and Stygothrombioidea. In his book on the families and genera of Australian water mites, Harvey (1998)

lists 413 species representing 89 genera from 22 families. This is an impressive diversity, almost equalling the number of named species of Australian caddisflies. Most taxa have been described from surface substrates of water bodies in southeastern Australia. The numbers of species and genera of water mites will likely grow as freshwater biologists begin to investigate the hyporheic zones and tropical areas of Australia.

Harvey (1998) provides a key to the genera of Australian water mites, while Cook (1986) has keys to many of the species. This LucID key includes a subkey to the families and subfamilies of water mites in Australia.

References:

Cook, D.R. 1976. Water mite genera and subgenera. Mem. Amer. Ent. Inst. 21: 1-860.

Cook, D.R. 1986. Water mites from Australia. Mem. Amer. Ent. Inst. 40: 1-568.

Harvey, M.S. 1998. The Australian Water Mites: a Guide to Families and Genera. CSIRO Publishing, Collingwood, Victoria.

Kerfoot, W.C. 1982. A question of taste: crypsis and warning coloration in freshwater zooplankton communities. Ecology 63: 538-554.

Krantz, G.W. 1978. A Manual of Acarology. 2nd edition. Oregon State University Book Stores, Corvallis, Oregon.

Proctor, H.C. 1997. Mating behavior of Physolimnesia australis (Halik) (Acari: Limnesiidae), a non-parasitic, rotifer-eating water mite from Australia. J. Arachnol. 25: 321-325.

Walter, D.E. and H.C. Proctor. 1999. Mites: Ecology, Evolution and Behaviour. University of New South Wales Press, Sydney, New South Wales.