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Subcohort Hydrachnidia

(True water mites)

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

[water mites.jpg]
Order: Acariformes

Updated: October 09, 2013      Freshwater Benthic Ecology and Aquatic Entomology Homepage


Linnean system of hierarchical classification (Williams & Feltmate, 1992):


The Hydrachnidia are not the only aquatic mites, but they are the most successful group of mites found in freshwater. The mites form a considerable part of the lasiophil fauna. They inhabit almost every aquatic habitat, and densities frequently exceed 200 mites/sq.metre. Water mites are frequently brightly coloured and sometimes relatively large (over 2 mm), making them more conspicuous than other aquatic mites, and thus exaggerating their already significant predominance. However, there is in fact a tendency for all the species recorded from the lasion to be small; 49 percent are under 1 mm long, whereas in the littoral fauna as a whole only 29 percent are as small as this.

Many water mites exhibit bright colour patterns, containing greens, blues, oranges or reds. Red predominates among species in the "primitive" Hydrovolzioidea, Hydrachnoidea, Eylaoidea, and Hydryphantoidea, and often the term the red water mites is used to denote this ancestral stem. Fish and invertebrates will eat water mites, but the brightly coloured species are apparently distasteful and predators learn to reject them.

The Hydrachnidia are among the most numerically abundant and taxonomically diverse of the freshwater aquatic mites. Several factors have probably contributed to their success:


Species of Hydrachnidia are common in such lentic waters as swamps, marshes, ponds, and the littoral and profundal zones of lakes. They are often associated with vegetation or with the top few millimetres of substrate, but they can also lead a planktonic existence. Water mites are common, too, in the erosional and depositional zones of rivers, and the air-water interface at the margins of various water bodies harbours a variety of these mites. Some species are adapted to live in such extreme environments as thermal springs, glacial meltwater rivers, temporary pools, waterfalls, and in groundwater buried within gravel banks of streams (interstitial habitats). A few species can inhabit oceans and inland saline waters, although most are limited to freshwater.

Water mites in lentic waters are often free-swimming and conspicuously coloured, but many interesting species are cryptic, clinging to vegetation or buried in the substrate. There is also a distinctive fauna deep within gravel beds of lotic waters. Wet moss and other vegetation in bogs, springs, seepage areas, and at the margins of various water bodies harbour a characteristic mite fauna.

Indicator value (Smith & Cook, 1991)

Water mites are among the most abundant and diverse benthic arthropods in many habitats. One square metre of substratum from littoral weed beds in eutrophic lakes may contain as many as 2000 deutonymphs and adults representing up to 75 species in 25 or more genera. Comparable samples from an equivalent area of substratum in rocky riffles of streams often yield over 5000 individuals of more than 50 species in over 30 genera (including both benthic and hyporheic forms). Mites have coevolved with some of the dominant insect groups in freshwater ecosystems, especially nematocerous Diptera, and interact intimately with these insects at all stages of their life histories.

Species of water mites are specialized to exploit narrow ranges of physical and chemical regimes, as well as the particular biologic attributes of the organisms they parasitize and prey upon. Preliminary studies of physicochemical and pollution ecology of the relatively well-known fauna of Europe have demonstrated that water mites are excellent indicators of habitat quality. The results of these studies, along with observations in sampling a wide variety of habitats in North America and elsewhere, lead to the conclusion that water mite diversity is dramatically reduced in habitats that have been degraded by chemical pollution or physical disturbance.

References and web URLs:

Freshwater Benthic Ecology and Aquatic Entomology Homepage                     Soil & Water Conservation Society of Metro Halifax (SWCSMH) Master Homepage

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