Water mites may be brilliant red or orange in colour, unusual among freshwater invertebrates, but they also display more subtle blues, greens and yellows. They are also unusual among mites in some lineages having movable, internalized eye lenses sunk deep within the prosoma rather than being set on the surface of the cuticle.
The palps of post-larval water mites vary depending on their diet. The egg-eating Hydryphantidae, Hydrodromidae, and Hydrachnidae have chelate (pincer-like) palps. The crustacean-eating Arrenurus (Arrenuruidae) have uncate palps to grasp the slim appendages of crustaceans. Most other water mite families have linear palps for grappling with prey animals.
Hydrachnidia are ubiquitous in nearly all freshwater habitats of every continent except Antarctica. Typical habitats include streams and marshes, but they may also be found in more obscure areas such as treeholes, hot springs, deep lakes and waterfalls. Some species have also adapted to marine environments.
Larvae are usually the only water mite life stage to have parasitic relationships with other organisms. Upon location of a host, larvae pierce host integument with their chelicerae and feed on hemolymph until fully engorged or brushed off. Common host groups include insects with aquatic or semi-aquatic juvenile stages, including, but not limited to, the Diptera (true flies), Odonata (dragonflies and damselflies), and Trichoptera (caddisflies). It was originally believed that water mite larvae located hosts by accidental contact, but recent studies have found they likely utilize a combination of visual, tactile, and chemical cues. Even though larvae are capable of sensing the presence of a nearby host, it has been suggested that they are unable to distinguish between host species, and rather select hosts solely based on spatial and temporal coincidence. The abundance of water mites in a region, as well as prevalence and intensity of host infection, are impacted by a multitude of environmental and biological factors, and have shown great geographic variation. In some cases, high infection intensities have significantly increased chances of host mortality and reduced fecundity. Water mite larvae have been considered as a potential biological control agents, although low natural infection intensities warrant supplementation with other control strategies in order to be effective.
Some water mites continue to be parasites in their post-larval stages. These are mainly associated with molluscs, such as mussels (Najadicola ingens and many species of Unionicola) and snails (two species of Dockovdia). That said, not all associations with other animals are parasitic; some Unionicola species merely use other animals as safe, well-oxygenated places to lay eggs and to pass their resting stages (protonymph, deutonymph).
Nymphs and adults of water mites are predatory. They prey on other water mites, small crustaceans (e.g. cladocerans, ostracods and copepods), the eggs, larvae and pupae of aquatic insects, and non-arthropod invertebrates such as rotifers, nematodes, and oligochaetes. The egg-eating water mites often prey on the eggs of the same insects they parasitise as larvae. To feed, water mites bite prey, inject saliva containing digestive enzymes to liquefy the tissue, and suck out the liquid.
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