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The Hydracarina, or water mites, include include a large diversity of more than 5000 named species and likely many more to be found in Africa, Asia, and South America.  They occupy subaquatic freshwater habitats including lakes, waterfalls, hot springs, and even tree holes (Proctor and Harvey  1998).  A few have become marine (Cook 1974). Most are free living carnivores that, similar to terrestrial arachnids, pierce their prey and suck out nutritious juices. A few are parasites.

Aposematism (“warning coloration”), the use of bright pigments such as red, orange, and yellow, along with taste unpalatable to predators, appears to be frequent among some hydrocarina.  In laboratory experiments, fish quickly learn to reject and avoid brightly colored water mites.  Skin glands on the dorsal surface of the mite skin secrete distasteful fluids when triggered by sensory hairs.  Chemical defense systems appear to be effective for several aquatic arthropods, including beetles (Kerfoot 1982).  In contrast, some individual largemouth bass (Micropterus salmoides) were found to specifically target mites (Hodgson et al. 2006).

See also the CFB zooplankton key

References:

Cook, D.R.  1974.  Water mite genera and subgenera.  Mem. Amer. Entomol. Inst. 21:1-860.

Hodgson, J.R., C.J. Hodgson, and J.Y.S. Hodgson  2006.  Water mites in the diet4 of largemouth bass.  J. Freshw. Ecol. 23:327-332.

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

Proctor, H., and M. Harvey  1998.  Tree of Life Hydracarina.  Water mites.  Version 09 August 1998.  http://tolweb.org/Hydracarina/2606/1998.08.09 in The Tree of Life Web Project, http://tolweb.org/