Hydrachnidia

The hydrachnids are a very diverse and abundant group (> 5000 species worldwide), with densities reaching up to 5000 individuals per m2 in streams. In some stream sections more than 50 species of hydrachnids may be found2. Some lake littorals comprise more than 70 species that reach densities of up 104 individuals m-2.

• In streams and rivers, the species composition of water mites is influenced by water temperature, chemistry, velocity and substratum composition. In lakes, the distribution of hydrachnids is related to trophic status, suitable microhabitats (i.e. rooted vegetation, algal mats) and their specific prey and hosts2.
• Although much is known about their diversity and distribution, little is known about their functional role in freshwater systems. There are reports of water mites feeding on other taxa, but how intense their predation effect is upon prey taxa, and whether they themselves are preyed upon, remains elusive.
• All water mites reproduce sexually, but cases of parthenogenesis have been hypothesised to explain the often unbalanced sex ratios1,2. Hydrachnidia have a heteromorphic parasitic larva, two pupa-like resting stages, and free-living sometimes predaceous deutonymphs and adults.
• Hydrachnids display a body-size range between 0.2 mm to 5 mm and are characterised by bright colours and variable body shapes. Most species have a globular shape, but some are dorso-ventrally or laterally flattened, elongated or balloon-shaped2.
• The identification of this group includes their mouthpart morphology. Some keys emphasize the examination of the shape of palps, rostrum and chelicerae. In addition, taxonomically relevant are the shape of the legs, the genital field, and more generally, the arrangement and shape of dorsal and ventral body plates.
• Specimens are mounted on slides onto mounting media, but for long-term preservation specific preparations are recommended1,2.