Atractides

Gerecke, Reinhard, 2003, Water mites of the genus Atractides Koch, 1837 (Acari: Parasitengona: Hygrobatidae) in the western Palaearctic region: a revision, Zoological Journal of the Linnean Society 138, pp. 141-378 : 141-142

ATRACTIDES : DIVERSITY AND BIONOMICS

Water mites of the genus Atractides have been found in all biogeographical regions except for Australia and Antarctica. When this revision started, a total of 308 species was accepted, with an additional 20 species regarded as synonyms, and another 20 names lacking taxonomic significance (assigned to taxa not belonging to the genus, or published as nomina nova replacing older names erroneously considered to be homonyms). One hundred and twenty-three species are known from Europe, 72 from Asia, 27 from North America, 33 from Central and South Central America and 54 from Africa ( Orghidan & Gruia, 1987; Otero-Colina, 1987; K. O. Viets, 1987; Wiles, 1991; Conroy & Bilyj, 1992,1998; Bader, 1994; Bader & Gerecke, 1996; Jin, 1997; Pešic¢ & Asadi, 2002).

For A. nodipalpis Thor, 1899 , the most common and best known European species, detailed information on life cycle, host range of parasitic larvae and habitat preference has been provided by Efford (1963, 1966), Ullrich (1978), Martin (1998) and Gerecke (2002). In Central Europe, this species mates and produces eggs in autumn, and larvae emerge in spring ( Martin, 1998). Contrary to previous statements (revision by Smith, 1998) the larval stage includes a parasitic phase, with chironomid midges as preferred hosts ( Efford, 1966; Martin, 1998). In early summer numerous deutonymphs are found; the adults emerge in late summer and autumn. No information is available on the morphology and duration of the proto- and tritonymph stages. Deutonymph and adult Atractides nodipalpis feed on small crustaceans and insect lar- vulae. The most apparent character of Atractides , the first leg with transformed distal segments, looks like a grasping organ, but has never been found involved in preying and feeding.

Several large-scale faunistic inventories have revealed that A. nodipalpis displays a certain tolerance of organic pollution. The species can be found in considerable numbers in slightly polluted running waters, but is absent from more heavily polluted habitats in Central Europe where Hygrobates fluviatilis (Ström) and H. nigromaculatus Lebert predominate ( Kowalik & Biesiadka, 1981; Meyer, 1986; Gerecke & Schwoerbel, 1991).

All the other species discussed here are collected much more rarely, and many are known only from single finds. Most were found only in clean water with well conserved substrates; they are probably sensitive to many forms of human impact. In general, Atractides species inhabit running water. Exceptions are A. ovalis Koenike, 1883 (the only known species with swimming setae on its legs), A. pavesii Maglio, 1905 , A. lacustris ( Lundblad, 1925) and A. moniezi Motaş, 1927 , found regularly in ponds and lakes in northern and Central Europe.

A description of the larval morphology of four further species, A. adnatus , A. panniculatus , A. macrolaminatus and A. walteri , has recently been provided by Martin (in press). Because of the paucity of information on the life cycle of all other species obtaining an idea of the spectrum of ecological adaption involves a brief investigation of the wide range of morphological diversity. There is a great variety of shapes of mouthparts and legs, as well as differences in idiosomal dimensions and integumentary structures. From field studies, one may deduce particular habitat patterns: many species are confined to springs, often with a preference for particular substrata ( Schwoerbel, 1959b; Bader, 1975; Biesiadka, 1979b; Cicolani et al., 1996; Gerecke et al., 1998). Others are found exclusively in phreatic and groundwater habitats ( Schwoerbel, 1986) often showing particular adaptations, such as smaller size, flattened shape of the idiosoma, depigmentation, and reduction of eyes. Among species coexisting in the benthos of a stream, considerable differences can be observed both in habitat preference and seasonal phenology (Gerecke, 2002).

Most of the extant information on taxa outside Europe includes morphological descriptions and distributional data only. As the result of taxonomic confusion within Europe, a separate systematics has been developed for all the other continents, which hampers our understanding of overall distribution. Few species are known to have a distribution area extending over more than one continent, although this suggests poor scientific communication rather than firm biogeographical borders. However, several studies have revealed the richness and diversity of Atractides species in tropical areas (e.g. K. Viets, 1935b; Cook, 1980).