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Revision of the subgenus Unionicola (Acari: Unionicolidae).

ABSTRACT. -- The subfamily Unionicolinae Oudemans contains a single genus Unionicola Haldeman, with more than 40 subgenera. The genus Unionicola is considered holophyletic with the subgenus Unionicola as the least derived group. All other subgenera represent divergent groups displaying unique characters, but synapomorphies and symplesiomorphies among subgenera permit their grouping into separate sister clades. Fourteen new subgenera are erected: Crowellatax, Mitchellatax, Poundsatax, Everittatax, Wilsonatax, Gledhillatax, Bakeratax, Downesatax, Davidsatax, Smithatax, Lasalleatax, Curryatax, Conroyatax, and Edwardsatax. Key words: Unionicola; taxonomic revision; Acari; water mites; new subgenera.

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The Unionicolinae Oudemans (Acari: Unionicolidae) represents a holophyletic group (Cook, 1974; Vidrine, 1986). A single genus, Unionicola Haldeman, contains more than 40 subgenera and more than 200 species that have a wide variety of interactions with other invertebrate organisms in freshwater ecosystems. Many species can be collected free-swimming, whereas many are resident parasites within freshwater molusks. This genus is cosmopolitan among permanent freshwater habitats, but many species remain unknown. Developing a hypothetical evolutionary history of the genus is necessary for testing hypotheses of coevolution (Vidrine, 1986, 1988).

This paper revises the subgenus Unionicola. The subgenus Unionicola contains many species of uncertain subgeneric placement. Using morphology and biogeography, these species of the subgenus are divisible into separate subgenera. Some species remain obscure and must be retained in the subgenus Unionicola until more is known about their morphology and biogeography. Fourteen new subgenera are erected. These named entities represent divergent clades within the genus and bring more homogeneity to the current subgeneric concepts within the genus.

RESULTS

Subgenus Unionicola Haldeman 1842

Type species. -- U. crassipes (Muller, 1776).

Diagnosis. -- Female genital field with two pairs of acetabular plates, and each plate similar in shape and apparently unmodified and bearing usually two or three acetabula; anterior female acetabular plates with two, long, inner setae, posterior plates with a single, long, inner seta; all four female acetabular plates closely appressed to one another with setae forming a central mass; male genital field with a single pair of acetabular plates forming a nearly circular field that is on the venter or absolutely posterior by not extending up onto the dorsum, and lacking thick setae or spines; pedipalps subcylindrical and well sclerotized; male and female walking legs similar and lacking obvious sexual dimorphism; typical species have large, moveable spines on tubercles on the first walking leg.

Distribution. -- Worldwide (freshwater habitats).

Discussion. -- There are 43 species in this subgenus, including some with relatively unmodified genital fields and legs. Unionicola nearctica Crowell and Davids, 1979, is here retained, as discussion continues concerning its validity (Conroy, 1984; Crowell, 1984; Davids et al., 1985; Vidrine, 1987; Proctor, 1989). Unionicola ovalis Imamura, 1954, is synonomized with U. miyazakii Imamura, 1953 (Hevers 1984). Unionicola unquiculata Walter, 1929, is a member of the subgenus Heversatax Vidrine, 1988, and Lundblad (1969) suggested that U. kantaka Cook, 1967, is a synonym. Most studied species are associated with Spongillidae or usually collected as free-swimming individuals. Occasionally transient specimens as well as encysted stages have been found in freshwater mussels (Bivalvia: Unionidae). The subgenus Unionicola remains polyphyletic, but I have not yet had the opportunity to view some of the rare species.

Additional species included. -- Unionicola affinis (Piersig, 1906), U. cyclophora Viets, 1913, U. digitata (Koenike, 1898), U. dresscheri Besseling, 1946, U. figuralis (Koch, 1836), U. fimbriata Viets, 1913, U. finisbellis Ramassotti, 1947, U. gilani Hevers, 1984, U. gracilipalpis (Viets, 1908), U. graciliseta (Viets, 1935), U. harpax (Koenike, 1898), U. iheringi (Koenike, 1890), U. inermis Lundblad, 1941, U. laurentiana Crowell and Davids, 1979, U. levipalpis Besseling, 1949, U. lindrothi Viets, 1981, U. longiseta Walter, 1915, U. lyncea (Koenike, 1895), U. megalopsis Viets, 1925, U. mexicana Cook, 1980, U. minor (Soar, 1900), U. minuta Viets, 1916, U. miyazakii Imamura, 1953, U. motasi Viets, 1959, U. nearctica Crowell and Davids, 1979, U. necessaria (Koenike, 1906), U. neoaffinis Cook, 1986, U. niigata Imamura, 1954, U. parvipora (Lundblad, 1920), U. parvula Lundblad, 1954, U. perpusilla Viets, 1954, U. pollicigera Viets, 1921, U. postmarginata Viets, 1925, U. pugionipalpis Viets, 1954, U. pusiligera Viets, 1954, U. schmackeri (Koenike, 1895), U. similis (Viets, 1935), U. singalensis (Daday, 1898), U. siolii Viets, 1954, U. tenuis (Lundblad, 1935), U. tridentifera Viets, 1921, and U. uncata Viets, 1916.

Subgenus Mitchellatax, new subgenus

Type species. -- U. curvitarsis Lundblad, 1941.

Diagnosis. -- Male and female genital fields resemble those of Unionicola; pedipalps resemble those of Unionicola; tibia and tarsus of first walking leg of male highly modified; tibia setose with two pairs of large, blunt setae, and an enlongate, thick seta at the distal end; tarsus strongly curved, with small claws.

Distribution. -- South America and Australia.

Discussion. -- Mitchellatax appears to represent a divergent group of mites with uniquely modified legs. Unionicola curvitarsis has the most pronounced modifications in the first walking leg, whereas the other species have only minor modifications. Leg modification has previously been used in the case of Heteratax Lundblad, 1941, as the primary character for separation of subgenera. These species are adequately figured in Lundblad (1942) and Cook (1986).

Additional species included. -- U. annulata Lundblad, 1947, U. longidens Lundblad, 1942, and U. pseudoannulata Cook, 1986.

Subgenus Poundsatax, new subgenus

Type Species. -- U. retractidens Lundblad, 1937.

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; genital field with five pairs of acetabula; pedipalps slightly elongate with tarsus bearing small clawlets; first walking legs significantly less setose than those of subgenus Unionicola; first walking leg with few setae and no tubercles on genu, tibia, and tarsus; tarsal claws of legs expanded distally and serrated at the tip.

Distribution. -- South America.

Discussion. -- Lundblad (1942) provided descriptions and illustrations of the anatomy of the monobasic taxon. Unionicola motasi Viets, 1959, resembles this species, but it is retained in the subgenus Unionicola.

Subgenus Everittatax, new subgenus

Type species. -- U. pectinata Wolcott, 1898.

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; first walking legs less setose than those of subgenus Unionicola; tarsal claws of first walking leg laterally flattened and pectinate.

Distribution. -- North America.

Discussion. -- Wolcott (1898, 1899) described and illustrated this species. It is divergent in tarsal claw structure of the first walking legs and in setal patterns of the legs. Unionicola dresscheri Besseling, 1946, also has uniquely serrate claws (Tuzovskii, 1985), but it is retained in the subgenus Unionicola.

Subgenus Wilsontax, new subgenus

Type species. -- U. plaumanni Lundblad, 1937.

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; pedipalps somewhat elongate and typical of Unionicola; first walking legs less setose than those of subgenus Unionicola; first walking leg with few, thickened, rather blunt setae on genu and tibia, with tarsus rather undifferentiated.

Distribution. -- North America and South America.

Discussion. -- Species of this subgenus have reduced chaetotaxy on their first walking legs, which distinctly separates them from members of the subgenus Unionicola and other subgenera. Lundblad (1942) and Vidrine (1984) adequately illustrated these species.

Additional species included. -- U. poirrieri Vidrine, 1984, and U. viciniseta (Lundblad, 1936).

Subgenus Gledhillatax, new subgenus

Type species. -- U. augustipalpis (Lundblad, 1937).

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; pedipalps somewhat elongate and thin, and typical of Unionicola, but with obvious clawlets distally; first walking legs significantly less setose than those of subgenus Unionicola; first walking leg with no elongate, obviously thickened, rather blunt setae on genu and tibia, with tarsus rather undifferentiated.

Distribution. -- South America.

Discussion. -- These species have reduced chaetotaxy on their first walking legs, which separates them from members of the subgenus Unionicola and other subgenera. Lundblad (1942) and Viets (1959) figured these species.

Additional species included. -- U. brevisuturata Viets, 1959, U. simplicipes (Lundblad, 1936), and U. longipes Lundblad, 1942.

Subgenus Bakeratax, new subgenus

Type species. -- U. chappuisi Walter, 1935.

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; pedipalps somewhat elongate and typical of Unionicola but with distal clawlets obscure; first walking legs significantly less setose than those of subgenus Unionicola; first walking leg with no elongate, obviously thickened, rather blunt setae on genu and tibia, with tarsus rather undifferentiated.

Distribution. -- Africa and Asia (India).

Discussion. -- These species have reduced chaetotaxy on their first walking legs, which distinctly separates these from members of the subgenus Unionicola and other subgenera. Cook (1966) and Baker (1981) provided adequate illustrations of these species.

Additional species included. -- U. bogerti (Daday, 1907), U. dadayi Cook, 1966, and U. incerta Bader, 1981.

Subgenus Downesatax, new subgenus

Type species. -- U. vidrinei Cook, 1986.

Diagnosis. -- Male and female genital fields similar to those of subgenus Unionicola; pedipalps somewhat elongate and typical of Unionicola, but the tarsus is relatively short with obvious clawlets; first walking legs significantly less setose than those of subgenus Unionicola, but more setose than Gledhillatax and Bakeratax; first walking leg with few elongate, obviously thickened, rather blunt setae on genu and tibia, with tarsus rather undifferentiated.

Distribution. -- Australia.

Discussion. -- Cook (1986) figured this species. Gledhillatax, Bakeratax, and Downesatax appear to be sister clades with extremely reduced chaetotaxy on their first walking legs. Their apparent distribution in Gondwana is possibly indicative of the ancient lineage of the group.

Subgenus Davidsatax, new subgenus

Type species. -- U. inflexa Viets, 1921.

Diagnosis. -- Male and female genital fields resemble those of Unionicola; pedipalps slightly elongate; first walking leg with distinctive setal pattern and arching distally in tibia, and arching centrally in tarsus.

Distribution. -- Africa.

Discussion. -- This is a possible sister subgenus with Wilsonatax and Heteratax Lundblad, 1941, because the chaetotaxy of their first walking legs is similar. This species is adequately figured in Lundblad (1949).

Subgenus Smithatax, new subgenus

Type species. -- U. minutissima Lundblad, 1947.

Diagnosis. -- Male and female genital fields similar to Unionicola; pedipalps slightly elongate and cylindrical, with basal segments not noticeably thicker than distal segments; first walking leg with distinctive setal pattern that is somewhat more reduced than that of the subgenus Unionicola; first walking leg with long, pointed setae on small tubercles.

Distribution. -- Australia.

Discussion. -- The species are adequately figured in Cook (1986). This is a possible sister subgenus with Downesatax.

Additional species included. -- U. alpa Cook 1986.

Subgenus Lasalleatax, new subgenus

Type species. -- U. brasiliensis (Lundblad, 1936).

Diagnosis. -- Male and female genital fields with five or six pairs of acetabula and resembling those of subgenus Unionicola; pedipalps slightly elongate and resembling those of subgenus Unionicolides Lundblad, 1937; first walking leg with a distinctive setal pattern that is less setose than that of the subgenus Unionicola.

Distribution. -- North America (Mexico) and South America.

Discussion. -- The species are adequately figured in Lundblad (1942), Viets (1954a), and Cook (1980). This subgenus appears to be a phenotypic intermediate between Unionicola, Wilsonatax, and Unionicolides.

Additional species included. -- U. conjuncta Viets, 1954a, and U. unidens Lundblad 1942.

Subgenus Curryatax, new subgenus

Type species. -- U. prehendens Viets, 1954b.

Diagnosis. -- Male and female genital fields resembling those of the subgenus Unionicola; pedipalps relatively short and thick with tibia bearing an elongate dorsal projection.

Distribution. -- South America.

Discussion. -- This species has an unusual pedipalp. The dorsal projection on the tibia of the pedipalp is unlike any structure thus seen in the genus. The pedipalp is figured in Viets (1954b).

Subgenus Conroyatax, new subgenus

Type species. -- U. setifera Cook, 1967.

Diagnosis. -- Male genital field with many large setae and divided into four plates; female genital field, coxal plates, and pedipalps resembling those of the subgenus Unionicola; pedipalps and coxal plates resemble those of Unionicola; chaetotaxy of first walking legs resemble those of the subgenera Heteratax, Wilsonatax, and Davidsatax; first walking leg with a pair of large, blunt setae on each of the segments except the tarsus.

Distribution. -- Asia (India).

Discussion. -- This species is adequately figured in Cook (1967). Conroyatax is apparently related to Heteratax, Wilsonatax, and Davidsatax.

Subgenus Edwardsatax, new subgenus

Type species. -- U. crassipalpis Walter, 1915.

Diagnosis. -- Male and female genital fields similar to those of the subgenus Unionicola; pedipalps thick with tarsus short and not tapered at distal tip; distal end of pedipalp tarsus with two small clawlets and an enlarged clawlet; first walking legs with few setae that are small and not on tubercles; male and female walking legs are similar.

Distrubution. -- Australia.

Discussion. -- This subgenus has distinctive short, thick pedipalps that vaguely resemble those of Crameratax Vidrine, 1988. Cook (1986) suggested that U. crassipalpis is closely related to members of the subgenus Armatax Bader, 1981, which have similar pedipalps. Species of Edwardsatax are adequately illustrated in Viets (1980) and Cook (1986).

Additional species included. -- U. flabelliseta Cook, 1986.

Subgenus Crowellatax, new subgenus

Type species. -- U. billieaehonore Crowell, 1990.

Diagnosis. -- Female with two pairs of acetabular plates, the anterior pair being only indistinctly separated from the posterior; male and female genital fields with five pairs of acetabula; setae in female genital field similar to those of the subgenus Unionicola; male genital field consisting of a pair of acetabular plates, each plate with a pair of inner, anterior setae; pedipalps similar to those of the subgenus Unionicola.

Distribution. -- New Zealand.

Discussion. -- Crowell (1990) placed this species in the subgenus Pentatax Thor. The similarities between females of Unionicola and Crowellatax indicate close relationship between these taxa; however, the female genital field is more fused with the two pairs of plates somewhat obscure in Crowellatax. Also, the male genital field with its prominent setae is unusual and unique in the genus Unionicola. Crowell (1990) adequately figured this species, which he obtained from sponges.

DISCUSSION

The genus Unionicola is holophyletic with the subgenus Unionicola as the least derived group. All other subgenera represent divergent groups displaying unique characters, but synapomorphies and symplesiomorphies among subgenera permit the grouping of them into separate sister clades. Fourteen new subgenera are erected: Crowellatax, Mitchellatax, Poundsatax, Everittatax, Wilsonatax, Gledhillatax, Bakeratax, Downesatax, Davidsatax, Smithatax, Lasalleatax, Curryatax, Conroyatax, and Edwardsatax.

The subgenus Unionicola contains a large number of divergent clades. The type species, U. crassipes, represents a specific phenotype, which shares a number of plesiomorphies with Neumania (Unionicolidae: Piontacinae). All other phenotypic groups are considered divergent, and many are named in this paper. However, several different phenotypes remain and may require further subgeneric designation. Unionicola dresscheri, with unusual serrated claws (Tuzovskii, 1985) may belong to another new subgenus, and Unionicola cyclophora is divergent and may represent still another subgenus.

Identification of distinctive clades within the genus and grouping them into sister subgenera allow for the development of a phylogenetic tree. The presence of several species and subgenera that do not clearly fit into an obvious morphological cluster causes vague areas to persist in the tree.

Nonmolluscan parasites have diverged at intervals in many directions, and this paper primarily sorts some of these divergent clades into subgenera. An overall study of phylogenetic affinities among the many subgenera is in progress, but general affinities among the subgenera in this paper are apparent.

The subgenus Lasalleatax represents a phenotypic intermediate between Unionicola and Ferradasatax Vidrine, 1988. Ancestors with traits resembling Lasalleatax may have given rise to the molluscan parasite fauna with distributions centered in the Southern Hemisphere (Gondwana).

The subgenus Pentatax Thor represents a phenotypic intermediate between Unionicola and Anodontinatax Vidrine, 1986. Ancestors with traits resembling Pentatax may have given rise to the molluscan parasite fauna with distributions centered in the Northern Hemisphere (Laurasia).

A number of subgenera (Downesatax, Gledhillatax, Wilsonatax, Bakeratax, Edwardsatax, and Smithatax) are closely related and possibly are divergent from a single ancestral clade. These bear reduced chaetotaxy on their first walking legs.

A number of subgenera (Crowellatax, Poundsatax, Mitchellatax, Everittatax, Curryatax, and Conroyatax) are quite divergent in one or more characteristics. These are obviously polyphyletic and represent several independent clades.

The dire need for additional specimens of many of the species from all parts of the world for study is apparent. Many tropical species are apparently unknown and may provide the source of plausible explanations of lines of divergence. The unravelling of the Unionicola systematic puzzle may play a formidable role in further developing the mechanisms by which vicariance biogeography and phyletic analysis can be used to explain the evolutionary history of not only this group but also other aquatic Acari.

ACKNOWLEDGMENTS

The subgenera were named to honor the following: Robert Crowell, Rodger Mitchell, Alan Pounds, Betty Everitt, James Wilson, Terry Gledhill, R. A. Baker, Barbara Downes, Cornelius Davids, Ian Smith, Mark LaSalle, Mary Curry, John Conroy, and Dale Edwards.

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MALCOLM F. VIDRINE

Division of Sciences, Louisiana State University at Eunice, Eunice, Louisiana 70535.
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Author:Vidrine, Malcolm F.
Publication:The Texas Journal of Science
Date:Nov 1, 1992
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