On the taxonomic status of the water mite genus Spongibates Wainstein, 1978 (Acari, Hydrachnidia, Hygrobatidae)

The identity of the water mite genus Spongibates Wainstein, 1978 has been examined. After examination of the holotype, we conclude that Wainstein described an aberrant Hygrobates species, very likely H. nigromaculatus. Therefore, Spongibates should be considered a junior synonym of Hygrobates.


Introduction
The water mite genus Spongibates has been described from Russia by Wainstein (1978). We noted some errors and discrepancies in the original publication. The aim of this paper is, therefore, to give a correct description of the genus and type species S. globosus Wainstein, 1978, and to discuss its taxonomic status.
Description -Idiosoma round, almost spherical, with five pairs of slit organs (Fig 1A-B plate. Posteromedial margin of Cx-I rounded, Cx-IV subtriangular in shape, with a nose-like protruding medial margin. Genital field with three pairs of acetabula. Coxal plates and genital field cover nearly all of the ventral surface. Capitulum with a short rostrum and several minute, leaf-like anterior projections on each side. Pedipalp large (Fig. 1C): P-1with a single short dorsodistal seta, P-2 with six thick setae and without ventrodistal projection, P-3 with four thick setae and one thin seta, P-2 and P-3 with denticulate ventral margin, P-4 ventral setae on the same level near middle of segment. Legs long, thin without swimming setae. First pair of legs 1.5 times, posterior pair of legs 2.6 times longer than idiosoma. Leg claws with long external clawlet and short internal one, lamella with concave ventral margin (Fig. 1D).
Habitat -The species has been found in a fresh-water sponge of the family Spongillidae.
Remarks -The idiosoma shape, unusual length of legs, structure of capitulum and genital plates are considered as diagnostic characteristics of the genus Spongibates by Wainstein (1978). In fact, the coxal plates and genital field in the female S. globosus cover nearly all the ventral surface (Fig. 1B). However, such a shape of the ventral surface is characteristic for juvenile adults and deutonymphs ( Fig. 2A) of the genus Hygrobates, while in mature specimens of this genus the coxal plates occupy about half or less than one half of the ventral surface (Figs. 2B-3A). For instance, the idiosoma length differs strongly between a juvenile (180-200) and a mature deutonymph (480-550) of H. nigromaculatus Lebert, 1879. In juvenile deutonymphs of this species the fourth legs are 2-3 times longer than the idiosoma, while in mature specimens the fourth legs are equal or only a little longer than the idiosoma. In mature Hygrobates females the coxal plates occupy less than one half of the ventral surface (Fig. 3A). The genital field of the female S. globosus (Fig. 1B) is similar to the genital field of H. nigromaculatus (Fig. 3B) and some other species of the genus (Gerecke et al. 2016). Also the leg claws of S. globosus (Fig. 1D) are similar to those of H. nigromaculatus (Fig. 3C).
Thus, based on the idiosoma shape, the unusual relative length of the legs and the structure of the genital plates, it is clear that Wainstein (1978) described a juvenile specimen of Hygrobates, very likely H. nigromaculatus (see below). Therefore, the genus Spongibates should be considered as a junior synonym of the genus Hygrobates.