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.

The terminology of idiosomal setae follows Tuzovskij (1987).The following abbreviations are used: P–1–5, pedipalp segments (trochanter, femur, genu, tibia and tarsus); I–Leg–1–6 , first leg, segments 1–6 (trochanter, basifemur, telofemur, genu, tibia and tarsus) i.e. III– Leg–4 = genu of third leg; L – length, W – width; ll – leaf-like projection; n = number of specimens measured; all measurements are given in μm.

Spongibates Wainstein, Zoologichesky Zhurnal, 57: 787. [In Russian] Type species: Spongibates globosus Wainstein, 1978 space5mm

Material examined — Holotype, slide 16260, deposited in the collection of Papanin Institute for Biology of Inland Waters (Borok, Russia).

Description — Idiosoma round, almost spherical, with five pairs of slit organs (Fig 1A-B). Integument finely striated. Lateral eyes beneath the integument. Setae Fch thicker than other idiosomal setae. Idiosomal setae associated with glandularia except for trichobothria Fp and Oi and setae Pi. Coxal plates I fused medially and with the capitulum to a large anteroventral 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).

Measurements (n=1) — Idiosoma L 585, W 520; pedipalp segments (P–1–5) L: 35, 115, 100, 160, 60; legs segments L: I-Leg-1-6: 70, 95, 130, 190, 215, 200; II-Leg-1-6: 75, 105, 145, 230, 260, 240; III-Leg-1-6: 80, 120, 210, 285, 315, 275; IV-Leg-1-6: 140, 160, 240, 340, 355, 300.

Distribution — Europe, Russia, Yaroslavl Province, Nekouz District, littoral zone of Rybinsk reservoir near settlement of Borok (Wainstein 1978).

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.

Three species of the genus Hygrobates have been found in Rybinsk reservoir (Tuzovskij 1974, 1996), i.e. H. longipalpis (Hermann, 1804), H. nigromaculatus and H. trigonicus Koenike, 1895. The female of Spongibates globosus (Wainstein, 1978) is most similar to the female H. nigromaculatus.

Measurements of female H. nigromaculatus (n=10) are as follows (Tuzovskij 2017): Idiosoma L 1000–1450; pedipalp segments (P–1–5) L: 30-42, 108-115, 85-90, 145-165, 40-48; legs segments L: I-Leg-1-6: 62-75, 90-115, 110-140, 160-210, 185-215, 165-205; II-Leg-1-6: 65-90, 100-115, 135-155, 210-240, 225-250, 195-240; III-Leg-1-6: 70-90, 100-150, 160-190, 260-290, 280-315, 245-275; IV-Leg-1-6: 130-165, 135-165, 220-250, 305-350, 310-350, 275-330. The length of the pedipalps and legs is without essential differences between S. globosus and H. nigromaculatus.

The unique character which distinguishes Spongibates is, according to Wainstein (1978), the presence of several minute leaf-like anterior projections on each side of the capitulum in the female of S. globosus, a feature not reported in other taxa of the family Hygrobatidae (Cook 1974, Gerecke et al. 2016). We suppose that Wainstein (1978) possibly described a juvenile aberrant female of H. nigromaculatus.

This research was performed in the framework of the state assignment of FASO Russia (theme No 0122-2014-0007). We express deep gratitude to anonymous referees for reviewing the manuscript.

Cook D.R. 1974. Water mite genera and subgenera. Memoirs of the American Entomological Institute, 21: 1– 860.

Gerecke R., Gledhill T., Pešić V., Smit H. 2016. 8. Acari: Hydrachnidia III. In: Gerecke R. (Ed.) Süβwasserfauna von Mitteleuropa, 7/2-3. Elsevier GmbH. Akademischer Verlag Berlin Heidelberg. p 1-429. doi:10.1007/978-3-8274-2689-5_1

Tuzovskij P.V. 1974. The distribution water mites in the littoral zone of the Rybinsk reservoir and adjacent water-bodies. Biology and productivity of freshwater invertebrates, 25 (28): 202-228. [in Russian]

Tuzovskij P.V. 1987. Morphology and Postembryonic Development in Water Mites. Moscow, Nauka. pp 172. [in Russian]

Tuzovskij P.V. 1996. Water mites of the Upper Volga. Tolyatti. pp 82. [in Russian]

Tuzovskij P.V. 2017. On the systematics of the water mites Hygrobates nigromaculatus Lebert, 1879 and H. setosus Besseling, 1942 (Acari, Hydrachnidia, Hygrobatidae). Zootaxa, 4277 (1): 17-31. doi:10.11646/zootaxa.4277.1.2

Wainstein, B.A. 1978. A new species and genus of the Family Hygrobatidae. Zoologichesky Zhurnal, 57 (5): 787-789. [in Russian]