Desertoniscus zaitsevi sp. nov.

(Figs 2–6)

Material examined. Holotype. male (ZMMU, Mc-1409), Russia, Republic of Kalmykia, 1 km N of Bolshoy Tsaryn (47.8835 N, 45.4175 E), 29.04.2017, leg. K. Gongalsky.

Paratypes. 1 male, 2 females (ZMMU, Mc-1410), 1 male, 2 females, same data as of the holotype (KG); 2 females (KG), Russia, Republic of Kalmykia, 7 km N of Bolshoy Tsaryn, Voskhod (47.8517 N, 45.4296 E), 29.05.2016, leg. K. Gongalsky (KG).

Diagnosis. Antennal flagellum with ratio of articles 1 to 2 being 1:3; male exopod of pleopod 1 with almost rounded apex; male ischiopod of pereiopod 7 subtriangular in shape.

Description. Maximum body length: male 10.5 mm; female 14.0 mm. Body length of holotype 10.2 mm. Body light grey-brown. Body elongate; pleon forming no continuous outline with pereion (Fig. 2). Cephalic lobes poorly-developed; distal part of head covered with γ-shaped dorsal setae (Fig. 3D). Median lobe with rounded distal edge (Fig. 3B). Dorsal surface of tergites smooth. Coxal plates of pereional segments 2–7 at base with white or yellow spots (Fig. 2); posterior edges of coxal plates of pereionites straight (Fig. 3A). Noduli laterales on pereionites 1–4 increasingly strongly removed from edges of coxal plates (Fig. 3A). Distal part of telson elongate; distal corner acutely triangular (Fig. 3C).

Antennula 3-segmented (Fig. 3E); article 1 wide and relatively long; article 2 ca 1.5 times shorter than 1st; article 3 almost as long and narrow as 1st, bearing tuft of setae at apex.

Antenna short, barely reaching pereionite 2 (Fig. 2); flagellum 2-segmented, proximal article ca 3 times shorter than distal one (Fig. 3F).

Left mandible (Fig. 4A) with molar process bearing two teeth and lacinia mobilis with straight edge; hairy lobe with row of ca. 10 penicils basal to lacinia. Right mandible smaller than left one, consisting of molar process with three teeth and lacinia mobilis with two teeth and hairy lobe with row of ca 10 penicils (Fig. 4B).

Maxillula (Fig. 4C) with two strong hirsute setae at medial corner of inner endite. Apical edge of outer endite bearing eight spines divided into two groups: four medial spines more slender and with split tips, and four lateral spines stouter and with simple tips. Tip of endite covered with brush of dense setae.

Edge of maxilla bilobate, medial half of apical edge of inner lobe with brush of dense short hairs (Fig. 4D). Inner margin with subapical tubercle. Maxilliped with outer corner of endite bearing two acute tips and numerous large spines near inner corner (Fig. 4E).

Pereiopod (Fig. 5A–5C). Ischipod of male pereiopod 7 subtriangular (Fig. 5C). Spines on distal parts of carpi on all legs with split tips.

Uropods (Fig. 3D) of same colour as dorsal surface of body. Exopods elongate, broadest at about proximal 1/3.

Male. All exopods at lateral margin with lungs. Exopod of pleopod 1 (Fig. 6B) with almost rounded tip and ca eight setae at apex, inner margin slightly concave and with minute setae. Endopod of pleopod 1 with dorsal furrow and row of spines (Fig. 6 C).

Pleopod 2 exopod triangular, with concave outer margin bearing more than 20 setae (Fig. 6D); endopod much longer than exopod, narrow, parallel-sided (Fig. 6D).

Pleopods 3–4 exopods (Fig. 4E–F) trapezoidal, 4th slightly smaller than 3rd.

Pleopod 5 exopod triangular, with broadly rounded corners (Fig. 6G).

Remarks. The species is assigned to Desertoniscus since it shows five pairs of pleopodal lungs, the flagellum of antennae consists of two articles and a rounded median lobe (Kwon 1993). The distinctive feature of the genus is its antennae only slightly surpassing the posterior margin of pereional tergite I (Verhoeff 1930). Based on the body shape, this species seems to be especially similar to D. schmalfussi Kashani & Allspach 2012 . However, these two species differ as follows: (i) the male exopod of pleopod 1 is with an almost rounded apex which is sharper in D. schmalfussi; (ii) the male ischiopod of pereiopod 7 is subtriangular in shape as compared to that bearing no sharp distal corner in D. schmalfussi; (iii) a sharp corner of the male exopod of pleopod 5 which is rounded in D. schmalfussi .

Distribution. The species has been found so far only between the Volga and Vostochnyi Manych rivers. It inhabits some steppe-clad parts of Kalmykia ( Artemisia austriaca, Festuca valesiaca, Tanacetum achilleifolium), being common in various biotopes and even the dominant isopod species in the region.

Etymology. The species name zaitsevi is dedicated to Dr. A. S. Zaitsev (Moscow, Russia), a prominent Russian soil zoologist who organized the studies of soil biota in northern Kalmykia and who took part in the field survey.