Litorogammarus dursi Marin & Palatov & Copilaș-Ciocianu 2023, sp. nov.

Litorogammarus dursi sp. nov. Figs 4–8 Material examined. Holotype: Ô (bl. 9.5 mm), ZMMU Mb-1243— Russian Federation, southwestern Caucasus, Krasnodar Krai, Novorossiysk area, lower part of Durso river, about 30–40 meters from its mouth, 44°40’44.0”N 37°33’41.6”E, a small expansion of the river with a weak current, among aquatic vegetation, with hand net, coll. Palatov D.M. & Marin I.N., 20.07.2021. Paratypes: Ô (bl. 8.5 mm), ♀ (bl. 7 mm), ZMMU Mb-1244, same locality and data as for holotype. Additional material: 3♀♀, 3 ÔÔ, LEMMI, same location and data as holotype. Etymology. The species is named after the Durso River, in the lower course of which the new species was discovered. Diagnosis. Head with deeply concave anteroventral lobe. Pleon with urosomite I with 2 robust setae on each margin and 2 median spine-like setae posterodorsally; urosomite II with 2 robust setae on each margin and 2 submedian spine-like setae (lateral spine stronger than median) dorsally; urosomite III with a row of 3–4 robust setae and 2 simple median setae. Antenna I with 6-segmented accessory flagellum. Antenna II without calceoli and aesthetascs. Upper lip with concave distal margin. Mandibles with large and flattened incisor teeth. Outer lobe of maxilla I very wide. Maxilliped with bluntly rounded outer and inner lobes. Epimeral plates with sharp and produced posteroventral angles. Description. Body length 8–11 mm. Head with deeply concave anteroventral lobe (Fig. 8 a); eye elongate reniform, well pigmented. Metasoma unarmed, with some short setules near dorsal articulation of segments. Urosomites I–III (Fig. 8 b) smooth, free, without any elevations; urosomite I with 2 robust setae on each margin and 2 median spine-like setae posterodorsally; urosomite II with 2 robust setae on each margin and 2 submedian spine-like setae (lateral spine stronger than median) dorsally; urosomite III with a row of 3–4 robust setae and 2 simple median setae (Fig. 8 b–d). Antenna I (Fig. 4 a) longer and slender than antenna II, especially in males, close to 1/3 of body length, with robust peduncle segments, articles ratio is 1.0:0.87:0.56; primary flagellum long with 26–30 segments, with aesthetascs, with 6-segmented accessory flagellum (Fig. 4 b); both primary and accessory flagellum covered with short distal setae. Antenna II (Fig. 4 c, d) sexually dimorphic: strong and densely setose with distally curved setae in males, with strong, forward pointing gland cone; peduncle articles II–V relatively stout, with ratio about 1.0:2.2:2.8, without calceoli and aesthetascs. Upper lip (labrum) (Fig. 5 a) with concave distal margin, covered with minute setae. Lower lip (labium) (Fig. 5 b) outer lobes covered with thin setae, bearing feebly developed inner lobes. Mandibles slightly asymmetrical; left mandible (Fig. 5 c): incisor with 9–10 small horizontally flattened teeth (Fig. 5 d), lacinia mobilis with 4 teeth; with a row of 10 serrated setae between lacinia and molar, few spatulate setae and a long seta at base of molar (Fig. 5 d), proximal article of palp without setae; article II with a tuft of long submarginal simple setae, distal article with 10 sets of 4–10 A-setae, 15 C-setae, 9–10 D-setae, almost equal in length, and 6 E-setae (Fig. 5 d); right mandible (Fig. 5 e): incisor process with 9 teeth, lacinia mobilis bifurcate, toothed, with a row of 6 serrated setae between lacinia and molar (Fig. 5 f). Maxilla I (Fig. 5 g) inner lobe broadening distally, distal margin straight, armed with numerous simple setae; outer lobe bluntly rounded distally, with about 20 hairbrush-like apical setae; palp asymmetrical, with a convex outer, and slightly concave inner margins, distal article about 3 times longer than basal right palp wide, with simple distal spines and lateral setae. Maxilla II (Fig. 5 h) inner lobe broad, distally rounded, with slightly produced blunt ventral margin, armed with numerous setae medially and apically; outer lobe wider than inner, expanding distally, distal margin blunt, rounded with apical setae. Maxilliped (Fig. 5 i): inner and outer plates bluntly rounded and slightly broadening distally, densely setose, inner lobe with 2–3 short and stout apical spines; palp 4- articulated, covered with numerous long simple setae, terminal article hooked. Gnathopod I (Fig. 4 e, f, g, h) with coxal plate distally slightly widened, concave, with 1 distal and 1–2 subdistal setae on anterior margin; basis sub-linear, with numerous long anterior and posterior setae; ischium about as long as wide, equal to merus; merus about 1.5 times longer than wide, with medium setae on posterior margin; carpus short, almost triangular, with a cluster of long setae on posterior margin; propodus (palm) pyriform in males (Fig. 4 e), about 2–2.5 times as long as wide, with oblique distoventral part of palmar margin in males and straight in females; palmar margin crenellated in its proximal (angular) part, with 5–7 palmar marginal spines and 1 mid-palmar spine in male (Fig. 4 f), and with 1 palmar angle spine and 1 short mid-palmar spine in females (Fig. 4 h); dactylus reaching about 40% length of propodus in males and less than 30% in females, posterior margin convex, with 2 long medial setae. Gnathopod II (Fig. 4 i, j, k, l) significantly larger in males; coxal plate subrectangular, with convex dorsal and ventral margins, with 1–2 setae on anterior margin subdorsally, without setae on lower margin and facial setae; basis sub-linear, with several long anterior setae and posterior setae; ischium about as long as wide, equal to merus; merus about as long as wide, with several long setae on posterior margin; carpus triangular, with slightly marked ventral lobe, armed with a cluster of long setae on posterior margin; propodus (palm) deeply trapezoidal, about 1.7 times as long as wide, with oblique distoventral part of palmar margin in males (Fig. 4 i) and rectangular elongated, about 2.5 times as long a wide, with straight palmar margin in females (Fig. 4 g); palmar margin crenellated in its proximal (angular) part, with row of 9–10 thin marginal spines and 4 mid-palmar spines in males (Fig. 4 j) and with 3 angular and 1 mid-palmar spines in females (Fig. 4 h); dactylus reaching about 40% length of propodus in males and less than 30% in females, posterior margin convex, with 2 long medial setae. Pereiopod III (Fig. 6 a) with segments covered with numerous anterior and posterior stout and small setae; coxal plate III almost rectangular, concave distally, with 1 seta on anteroventral margin; basis sub-linear, about 3.5 times as long as wide; ischium as long as wide; merus about 3 times as long as wide; carpus about 4 times as long as wide, with anterior margin smooth with distodorsal setae only; propodus about 5 times as long as wide, with smooth anterior margin; dactylus (Fig. 6 d) relatively stout, curved, with 1 seta at hinge of unguis. Pereiopod IV (Fig. 6 c) with segments covered with numerous anterior and posterior stout and small simple setae; coxal plate IV almost quadrate, e distal margin, concave, with 4–5 small setae along lower anterior margin; basis sub-linear, about 3.5 times as long as wide; ischium as long as wide; merus about 3 times as long as wide; carpus about 4 times as long as wide, with smooth anterior margin and distodorsal spines only; propodus about 4 times as long as wide, with smooth anterior margin; dactylus (Fig. 6 d) stout, curved, with 1 simple seta at hinge of unguis. Pereiopod V (Fig. 6 e) slightly shorter than PpVI–VII; coxal plates V bilobate, with 1 small seta on anterior lobe and 1 long spine on posterior lobe; basis relatively wide, about 1.5 times as long as wide, with dorsal margin slightly convex, armed 5–6 small stout spines, with well-developed ventral lobe armed with short spines, posteroventral angle bluntly rounded; ischium subquadrate, about as long as wide; merus about 2–2.5 times as long as wide; carpus about 3 times as long as wide; carpal and meral segments with strong spines and spiniform setae along anterior and posterior margins; propodus about 7–8 times as long as wide, with clusters of small spines along anterior margin and long simple spines along posterior margin; dactylus (Fig. 6 f) stout, curved, with 1 seta at hinge of unguis. Pereiopods VI–VII (Fig. 6 f, g) almost similar in shape, PVII slightly stronger than PVI; coxal plates VI–VII bilobate, with anterior lobe bearing 1–2 spines or unarmed, with 1–2 spines on posterior lobe; basis wider in proximal part and narrowing distally, with relatively short spines along anterior margin and short spiniform setae along posterior margin; ischium subquadrate, about as long as wide; merus about 2.5 times as long as wide; carpus about 3.5–4 times as long as wide; propodus about 6 and 5 times as long as wide in PVI and PVII, respectively; carpal, meral and propodal segments with strong spines and stout setae along anterior and posterior margins; dactylus (Fig. 6 h, j) stout, curved, with 1 seta at hinge of unguis. Coxal gills balloon-shaped, present on GnII and PpIII–VI. Epimeral plate I (Fig. 7 a) with triangular posteroventral corner, posterior margin with 1 spine and 1 stout setae, ventral margin slightly convex. Epimeral plate II (Fig. 7 b) ventrally convex, anterior part with 4 submarginal spines, posterior corner triangularly produced, bearing several small setae on posterior margin. Epimeral plate III (Fig. 7 c) almost straight ventrally, with 4 submarginal spine-like setae in its anterior part, with produced bluntly pointed posteroventral corner. Pleopods I–III (Fig. 7 e) similar in shape; both rami almost subequal, with plumose setae, basal segment with 2 coupling slender hooks and 1–2 long setae in retinacules (Figs 7 f, 8 e). Uropod I (Fig. 7 g, h) with peduncle armed with 1 large proximoventral spine, with 3–4 inner and 3 outer basiofacial spines, with 2 subdistal spines; inner ramus equal to outer ramus; outer ramus (exopodite) with 2 dorsal and 4 subdistal dorsal spines; rami equal in size, inner ramus (endopodite) with 3 dorsal and 3 apical spines, outer ramus (exopodite) with 3 dorsal and 3 apical spines. Uropod II (Fig. 7 i, j) smaller than uropod I; peduncle with 3 inner and 2 outer basiofacial spines, with 2 subdistal dorsal spines; outer ramus (exopodite) equal to inner ramus (endopodite); inner ramus with 3 dorsal, 1 mesial and 5 apical spines; outer ramus with 2 dorsal, 1 mesial and 4 apical spines. Uropod III (Fig. 7 k, l) of parviramous type; peduncle about 1.2 times longer than wide, with 2 clusters of subdistal spines; inner ramus (endopodite) reduced, scale-like, about 6 times shorter than outer ramus (endopodite) in males, with a row of 5–7 subdistal spines and several small setae; outer ramus (endopodite) 2-segmented, with proximal segment armed with 7 groups of spines and/or short setae along outer margin, with 4 groups of short spines and plumose setae along inner margin; distal segment short, but distinct, about 6–7 times shorter than proximal segment, about 2 times longer than subdistal spines accompanying it. Telson (Fig. 7 d) entirely cleft; lobes elongated, suboval, about 1.8 times as long as wide, each lobe armed with a group of 3–4 strong apical spines and 2 small slender setae, and with 1, rarely 2 groups of 2–3 dorsal submedian strong spines, accompanying with simple setae. Sexual dimorphism. The stronger and densely setose antenna II and larger propodus of gnathopod II easily distinguish males from females (Fig. 4 c, d); propodus (palm) of gnathopod I is subrectangular in females (Fig. 4 g). Coxal gills are quite linear in females. Uropod III is also larger in males than in females (Fig. 7 k, l); inner ramus (endopodite) is about 4–5 times in females vs. about 6 times in males. Coloration. Body and appendages translucent. Eyes cornea silver or brown. Body size. The largest collected ♀ has tbl. 9.0 mm; the largest collected Ô has tbl. 11.0 mm. GenBank accession number. OQ407693–OQ407694 (this study). Habitat and distribution. The species is presently known only from its type locality—the lower part of the small coastal Durso River flowing into the Black Sea, southwestern Caucasus, Russia. All studied specimens were collected in the area of the river expansion with a relatively weak water current and numerous coastal aquatic vegetation, where this species is probably living. No specimens of the new species were collected in the river part with stronger currents or estuarine part with brackish water, under pebble, stones or boulders. Taxonomic remark. Litorogammarus dursi sp. nov. possess very unusual mouthparts, such as concave distal margin of the upper lip, large and flattened incisor teeth of mandible, very wide outer lobe of maxilla I and bluntly rounded outer and inner lobe of maxilliped, as well as combination of other features, such as armature of urosome and form of EpI–III easily separate this species from the congeners and all other Echinogammarus -like gammarids. Distally produced anterolateral lobe of the head also allow to distinguish the species among the congeners (Table 3).Published as part of Marin, Ivan, Palatov, Dmitry & Copilaș-Ciocianu, Denis, 2023, The remarkable Ponto-Caspian amphipod diversity of the lower Durso River (SW Caucasus) with the description of Litorogammarus dursi gen. et sp. nov., pp. 483-517 in Zootaxa 5297 (4) on pages 491-498, DOI: 10.11646/zootaxa.5297.4.2, http://zenodo.org/record/800912