The specific epithet tepehuana refers to the great Tepehuan ethnic group, whose current settlement is in southern Durango. The name Tepehuan comes from Náhuatl and has two meanings: “owners of the hills”, tepetl (hill) and huan (possessive), and “winner of battles”, tepehuani.

Holotype male, body length 5.4 mm (CNCR 35295), from Tunal River in La Ferrería Durango, Mexico (23°57.905'N, 104°39.817'W) 12 June, 2016. Collectors: A. Marrón-Becerra and G. Rivas. Paratypes (n = 10 males, n = 10 females): males mean size 5.7 ± 0.6 mm, female body length 5.6 ± 0.6 mm (CNCR 35296, permanent slides and CNCR 35297 SEM preparations), same locality, date and collectors as holotype.

Mexico, Durango, Tunal River in La Ferrería: 23°57.905'N, 104°39.817'W (Fig. 1), above 1874 m a.s.l., scarce water but with high density of macroalgae.

Pleonite 1 and 2 with dorsoposterior carina. Coxa 4 excavated posteriorly. Eyes pigmented. Antenna 1 shorter than Antenna 2 without accessory flagellum. Antenna 2 less than one-half body length. Maxilla 1 palp short, reaching less than half distance between base of palp and tip of setae of outer plate, with one stout distal seta; inner plate slender with three or four strong pappose distal setae. Maxilla 2 inner plate with two or three strong pappose setae on inner margin. Gnathopod 1, propodus hammer-shaped, palm slope transverse, inner face with three or four pappose setae, comb scales on distoposterior and distoanterior border. Gnathopod 2, basis hind margin with two setae. Uropods without curved setae. Uropod 3, peduncle and ramus subequal in length, styliform. Telson slightly longer than wide, narrowing posteriorly, with two long simple setae widely separated.

Body , tergites of pleon 1 and 2 with dorsoposterior carina (Fig. 2A, B).

Mean body length of males: 5.7 ± 0.6 mm (n = 10).

Mean cephalothorax length: 0.6 ± 0.03 mm (n = 10).

Epimeral plates 1–3 acuminate.

Coxae 1–3 (Fig. 2A, B) subequal in shape, subrectangular, longer than wide; coxa 1 shorter than coxae 2 and 3; coxa 4 wider than coxae 1–3 with a deep posterior excavation; coxae 1–4 slightly overlapping anterior coxa, distal margin rounded with small setae widely separated, acumination absent. Coxae 5–7 reduced, shorter than coxae 5 and 6, bilobate except coxa 7; coxa 5 with two subequal lobes, posterior lobe slightly longer than anterior; coxa 6 with two unequal lobes, anterior lobe reduced; coxa 7 with a single lobe, anterior lobe absent.

Head typically gammaridean (Fig. 2A, B), smooth surface, length less than combined length of the first two thoracic segments, reaching the half of the second pereionite, rostrum absent. Eyes pigmented, medium, rounded, located between insertions of antennae 1 and 2.

Antenna 1 (Figs 2A, B, 3A) less than one-half the body length, shorter than antenna 2 (80% length of A2), but longer than peduncle of antenna 2, reaching more than one-half of the third pereionite; peduncle longer than head, reaching beyond half of the length of first pereionite, article 1 and 2 subequal in length, article 1 wider than articles 2 and 3, article 2 longer and wider than article 3, article 3 shorter and thinner than articles 1 and 2, proportions (1.5:1.4:1), article 1 close to the middle length of the ventral surface with two short cuspidate setae, one smaller, and one cluster with three cuspidate setae at distal end; flagellum with 9–11 articles reduced gradually toward the distal portion, flagellum longer than peduncle; aesthetasc on flagellum, present on articles 4–11, 4–7 (one pair), 8–11 (one aesthetasc). Accessory flagellum absent.

Antenna 2 (Figs 2A, B, 3B) almost 1.2× longer than antenna 1, slightly longer than one-third body length, reaching half of the fifth pereionite; peduncle reaching the second pereionite, peduncle articles increase gradually in length and decrease in width, article 3 shorter but wider than articles 4 and 5, article 4 a little longer than the length of article 3, article 5 slightly longer than article 4; flagellum with 11–12 articles, slightly longer than peduncle, almost 1.25× the length; without aesthetasc on flagellum.

Buccal parts : upper lip (Fig. 3E) with distal margin rounded and numerous setules present, longer and more distant toward the lateral end, two setae plus accessory setae near the distal margin on both sides (left and right) symmetrically.

Lower lip (Figs 3F, 8G), outer lobes without notches or excavations; mandibular projection of outer lobes rounded; without inner lobes.

Mandibles (Figs 3G, H, 8A, B) without palp, asymmetric. Incisor toothed, six to eight teeth present. Left lacinia mobilis similar to incisor process, with five or six teeth; setal row on left mandible with three or four main pappose setae plus accessory setae. Right mandible with six or seven teeth, lacinia mobilis reduced than the left one, with two pairs of asymmetrical, L-shaped teeth, fused at the base, proximal pair shorter than distal, inner margin denticulate; near the lacinia mobilis base, with one pair of setae; setal row with two main pappose setae plus accessory setae and with setulae near the molar process. Molar process large, cylindrical, and triturative; left mandible lateral view almost rectangular (Fig. 3G) and right mandible with a 60° angle (Fig. 3H); with accessory pappose setae present in both molars.

Maxilla 1 (Figs 3C, 8D, E) with short palp, vestigial, uniarticulate, longer than wide, distally pointed with one short and stout distal seta (Fig. 8D), palp length almost exceeds half of the distance between base of palp and base of seta on outer plate, but less than half the distance between base of palp and tip of seta on outer plate (Fig. 8E); inner plate slender, shorter than outer plate, with three or four pappose distal setae (two distal and one or two subdistal); outer plate with nine stout serrate setae (Fig. 8E).

Maxilla 2 (Figs 3D, 8C) with plates subequal in length, width, and shape; inner plate shorter and slender, with two or three pappose setae on mid-distal margin, and with seven shorter serrulate setae on distal margin; outer and inner plates with abundant setules.

Maxilliped (Figs 3I, 8F) with inner plate longer than outer plate; distal margin slightly convex, almost flat, in both plates; inner plate distal margin with three cuspidate setae of equal size and with plumose setae; outer plate, inner and distal margins with numerous simple setae. Palp composed of four articles subequal in the maximum length; first article with three simple setae at the inner distal end and one pair at the outer distal end, one on each side; second article with numerous simple setae on the inner margin and three on outer distal end; third article with several setae on distal margin, distal end on the outer margin with three simple setae and comb setae; fourth article ungiform, longer than nail, with comb setae, inner margin near the distal half with three setae and one near nail base on outer margin; nail reaching almost two-thirds of the fourth article with serration at distal half.

Gnathopod 1 (Figs 4A, 6A–C) subchelated, hammer shaped, shorter than gnathopod 2. Basis elongated, maximum length close to 3× longer than the maximum width; near to the half of posterior margin with one seta, distal end with two clusters of two setae. Ischium short, almost as long as wide, length almost same as maximum width of basis and maximum length of merus; distal posterior end with two clusters of two setae. Merus longer than wide, almost half of the length of ventral surface with comb scales; distal margin with four setae. Carpus longer than wide, longer and slightly wider than propodus, with strong short and wide posterior lobe forming a scoop-like structure open to the inside; lobe, inner surface with three serrate setae, external surface on the distal half of lobe, near to the margin, with comb scales, anterior distal end with three setae. Propodus 1.5× longer than wide; inner surface near to the distal margin with four serrate setae (three in a row); distal anterior end with two clusters of approximately five setae; distal anterior and posterior surfaces with comb scales; palm transverse, posterior distal end with a robust seta and cup for dactyl. Dactyl claw-like; nail present; anterior surface near to the proximal half with one plumose seta, with comb scales over the anterior surface.

Gnathopod 2 (Figs 4B, 6D, E) subchelated; palm slightly oblique. Basis elongate, more than 3× longer than wide; posterior margin with two long setae. Ischium short, subquadrate, shorter than merus. Merus short; distal end of posterior margin with eight simple setae; distal half of the posterior inner and outer surfaces with comb scales. Carpus shorter than propodus; anterodistal end with two setae; posterior lobe scoop-like, elongate, length similar to the merus maximum length, almost 1.5× the width of merus, with several submarginal pappose setae and comb scales. Propodus robust, almost 1.5× as long as wide, subrectangular; palm slightly shorter than posterior margin, slope slightly irregular, with some long simple setae, several short, and several medium setae; distal margin of palm with one truncated process and presence of one slightly posterior excavation at base, near to the insertion of dactyl; palm posterior distal end with two strong setae, comb scales and cup for dactyl. Dactyl claw-like, congruent with palm, without comb scales; outer margin proximal third with a plumose seta; inner margin crenulate.

Pereopods 3–7 (Figs 4E–I, 7D–I) simple, gradually longer posteriorly. Pereopod 5 shorter than pereopods 4 and 6.

Pereopod 3 (Figs 4E, 7D) with basis elongate; mid-posterior margin with two simple setae; anterodistal and posterodistal ends with simple setae. Ischium subquadrate; posterodistal end with one pair of setae. Merus longer than ischium (more than twice the length); posterior margin with three setae; anterior margin with one seta; anterodistal and posterodistal ends with one cluster of four setae. Carpus shorter and slenderer than merus; posterior margin with four stout setae; posterodistal end with at least five slender setae, longer than the ones from posterior margin; anterodistal end with at least two setae. Propodus almost as large as the posterior margin of merus, slenderer than carpus; posterior margin with eight setae; anterodistal end with three simple setae. Dactyl claw-like; nail present; first proximal third of the anterior margin with one plumose seta; posterior margin with one simple seta close to the nail.

Pereopod 4 (Figs 4F, 7E) similar in shape to pereopod 3 but slightly longer; coxa 4 wider than coxa 3, with a posterior excavation; basis posterior margin with one simple seta.

Pereopods 5–7 (Figs 4G–I, 7F–H) similar in shape; basis posterior lobe rounded and denticulate. Pereopod 7 (Fig. 7I) with basis lobe widely expanded, almost reaching ischium distal margin; wider than lobes of pereopods 5 (Figs 4G, 7F) and 6 (Figs 4H, 7G); width almost 0.75× width of basis (measured at cleft between basis and basis lobe); posterior margin with 14 serrations, each with one setule but one or two serrations with one stout seta in the distal margin; anterior margin with three stout setae and one at distal end.

Pleopods 1–3 (Fig. 5E) not modified, biramous, elongated, rami multi-annulated, with numerous plumose setae; inner margin of peduncle with two short retinacula (coupling hooks) at distal end.

Uropod 1 (Figs 5A, 8H) longer than uropod 2 (Fig. 8H); peduncle longer than rami, proximal half of the dorsal margin with three or four dorsal setae, inner and outer distal end with one seta; rami subequal, inner ramus slightly shorter, with two dorsal setae and four distal setae, outer ramus with three dorsal and three distal setae; male without curved setae on inner ramus.

Uropod 2 (Figs 5B, 8H) longer than peduncle of uropod 1; peduncle as long as rami, with two dorsal setae over the distal half and one at distal end; rami subequal, inner ramus with two dorsal and four distal setae, outer ramus with two or three dorsal and three distal setae.

Uropod 3 (Figs 5C, 8H–I) slightly shorter than peduncle of uropod 2; peduncle rectangular, wider than ramus with four strong distal setae of variable length, inner ramus absent, outer ramus uniarticulate slender, slightly shorter than peduncle, basal width near 3× the apex of ramus, with three or four slender apical setae and one connate seta.

Telson (Figs 5D, 8J) entire, slightly longer than wide, narrowing posteriorly, with two long simple setae widely separated; outer surface bearing two clusters of three plumose setae near the half distal portion, close to the margin, symmetrically distributed.

Coxal gills sac-like, present on segments 2–6 (Fig. 6D). Sternal gills tubular, present on segments 3–7.

Female (Fig. 2C). Similar to male. Gnathopod 1 (Figs 4C, D, 7A–C) with carpus with five setae on the inner face lobe; propodus with four setae in a row over the inner face. Gnathopod 2 smaller than male gnathopod 2, parachelated, palm reverse oblique; basis posterior margin with two setae; propodus slightly longer than twice its maximum width, outer face with three setae in a row and three large setae near the palm, anterior and posterior distal half with comb scales. Pereonite 2 with one anterior excavation or notch for the amplexus. Pereopod 7 lobe with 13 serrations and setules, and two stout setae on the distal margin. Oostegites subtriangular, with setae curled on the margins, reaching almost one-half length of merus (Fig. 4C).

Intraspecific variation: Maxilla 1, inner plate usually with four setae, the smaller adults could have three setae and the young ones two setae. Maxilla 1 palp, length quite variable during the molt process. Maxilla 2 inner plate with two or three setae, even in the same organism.

Freshwater, epigean.

La Ferrería, Durango, Tunal river (23°57.905'N, 104°39.817'W).

Hyalella tepehuana sp. nov. is the first species described from northern Mexico. It is easily distinguished from other species of Hyalella from the USA, Mexico, and the Caribbean by the atypical number of setae on the inner plate of maxilla 1 (four setae) and maxilla 2 (three setae), and by the shape of the telson. The species morphologically most similar to H. tepehuana sp. nov. is H. faxoni Stebbing, 1903 from the Volcan Barva in Costa Rica, but Hyalella tepehuana sp. nov. differs by the presence of dorsal processes on pereionites 1 and 2, and by the following characters: number of articles on antennae 1 and 2 (9–11 and 11 or 12, respectively, in H. tepehuana sp. nov. versus 12 and 15–17, respectively, in H. faxoni); number of setae on propodus inner face of gnathopod 1 (four in H. tepehuana sp. nov. versus five in H. faxoni); number of setae on the posterior basis of male gnathopod 2 (two in H. tepehuana sp. nov. versus four in H. faxoni); shape of uropod 3 (styliform in H. tepehuana sp. nov. versus globose in H. faxoni); and telson shape (longer than wide in H. tepehuana sp. nov. versus quadrate and wider than long in H. faxoni). Furthermore, in the new species, the fourth article (= dactyl) of maxilliped is more slender than in H. faxoni, according to González and Watling (2002b) (Table 1). Unlike H. faxoni, Hyalella tepehuana sp. nov. and H. azteca have a dorsal process on perionites 1 and 2. The differences between these last two species are considerable, based mainly on the morphology of palp of maxilla 1, the number of setae on inner plate of maxilla 1 and maxilla 2, the number of setae on uropods, the shape of the telson and the distance between the distal setae on the telson, according to the redescription by González and Watling (2002a) (Table 1). These differences seem sufficient to distinguish Hyalella tepehuana sp. nov. from H. azteca and other species from North America.

Due to the subtle variations within species, and lack of morphological studies and formal descriptions, the identification of species of Hyalella in North America is complex. Hence, the new characters proposed by Soucek et al. (2015) to distinguish species are useful: proportion of length of ramus uropod 3 versus the length of stout setae in peduncle, and proportion of gnathopod 2 merus width versus carpus lobe width. The ramus of uropod 3 in Hyalella tepehuana sp. nov. is larger than peduncle stout setae. The relative proportions of the merus width and carpus lobe of the ganthopod 2 of H. tepehuana sp. nov. are similar as H. spinicauda in Michigan and Wisconsin, USA, and some localities in Canada, and different from the proportions found in H. azteca (1.5×).

Hyalella tepehuana sp. nov. is also similar to the recently described H. wakulla Drumm & Knight-Gray, 2019, from Florida, USA. These two species have a similar body length, about 5.5 mm, by which they may be considered to be smaller ecomorphs; however, the main differences between these two species are the number of articles in antennae 1 and 2, and the number of setae on the buccal parts: Hyalella tepehuana sp. nov. has more articles in antenna 1 (9–11) and antenna 2 (11–12) while H. wakallua has fewer articles in antenna 1 (eight) and antenna 2 (nine). The new species also bears a maximum of four setae on the inner plate of maxilla 1 and bears setules on the palp, whereas, H. wakulla bears three setae on inner plate of maxilla 1, and the palp lacks setules. Also, H. tepehuana sp. nov. has a maximum of three pappose setae on the inner plate of maxilla 2, while H. wakulla has only two, and the maxilliped fourth article in H. tepehuana sp. nov. has fewer than four subterminal setae on the medial margin in contrast to H. wakulla, which has a maximum of four subterminal setae in adults. We consider the differences presented here sufficient to consider Hyalella tepehuana sp. nov. as a new species.