Hyalella parva Marrón-Becerra & Hermoso-Salazar 2023, n. sp.

Hyalella parva n. sp. Figures 2–4 Etymology. The specific epithet parva refers to the small size in the species. Material examined: Holotype male, size 3.35 mm (head size 0.38 mm) (Cat. No. CNCR 36163), from lake Nahá, Chiapas, Mexico (16°59’24.1”N, 91°35’56.7”W), 832 m asl, dissolved O 2 4.9 mg L- 1, pH 7.69, Temperature 20.6 °C, 17 March 2017. Collectors: F. Charqueño-Celis, L. (n = 1 male, n = 1 female): paratype female size 3.5 mm (head size 0.34 mm) (Cat. No. CNCR 36164), permanent slides CNCR (36165), locality same as holotype. Type locality: Nahá, Lacandona, Chiapas, Mexico (16°59’24.1”N, 91°35’56.7”W). Habitat: Freshwater epigean, collected in filamentous algae. Diagnosis: Pleonites 1 and 2 without dorsoposterior carina. Coxa 4 excavated posteriorly. Eyes pigmented, black. Antenna 1 shorter than antenna 2 without accessory flagellum. Antenna 2 less than one-half of body length. Maxilla 1 palp short, reaching less than half of the length between the base of palp and tip of setae of outer plate, with one short stout distal seta; inner plate slender with two strong pappose apical setae. Maxilla 2 inner plate with two strong pappose setae on inner margin. Gnathopod 1, propodus hammer-shaped, palm with a transverse slope, inner face with two pappose setae in a one row plus one small seta, comb scales on distoposterior and distoanterior border. Gnathopod 2 basis hind margin with one long and slender seta, palm without posterior excavation at base of truncated process (angle step). Uropod 1 without curved setae. Uropod 3, peduncle longer than ramus, styliform. Telson longer than wide, distal margin rounded with two long simple setae widely separated. Description of male (holotype, CNCR 36163). Size 3.35 mm (head size 0.38 mm). Body, tergites of pleon 1 and 2 without dorsoposterior carina (Fig. 2A). Epimeral plates 1–3 acuminate. Coxae 1–3 (Fig. 2A) subequal in shape, subrectangular, longer than wide; coxa 1 shorter than coxae 2–3. Coxa 4 wider than coxae 1–3 with 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 1–4, bilobate except coxa 7, lobe of basis overlapping posterior lobe of coxa, except coxa 7; coxa 5 with two subequal lobes, anterior lobe slightly shorter than posterior, overlapping anterior and posterior coxae; coxa 6 with two unequal lobes, anterior lobe reduced; coxa 7 with one single lobe, anterior lobe absent. Head typically gammaridean (Fig. 2A), with smooth surface, total length less than the combined length of the first two pereonites, reaching to two-thirds of first pereonite, almost reaching second pereonite, rostrum absent. Eyes pigmented, medium, subtriangular, and black, located between the insertions of antennae 1 and 2. Antenna 1 (Figs. 2A–B) less than half the body length, shorter than antenna 2 (85% length of antenna 2), but longer than peduncle of antenna 2, almost reaching to two-thirds of third pereonite; peduncle as long as head, reaching first pereonite, article 1 and 2 subequal in length, article 1 wider than articles 2–3, article 2 longer and wider than article 3, article 3 shorter and thinner than articles 1–2 (proportions 1.75:1.3:1), article 1 almost at half of ventral surface with one short cuspidate seta and one cluster with two cuspidate setae at distal end; flagellum with seven articles reduced gradually toward distal portion, flagellum longer than peduncle; one pair of aesthetascs on articles 4–6 (one pair). Accessory flagellum absent. Antenna 2 (Figs. 2A, C) almost 1.2 times longer than antenna 1, shorter than one-third the body length, slightly overreaching the fourth pereonite; peduncle almost reaching the middle of first pereonite, peduncle articles gradually increasing in length and decreasing in width, article 3 shorter but wider than 4 and 5, article 4 longer than article 3, article 5 longer than article 4 (almost 1.5 times); flagellum with five articles, length subequal than peduncle; flagellum without aesthetascs. Mouthparts: upper lip (Fig. 2D) distal margin rounded and numerous setules present, being longer and more distant towards lateral end, without bifid setae. Lower lip (Fig. 2E) subquadrate, outer lobes without notches or excavations; mandibular projection of outer lobes rounded; without inner lobes. Left and right mandibles (Figs. 2F–G) without palp, asymmetric. Incisor with six teeth. Left lacinia mobilis similar to incisor process, with five teeth; setal row on left mandible with three main pappose setae plus accessory setae. Right lacinia mobilis more reduced than left, with two pairs of asymmetric L-shaped teeth, fused at the base, proximal pair longer to distal, inner margin denticulate; near lacinia mobilis base with setules on inner margin; setal row on right mandible with two main pappose setae plus accessory setae and with setules near molar process. Molar process large, cylindrical, and triturative, with few setules at base; left mandible, molar lateral view almost rectangular and right mandible molar angled 60°; bot molars with accessory pappose setae. Maxilla 1 (Fig. 2H), palp short, vestigial, uni-articulated, longer than wide, distally pointed with one short stout distal seta, reaching less than half of palp length, setules present; palp length almost reaching one-third of distance between base of palp and base of seta on outer plate, but less than half distance between base of palp and tip of seta on outer plate (Fig. 2H); inner plate slender, shorter than outer plate, with two pappose apical setae; outer plate with nine apical stout serrate setae and with setules. Maxilla 2 (Fig. 2I), plates subequal in length, width, and shape; inner plate shorter and slightly slender, with two pappose setae on mid-distal margin, and with four long serrulate setae on distal margin; outer and inner plates with large simple distal setae (type A2) and few setules on both distal surfaces and margins. Maxilliped (Fig. 2J), inner plate shorter 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, inner margin with several pappose setae; outer plate, inner and distal margins with numerous simple setae. Palp composed of four articles subequal in length; first article with two simple setae at inner distal end and one seta at outer distal end; second article with numerous simple setae on inner margin and three at outer distal end; third article with several setae on distal margin, distal end on outer margin with six simple setae, in two clusters of three, and comb scales, inner margin with at least eight setae and six at distal end; fourth article unguiform, slightly longer than nail, with comb scales, inner margin with three setae near distal half, outer margin with one seta near nail base; nail overreaching two-thirds of fourth article in length, with serration on distal half. Gnathopod 1 (Fig. 3A) subchelate, propodus hammer-shaped, shorter than gnathopod 2. Basis elongated, maximum length almost 3.5 times longer than maximum width; posterior margin with one medial seta, posterior distal end with one cluster of two setae. Ischium short, subquadrate, longer than wide, length longer than maximum width of basis and as long as maximum length of merus; distal posterior end with one cluster of three setae. Merus longer than wide, almost at midway of the posterior margin with comb scales; almost at distal margin with three setae. Carpus longer than wide, slightly longer and slightly wider than propodus; with strong, short, and wide posterior lobe, forming one scoop-like structure open to inside; inner surface of lobe with one serrate seta, external surface on distal half of lobe, near margin, with comb scales and serrate setae, anterior distal end with four setae. Propodus maximum length almost 1.5 times longer than wide; inner surface near distal margin with two serrate setae in one row and one extra seta; anterior distal end with two clusters of approximately three setae; distal half of anterior and posterior surfaces with comb scales; palm transverse, posterior distal end with one robust seta and cup for dactyl. Dactyl claw-like; nail present; anterior surface near proximal third with one plumose seta, with comb scales on anterior surface. Gnathopod 2 (Fig. 3B) subchelate. Basis elongate, more than 3.5 times (3.7 times) longer than wide; posterior margin with one long and slender seta. Ischium short, subquadrate, shorter than merus. Merus short; distal end of posterior margin with three simple setae, medial setae absent; less of distal half of posterior inner and outer surfaces with comb scales. Carpus shorter than propodus, length similar to merus maximum length, slightly exceeds 1.5 times width of merus; anterior distal end with two setae; posterior lobe scoop-like, elongate, almost 1.5 times width of merus, with several submarginal pappose setae and comb scales. Propodus robust, ovate, length 1.5 times maximum wide; palm oblique, shorter than posterior margin; slope slightly irregular, with few long, short, and medium simple setae; distal margin of palm with one truncated process near insertion of dactyl and without posterior excavation at base (angle step); palm posterior distal end with two strong setae, comb scales, and cup for dactyl; cup for dactyl 0.5 times length between cup and truncated process. Dactyl claw-like, congruent with palm, occupying half of cup for dactyl, without comb scales; outer margin proximal third with one plumose seta; inner margin slightly crenulated with separated setules. Pereopods 3–7 (Figs. 3C–G) simple, gradually longer posteriorly. Pereopod 5 shorter than 4 and 6. Pereopod 3 (Fig. 3C), basis elongate; posterior margin with one medial simple seta; anterodistal and posterodistal ends with one simple seta. Ischium subquadrate; posterodistal end with one pair of setae. Merus longer than ischium (almost 2.5 times the maximum length); anterior margin without robust seta; posterior margin with three slender simple setae; anterodistal and posterodistal ends with one cluster of 2–3 slender setae. Carpus shorter and slenderer than merus; posterior margin with one cluster of two stout setae plus accessory seta; posterodistal end with at least two slender setae, slender than ones from posterior margin; anterodistal end with at least two setae. Propodus shorter than anterior margin of merus, slenderer than carpus; posterior margin with four stout setae plus accessory seta; anterodistal end with three simple setae. Dactyl claw-like, length more than half of propodus; nail present; first proximal third of anterior margin with one plumose seta; anterior and posterior margins with one simple seta near base of nail. Pereopod 4 (Fig. 3D) similar in shape to pereopod 3 but slightly longer; coxa 4 wider than coxa 3 with posterior excavation; basis posterior margin with one simple seta; propodus posterior margin with six setae. Pereopods 5–7 (Figs. 3E–G) similar in shape; basis posterior lobe rounded and denticulated. Pereopod 7 (Fig. 3G) basis lobe widely expanded posteriorly, overreaching ischium distal margin; wider than lobes of pereopods 5 and 6; basis lobe, width almost 2 times as width as basis (measured at cleft between basis and basis lobe); posterior margin with five serrations, each with one setule, and without stout setae on distal margin; anterior margin with three cluster of two stout setae and one cluster at distal end. Dactyl length less than half of propodus. Pleopods 1–3 (Fig. 4A) not modified, biramous, elongated, rami multi-annulated, with numerous plumose setae; pleopod 1, inner margin of peduncle at distal end with two short retinacula (coupling hooks). Uropod 1 (Fig. 4B) longer than uropod 2 (Fig. 4C); peduncle longer than rami, proximal half of dorsal margin with two dorsal setae, inner and outer distal ends with one seta; rami subequal, inner ramus slightly longer, with two dorsal and four distal setae, outer ramus with three dorsal and four distal setae; male without curved seta on inner ramus. Uropod 2 (Fig. 4C) slightly shorter than peduncle of uropod 1; peduncle slightly shorter than rami, with one dorsal seta on distal half and one at distal end; rami subequal, inner ramus slightly longer with two dorsal and five distal setae, outer ramus with two dorsal and four distal setae. Uropod 3 (Fig. 4D) slightly shorter than peduncle of uropod 2; peduncle subrectangular, wider than ramus with two robust distal setae of variable length; inner ramus absent, outer ramus uniarticulate, slender, shorter than peduncle, 2.5 times the length of the larger seta of peduncle, basal width near 2.5 times as wide as the apex width of ramus, with two slender apical setae and one connate seta. Telson (Fig. 4E) entire, longer than wide, distal margin rounded with two long simple setae widely separated; dorsal surface with two clusters of three plumose setae near middle distal portion, near margin, symmetrically distributed. Coxal gills sac-like, present on segments 2–6. Sternal gills tubular, present on segments 3–7. Female (paratype, CNCR 36164). Size 3.5 mm (head size 0.34 mm). Similar to male. Antenna 1, flagellum with six articles. Antenna 2, flagellum with seven articles. Gnathopod 1 (Fig. 3H), carpus with one seta on inner face lobe; propodus with one seta on inner face, and without medial seta on posterior margin. Gnathopod 2 (Fig. 3I) smaller than male gnathopod 2, parachelated; palm reverse oblique, basis posterior margin with one seta; propodus longer than twice maximum width (2.4 times); outer face with 1–2 setae in one row and one large seta near palm; anterior and posterior distal half surface with comb scales, and one medial seta on posterior margin. Pereonite 2 with one anterior excavation or notch for amplexus. Pereopod 7 lobe with 10 serrations and setules, with two stout setae on distal margin plus two long setules. Oostegites subtriangular, with curled setae on margins, almost reaching one-half of the length of merus. With five eggs.Published as part of Marrón-Becerra, Aurora & Hermoso-Salazar, Margarita, 2023, Description of three new species of Hyalella Smith, 1874 (Crustacea: Amphipoda) from Southeast Mexico, pp. 71-93 in Zootaxa 5323 (1) on pages 73-78, DOI: 10.11646/zootaxa.5323.1.4, http://zenodo.org/record/820395