Holotype male, Argentina, Province of Misiones, Yabotí Biosphere Reserve, San Pedro and Guaraní Departments, Salto Isipós (27°13.19'S; 54°02.73'W) (MLP 26978), October, 19, 2011. César, I. I. and Martín, S. M., collectors.

15 males, 15 females, 7 ovigerous females, and 70 juveniles (MLP 26979), same data as holotype.

Argentina, Province of Misiones, Yabotí Biosphere Reserve, San Pedro and Guaraní Departments, Salto Isipós (27°13.19'S; 54°02.73'W).

Body surface smooth. Coxa 4 excavated posteriorly. Eyes pigmented. Antenna 1 shorter than antenna 2. Antenna 2 less than half the body length.Maxilla 1 palp short, reaching to less than half the distance between base of palp and tip of setae on outer plate; inner plate slender, with two strong, pappose apical setae. Maxilla 2 with two strong pappose setae on inner margin. Gnathopod 1 propodus length less than twice maximum width, hammer-shaped, inner face with six to nine serrate setae, comb scales on distoposterior border. Gnathopod 2 propodus ovate, palm shorter than posterior margin, distal margin of palm irregular. Pereiopods 3 and 4 merus and carpus posterior margin with three hind marginal clusters of short setae; propodus posterior margin with five groups of setae. Uropod 3, peduncle slender (rectangular), wider than ramus, with five strong distal setae, basal width more than twice apex of ramus. Telson as long as wide, entire, apically rounded, bearing two long simple setae symmetrically distributed on distal margin, and three small submarginal setae close to each main setae. Sternal gills present on segments 2 to 7.

(Figs 2 to 5). Mean body length: 5.9 ± 1.09 mm; mean cephalothorax length: 0.61 ± 0.08 mm (n = 15). Body surface smooth. Epimeral plate 1, 2, and 3 acuminate. Coxae 1 to 4 subequal in size and shape, slightly overlapping. Acumination in coxae absent. Coxa 1 similar to 2 and 3. Coxa 3 narrower than 4. Coxa 4 as wide as deep, excavated posteriorly. Coxa 5 posterior lobe deeper than anterior lobe. Coxa 6 posterior lobe deeper than anterior lobe, anterior lobe small.

Head typically gammaridean, as long as the first two thoracic segments, rostrum absent. Eyes pigmented, medium, rounded, located between insertion of antenna 1 and antenna 2 (Fig. 2A).

Antenna 1 (Fig. 2B) less than half the body length, shorter than antenna 2 but longer than peduncle of antenna 2; peduncle as long as head; article 3 shorter than article 1 and article 2 in length; flagellum of 10–11 articles, longer than peduncle; aesthetascs on flagellum, present on articles 4 (2 aesthetascs), 5 (2), 6 (3), 7 (1), and 8 (1).

Antenna 2 (Fig. 2C) less than half of the body length, peduncle longer than head, article 4 shorter than article 5, setal groups on articles 4 and 5 scarce, flagellum with 13–14 articles and longer than peduncle.

Mandible without palp; incisor toothed; left lacinia mobilis with five teeth; setal row on left mandible with three main pappose setae plus accessory setae (Fig. 2D), right mandible with two main pappose setae plus accessory setae; molar large, cylindrical, and triturative; accessory seta present (Fig. 2E).

Upper-lip ventral margin round (Fig. 2F).

Lower-lip outer lobes rounded without notches or excavations, mandibular projection of outer lobes truncated (Fig. 2G).

Maxilla 1 (Fig. 2H) palp short, uniarticulate, reaching to less than half the distance between base of palp and tip of setae on outer plate, distally pointed; inner plate slender, smaller than outer plate, with two strong, pappose apical setae; outer plate with nine stout and serrate setae.

Maxilla 2 (Fig. 2I) inner plate subequal in length and width to outer plate, with two strong pappose setae on mid-inner margin; outer and inner plates with abundant setules.

Maxilliped (Fig. 3A) inner plates apically truncated, with three connate setae and pappose setae apically and medially; outer plates larger than inner plates, apically truncated, apical, medial, and facial setae simple. Palp of four articles: article 2 wider than long, medial margin with long simple setae; article 3 outer distal face (at the base of article 4) with several long simple setae, inner distal face with long plumose setae, inner distal margin with long setae, outer margin with one or two strong and long plumose setae; dactylus unguiform, shorter than article 3, distal setae simple and shorter than nail, inner margin with setae, distal nail present.

Gnathopod 1 (Fig. 3B and C) subchelate; carpus longer than wide, longer and wider than propodus, with strong and wide posterior lobe, and forming a scoop-like structure, open to the inside, inner face with five serrate setae; propodus (Fig. 3D) length less than two times maximum width, hammer-shaped, with no setae on anterior border, with three simple setae on posterior border; inner face (Fig. 3E) with six to nine serrate setae, several small triangular setae, comb scales on distoposterior border, palm slope transverse, margin convex, posterior distal corner with robust setae, dactylus claw-like with comb scales.

Gnathopod 2 (Fig. 4A) subchelate; basis hind margin with two long setae; merus with less than seven setae on posterior margin, posterodistal margin straight, distal corner rounded; carpus posterior lobe elongated, produced between merus and propodus, distal end of carpal lobe with cuticular denticles and with several serrate setae; propodus ovate, distoposterior border with comb scales, palm (Fig. 4B) shorter than posterior margin, slope oblique, margin irregular, bearing several strong short setae, anterior edge with a wide truncated or rounded process, posterior distal corner with strong setae and with cup for dactylus; dactylus claw-like, as long as palm, with seven short simple setae symmetrically distributed on inner border. Triangular space between propodus and dactylus when dactylus is closed.

Pereiopods 3 to 7 simple. Pereiopods 3 (Fig. 4C) and 4 (Fig. 4D) with merus and carpus posterior margin having three hind marginal clusters of short setae; propodus posterior margin with five groups of setae; dactylus less than half the propodus length. Pereiopods 5 to 7, all similar in structure and successively slightly longer; dactylus less than half the propodus length. Pereiopod 5 (Fig. 4E) longer than pereiopod 4, basis posterior lobe longer than wide, smaller than posterior lobe of pereiopod 7. Pereiopod 6 (Fig. 4F) longer than pereiopod 4, basis posterior lobe longer than wide, larger than posterior lobe of pereiopod 5 and smaller than posterior lobe of pereiopod 7. Pereiopod 7 (Fig. 4G) slightly longer than pereiopod 6, basis posterior lobe longer than wide.

Pleopods (Fig. 5A) not modified; peduncle slender; longest ramus longer than peduncle.

Uropod 1 (Fig. 5B) longer than uropod 2; peduncle longer than rami, with 4-5 dorsal setae; rami subequal; inner ramus with 2–3 dorsal setae and 5 distal setae; male without curved setae on inner side of the ramus; outer ramus with 3–4 dorsal setae and 5 distal setae.

Uropod 2 (Fig. 5C) peduncle as long as rami, with 3 dorsal setae; rami subequal; inner ramus with 3 dorsal setae and 6 distal setae, outer ramus with 4 dorsal and 4 distal setae.

Uropod 3 (Fig. 5D) as long as peduncle of uropod 2; peduncle slender (rectangular), wider than ramus, with 5 strong distal setae of variable length, inner ramus absent; outer ramus uniarticulate, as long as peduncle, basal width more than twice apex of ramus, with 4–5 simple slender apical setae and one connate seta.

Telson (Fig. 5E) as long as wide, entire, apically rounded, bearing two long simple setae symmetrically distributed on distal margin, and three small setae close to each main seta.

Coxal gills sac-like, present on segments 2 to 6. Sternal gills tubular, present on segments 2 to 7.

(Fig. 6). Mean total length: 4.52 ± 0.71 mm; mean cephalothorax length: 0.51 ± 0.07 mm (n = 15). Antenna 1 (Fig. 6A) flagellum of 9–10 articles. Antenna 2 (Fig. 6B) similar in length and shape to male, flagellum of 15 articles.

Gnathopod 1 (Fig. 6C and D) similar in size and shape to gnathopod 2. Propodus inner face with six serrate setae (Fig. 6E). Gnathopod 2 (Figs 6F and G) propodus of length less than two times maximum width, normally subchelate, palm transverse. Propodus inner face with two serrate setae.

Freshwater, epigean.

Argentina, Province of Misiones, Yabotí Biosphere Reserve, San Pedro and Guaraní Departments, Salto Isipós (27°13.19'S; 54°02.73'W).

The species name misionensis refers to the location where this new member was found, the Province of Misiones in Argentina.

H. misionensis has some morphological similarities to H. pampeana Cavalieri, 1968, a common freshwater amphipod of the Province of Buenos Aires. The principal similarity is the shape of gnathopod 2 in the males, with a triangular space between the propodus and the dactylus in both species; the number of antennal segments (H. misionensis, antenna 1: 10–11 articles and antenna 2: 13–14 articles; H. pampeana, antenna 1: 11–12 articles and antenna 2: up to 18); and the total length (H. misionensis: 5.9 mm, H. pampeana: 5 mm). Although, the two species differ in the presence of a curved seta in the inner ramus of uropod 1 in the males of H. pampeana; this seta is absent in H. misionensis. The width of the propodus of gnathopod 1 is about 3/4 of its length in H. pampeana, but about 2/3 of its length in H. misionensis. In addition, the inner face of propodus in gnathopod 1 of H. pampeana has 5 to 6 pappose setae, but in H. misionensis has 6 to 9 serrate setae. The setation of the telson is also different with 2 to 5 setae of variable length in H. pampeana and only two simple setae in H. misionensis. Comparisons between morphological characters of Hyalella species of Argentina and Falkland Islands are presented in Table 1.

The study area where Hyalella misionensis was found is geographically close to Brazil, where fourteen species of the genus have been reported (Bueno et al. 2013). In Table 2 we compared the main morphological characters of H. misionensis with those of the Brazilian Hyalella species that were geographically close to where the newly described species was found: H. castroi Gonzalez, Bond Buckup & Araujo, 2006, H. pleoacuta Gonzalez, Bond Buckup & Araujo, 2006, H. gracilicornis Faxon, 1876, H. longistila Faxon, 1876 and H. warmingi Stebbing, 1899 (Gonzalez and Watling 2003, Gonzalez et al. 2006).