Holotype: male, Brazil, Rio Grande do Sul state, Roque Gonzales municipality, wetland, Ijuí watershed, Uruguay hydrographic region, (28°13'55.6"S, 54°58'37.3"W) (MNRJ 23384), allotype female (MNRJ 23385), July, 7, 2002, Stenert, C. coll.

MNRJ 23386 (5 males; 5 females; 5 juveniles), UFLA 0187 (10 males; 10 females; 10 juveniles) with the same data as the holotype.

Brazil, Rio Grande do Sul state: Roque Gonzales municipality, 28°13'55.6"S, 54°58'37.3"W, wetland, Ijuí watershed, Uruguay hydrographic region, ca 200 m high, July 7 2002, Stenert, C. coll.

Body surface smooth. Eyes absent. Antenna 1 longer than antenna 2, flagellum with 18–23 articles. Antenna 2 less than half body length, flagellum with 14-16 articles. Maxilliped with distal nail longer than dactylus. Gnathopod 1 propodus length less than twice maximum width, hammer shape, inner face with 7 pappose setae, without comb-scales. Gnathopod 2 carpus wider than long, posterior lobe elongated without comb-scales or denticles in the border; propodus ovate, without comb-scales, palm sub-equal to posterior margin, slope oblique, palm with two rows of several cuspidate setae with an accessory setae and simple setae. Peraeopod 5 smaller than others; peraeopod 6 and 7 much more longer than others. Uropod 1 inner ramus of male with a short curved seta, four cuspidate setae with an accessory seta apically, with one of them almost half length of the outer ramus. Uropod 3 shorter than telson, peduncle wider than ramus, with one cuspidate seta with an accessory seta distally. Telson wider than long, with two long simple apical setae. Sternal gills present on segments 3 to 7.

Mean body length: 5.03 ± 0.85 mm, mean head length: 0.46 ± 0.07 mm (n=10). Body surface smooth; epimeral plates not acuminate (Fig. 1A, Fig. 6A).

Head smaller than 2 thoracic segments, rostrum absent. Eyes absent (Fig. 6B).

Antenna 1 (Fig. 1B) longer than antenna 2, more than half body length; peduncle surpassing head length; flagellum with 18 to 23 articles; aesthetascs (Fig. 1C) ocurring in pairs distally on flagellum after article 5.

Antenna 2 (Fig. 1D) peduncle not surpassing the second pereionite, less than half body length, peduncle slender, longer than head; flagellum with 14 to 16 articles, longer than peduncle.

Mandible basic amphipodan (in the sense of Watling 1993), but without palp; incisor toothed; left lacinia mobilis with six teeth (Fig. 1E); seta row on left mandible with five main pappose setae plus accessory setae, right mandible with three main pappose setae plus accesory setae; molar large, cylindrical and triturative with setules around its circumference.

Upper lip (Fig. 1F) margin rounded; distal border covered by setules on dorsal and ventral faces.

Lower lip (Fig. 1G) outer lobes rounded, with setules on dorsal and ventral faces.

Maxilla 1 (Fig. 1H) palp uniarticulate, short, longer than wide, covered by several simple setae and reaching less than half length the distance between the base of palp and tip of setae on outer plate. Inner plate slender, shorter than outer plate, with two long papposerrate apical setae, several simple setae on inner margin; outer plate with 8–9 long serrate setae.

Maxilla 2 (Fig. 1I) inner plate shorter than outer plate, with two long papposerrate, eight serrulate and several simple apical setae; outer plate with abundant long simple setae; outer and inner plates with several setules.

Maxilliped (Fig. 4B) inner plate with three strong cuspidate setae apically, several pappose setae on apical and inner borders, inner plate recovered by abundant short setule; outer plate larger than inner plate, recovered by setule and with three pappose setae and several simple setae; palp longer than outer plate, four articles; article 1 wider than long, outer and inner faces with short simple setae; article 2 wider than long, inner face with several long simple setae; article 3 wider than long, outer and inner faces with several long simple setae and outer face with four pappose setae; dactylus unguiform recovered by short simple setae, shorter than third article, inner border with several simple setae; distal nail longer than dactylus.

Gnathopod 1 (Fig. 2A) subchelate; coxal plate wider than long, with simple setae on the border; basis, ischium and merus with serrate setae dorsally; carpus longer than wide, shorter than propodus, with lateral distal lobe produced and forming a scoop-like structure, border pectinate with several serrate setae, without denticles and comb-scales in their basis; border propodus width 3/4 of maximum length, hammer-shaped (Fig. 2B), without setae on anterior border, without comb-scales, inner face with 7 serrate setae, with simple setae on the disto-posterior border; palm slope transverse, margin slightly concave, posterior distal corner with two cuspidate setae with an accessory seta; dactylus claw-like without comb-scales, one plumose seta dorsally and few setae ventrally.

Gnathopod 2 (Fig. 2C) subchelate; basis hind margin with five groups of simple setae; merus with few setae on posterior margin; carpus wider than long, posterior lobe slim produced between merus and propodus, border pectinate with several short serrate setae, without denticles or comb-scales; propodus ovate (Fig. 2D), length 1.4 maximum width, without comb-scales; palm sub-equal than posterior margin of propodus, slope oblique, palm with two rows of several cuspidate setae with an accessory setae and simple setae, posterior distal corner with few simple setae and with a cup for dactylus; dactylus claw-like, congruent with palm, with few endal setae and a plumose seta dorsally, few setae ventrally, without comb-scales.

Peraeopods 3 (Fig. 3A) and 4 (Fig. 3B) merus and carpus posterior margin with clusters of simple setae; propodus posterior margin with six to seven groups of simple setae; dactylus less than half-length of propodus. Peraeopods 5 to 7 dactylus less than half-length of propodus; merus, carpus and propodus posterior margin with 4-5 marginal clusters of 2-9 cuspidate setae with an accessory setae. Peraeopod 3 sub-equal to peraeopod 4; peraeopod 5 (Fig. 3C) smaller than others; peraeopods 6 (Fig. 3D) and 7 (Fig. 3E) much longer than others. Coxal plates - peraeopod 3: longer than wide, width about half its length; peraeopod 4: wider than long; peraeopod 5: wider than long, with two lobes; peraeopod 6: ovate; peraeopod 7: wider than long. All coxal plates with simple setae on the border.

Pleopods (Fig. 4A) peduncle smaller than flagellum, without coupling spines; rami with several plumose setae; plumose setae of the last article longer 1.4 times than peduncle.

Uropod 1 (Fig. 4C) peduncle 1.7 times longer than rami; outer ramus longer than inner ramus; outer ramus with six cuspidate setae with an accessory seta, four cuspidate setae with an accessory seta apically, one smaller and three more longer, one of them with almost half length of the outer ramus; inner ramus with two dorsal cuspidate setae with an accessory seta on inner margin, male with a short curved seta apically on the ramus, five cuspidate setae with an accessory seta apically, three smaller and two more longer, one of them more than half of the length of the inner ramus; peduncle setation present.

Uropod 2 (Fig. 4D) shorter than uropod 1; ramus and peduncle of the same length; inner ramus with three dorsal setae and four distal setae, one more than half the length of the inner ramus; outer ramus with four dorsal setae and four distal setae, one more than half the length of the outer ramus; peduncle wider than ramus with four cuspidate setae with an accessory setae.

Uropod 3 (Fig. 4E) shorter than telson, shorter than peduncle of uropod 1 and uropod 2; inner ramus absent; outer ramus uniarticulate; peduncle longer than wide with one cuspidate seta with an accessory seta distally; ramus shorter than peduncle; basal width 1.8 times the width of ramus apex, with five cuspidate setae with an accesory seta and one long simple seta, longer than peduncle.

Telson (Fig. 4F) entire, apically rounded, more than 1.2 times wider than long, with two long simple apical setae; sometimes with plumose setae laterally.

Coxal gills sac-like present on pereonites 2 to 6. Sternal gills tubular present on pereonites 3 to 7.

Mean body length: 4.8 ± 0.43 mm, mean head length: 0.48 ± 0.03 mm (n=10) (Fig. 6C). Antenna 1 flagellum with 19 to 20 articles; antenna 2 similar in shape to male; flagellum with 15 to 16 articles.

Gnathopod 1 (Fig. 5A) similar in size and shape to gnathopod 2; without comb-scales; propodus (Fig. 5B) longer than wide; similar to male gnathopod 1 except that propodus is less narrow and shorter. Gnathopod 2 (Fig. 5C) different from male gnathopod 2 in shape and smaller; propodus (Fig. 5D) length 1.1 times maximum width, subchelate, inner face with five serrate setae, palm transverse, without comb-scales.

The specific name, imbya, honors the indigenous tribe Mbyá-Guarani that inhabited the local before the colonization of european immigrants.

Freshwater, hypothelminorheic.

The area where specimens of Hyalella imbya were collected was severely altered in the last recent years (Fig. 7). The area suffered during decades with agriculture, but the recent impacts were even more harmful. Such area is drained to a tributary stream which flows to Ijuí river (its margin was about 3.5 km far from the sampled area). In 2011 the riparian vegetation of this tributary was removed (Fig. 7B) and a reservoir was filled, flooding the deforested area (Fig. 7C). The phreatic level was altered since the distance between the sampled area and the nearest reservoir´s margin was reduced to about 2 km. In a visit maid on March 30, 2012 by two of the authors (S. G. Rodrigues and A. A. P. Bueno) to the same area no specimen was found. The area was completely dry and no spring was observed. It seems that changes in the hydrological parameters due to the building of São José reservoir altered the species habitat. Further considerations regarding such impacts will be discussed later.