A new species of Hyalella (Crustacea, Amphipoda, Dogielinotidae) from the Atlantic Forest of Misiones, Argentina

Abstract The freshwater genus Hyalella Smith, 1874 has a distribution restricted to the Western Hemisphere with most species being found in South America. In this report we describe a new species of Hyalella from the Atlantic Forest of the Misiones province, Argentina.


Introduction
The genus Hyalella includes approximately 70 valid species distributed in only the Americas (Baldinger 2004, WoRMS 2014. The hyalellids inhabit different freshwater environments, associated with either the bottom sediments (benthic fauna) or the aquatic vegetation (Poi de Neiff 1992), where these amphipods constitute a fundamental link in the transfer of matter and energy in those ecosystems (Casset et al. 2001, González et al. 2006, Castiglioni and Bond-Buckup 2008.
Currently, nine species of Hyalella have been recorded for Argentina: H. curvispina Shoemaker, 1942, H. fossamancinii Cavalieri, 1959, H. pampeana Cavalieri, 1968, H. neonoma Stock & Platvoet, 1991, H. falklandensis Bousfield, 1996, H. rionegrina Grosso & Peralta, 1999 Grosso & Peralta, 1999, H. kochi González & Watling, 2001, and H. bonariensis Bond-Buckup, Araujo & Santos, 2008(Santos et al. 2008, De los Ríos-Escalante et al. 2012. Although studies on the genus have increased in recent years, essential aspects of the taxonomy and ecology of Hyalella in Argentina still remain poorly known. The Atlantic Forest of South America -a species-rich and ecologically highly complex system -is considered one of the biodiversity "hot spots" of the world (Myers et al. 2000). In Argentina, the Atlantic Forest includes the province of Misiones, where part of the remaining forest biome is partially protected by the Yabotí Biosphere Reserve.
The aim of this work was to describe a new species of freshwater amphipod of the genus Hyalella from the Atlantic Forest in Misiones, Argentina.

Materials and methods
The Yabotí Biosphere Reserve is located in the eastern central region of the Misiones province (Fig. 1). The climate is hot and humid without dry season, with an annual mean precipitation of 2000 mm and an annual mean temperature of 21 °C (Cabrera 1971). Amphipods were collected by hand from the epilithic vegetation (bryophytes) growing on the rocks of the waterfall Salto Isipós, near Paraíso stream (27°13.19'S; 54°02.73'W).
In the laboratory, the cephalothorax length (CL) and total length (TL) of 30 specimens (15 males and 15 females) was measured under a stereoscopic microscope with a milimetric scale (LEICA EZ4). The anatomical pieces were placed in semipermanent slides and the drawings realized by means of a drawing tube mounted on a microscope ocular (LEICA DMLS). The terminology used for the setae of the appendices follows Zimmer et al. (2009). Morphological description is generalized from 10 individuals dissected (5 males and 5 females). We have mentioned the variations when appropiate.
Type material is deposited on Colección de Carcinología, División Zoología Invertebrados (DZI), Facultad de Ciencias Naturales y Museo (FCNyM), Universidad Nacional de La Plata (UNLP), Argentina. Diagnosis. 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. Description of male (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.

Taxonomy
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 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.
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.
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.
Remarks. 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  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, 1876and H. warmingi Stebbing, 1899(Gonzalez and Watling 2003, Gonzalez et al. 2006).