Hyalella gracilicornis (Faxon, 1876)

González, Exequiel R. & Watling, Les, 2003, A new species of Hyalella from Brazil (Crustacea: Amphipoda: Hyalellidae), with redescriptions of three other species in the genus, Journal of Natural History 37 (17), pp. 2045-2076 : 2046-2052

Hyalella gracilicornis (Faxon, 1876)

(figures 1–5)

Allorchestes dentatus var. gracilicornis Faxon, 1876: 374–375 , figure 36.

Hyalella gracilicornis: Barnard and Barnard, 1983 (not dentata, not inermis): 708.

Hyalella (Mesohyalella) gracilicornis: Bousfield, 1996: 192 .

Type material. Museum of Comparative Zoology , Harvard University .

Type locality. Campos , Rio de Janeiro, Brazil (21 ° 28 ∞ S, 41 ° 13 ∞ W).

Material examined. Represa Belvedere, Campus da UFV, Minas Gerais State, Brazil (18 ° 55 ∞ S, 48 ° 1 ∞ W). MNRJ, 9533.

Diagnosis. Body surface smooth. Coxa 4 excavated posteriorly. Eyes pigmented. Antenna 1 shorter than antenna 2. Antenna 2 more than half length of body. Mandible incisor toothed. Maxilla 1 palp longer than wide, reaching half of the distance between base of palp and tip of setae on outer plate; inner plate slender, with two strong and pappose apical setae. Maxilla 2 inner plate with one strong pappose seta on inner margin. Gnathopod 1 propodus length less than twice maximum width (quadrangular), hammer-shaped, inner face with four pappose setae, setose scales on disto-posterior and disto-anterior border. Gnathopod 2 propodus ovate, palm shorter than posterior margin, slope slightly transverse, anterior corner with wide truncated or rounded process. Peraeopods 3 and 4 merus and carpus posterior margin with three to four clusters of long setae; propodus posterior margin with two to four groups of setae. Uropod 3 peduncle with two strong distal setae; outer ramus longer than peduncle, basal width more than twice tip of ramus. Telson as wide as long, apically pointed, with two closely spaced, long simple setae, symmetrically distributed on apex. Sternal gills on peraeonites 2–7.

Female. Gnathopod 1 similar in size but different in shape to gnathopod 2. Gnathopod 2 different from male gnathopod 2 in shape and smaller, propodus length more than three times maximum width, normally subchelate, palm transverse.

Description of male. Size 5.9 mm. Body surface smooth (figure 1). Epimeral plates 1–3 acuminate. Coxae 1–4 subequal in size and shape, slightly overlapping. Acumination in coxae absent. Coxa 1 subequal to coxae 2 and 3. Coxa 3 narrower than coxa 4. Coxa 4 deeper than wide, excavated posteriorly. Coxa 5 posterior lobe deeper than anterior. Coxa 6 anterior lobe small.

Head smaller than first two thoracic segments, typically gammaridean, rostrum absent. Eyes pigmented, large, round, located behind insertion of antenna.

Antenna 1 less than half of body length, shorter than antenna 2, longer than peduncle of antenna 2; peduncle longer than head, article 1 longer than article 2, article 3 shortest; flagellum with 12 articles, longer than peduncle, basal article not elongated; asthetascs occurring distally on flagellum after article 5 (figure 2 A1).

Antenna 2 more than half body length; peduncle slender, longer than head, article 4 shorter than article 5, setal groups on article 4 and 5 scarce; flagellum of 19–25 articles, longer than article 5, basal article not elongated (figure 2 A2).

Basic amphipodan mandible (in the sense of Watling, 1993); incisor toothed; left lacinia mobilis with four teeth; setae row on left mandible with three main setae, without accessory setae, right mandible with two main setae plus accessory setae; molar large, cylindrical and triturative, accessory seta present. Labrum ventral margin round. Lower lip outer lobes rounded without notches or excavations, mandibular projection of outer lobes round (figure 2 Mr, Mls, Bl; figure 3 U, L).

Maxilla 1 palp uniarticulate, longer than wide, reaching half of the distance between base of palp and tip of setae on outer plate, distal setae feeble; inner plate slender, smaller than outer plate, with two strong and pappose apical setae; outer plate with nine stout and serrate setae (figure 4 X1r, X1l).

Maxilla 2 inner plate subequal in length and width to outer plate, one strong pappose seta on inner margin, outer and inner plates with scarce setules (figure 4 X2r).

Maxilliped inner plates apically truncated, with three connate setae, pappose setae apically and medially; outer plates larger than inner plates, apically truncated, apical, medial and facial setae simple; palp longer than outer plate, four articles; article 2 as wide as long, medial margin with long simple setae; article 3 outer distal face with few 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 third article, distal setae simple and as long or longer than nail, inner border with setae, distal nail present (figure 4 S).

Gnathopod 1 subchelate; carpus longer than wide, longer and wider than propodus, with strong and wide posterior lobe, produced and forming a scoop-like structure, open to the inside, inner face with one to three pappose setae, border pectinate and with several pappose setae; propodus length less than twice maximum width (quadrangular), hammer-shaped, with no setae on anterior border, inner face with four pappose setae, five to nine small triangular setae, setose scales on disto-posterior and disto-anterior border, palm slope transverse, margin concave, posterior distal corner with robust setae; dactylus claw-like, no endal setae, congruent with palm (figure 2 G1).

Gnathopod 2 subchelate; basis hind margin with one to three groups of setae; merus with fewer than seven setae on posterior margin, postero-distal margin concave, distal corner rounded; carpus posterior lobe elongated, produced between merus and propodus, border pectinate with several pappose setae; propodus ovate, scales on disto-posterior border, palm shorter than posterior margin, slope slightly transverse, margin irregular, with few strong short setae, few long setae, and several medium-sized setae, anterior corner with a wide, truncated or rounded process, posterior distal corner with strong setae, and with cup for dactyl; dactylus claw-like, congruent with palm, with a few endal setae (figure 2 G2).

Peraeopods 3–7 simple. Peraeopods 3 and 4 merus and carpus posterior margin with three to four hind marginal clusters of long setae; propodus posterior margin with two to four groups of setae; dactylus less than half length of propodus. Peraeopods 5–7 all similar in structure and slightly longer successively; dactylus less than half length of propodus. Peraeopod 5 subequal to peraeopod 4, basis posterior lobe as wide as deep, smaller than posterior lobe of peraeopod 7, merus with two hind marginal setae, proximal setae smaller than more distal setae. Peraeopod 6 longer than peraeopod 4, basis posterior lobe deeper than wide, smaller than posterior lobe of peraeopod 5, and smaller than posterior lobe of peraeopod 7. Peraeopod 7 subequal to peraeopod 6, basis posterior lobe wider than deep (figure 5 P3, P4, P5; figure 3 P6, P7).

Pleopods not modified; peduncle slender; longest ramus longer than peduncle.

Uropod 1 longer than uropod 2; peduncle subequal to rami; rami subequal; inner ramus with two dorsal and five distal setae, two of them longer, male without curved setae on inner side of the ramus; outer ramus with four dorsal and four distal setae; peduncle setation present (figure 5 R 1, R 1t).

Uropod 2 rami subequal; inner ramus with two dorsal and five distal setae; outer ramus with three dorsal and three distal setae; peduncle setation present (figure 3 R 2, R 2t).

Uropod 3 longer than urosomite 3, shorter than peduncle of uropod 1, as long as peduncle of uropod 2; peduncle slender (rectangular), same width as ramus, with two strong distal and one marginal setae; inner ramus absent; outer ramus uniarticulate, longer than peduncle, basal width more than twice tip of ramus, with one or two simple, apical, slender setae, and two connate setae (figure 5 R 3).

Telson as wide as long, entire, apically pointed, with two closely spaced, long, simple setae, symmetrically distributed on apical margin (figure 3 T).

Coxal gills sac-like, on peraeopods 2–6. Sternal gills tubular, on peraeonites 2–7.

Characters of female that differ from male. Size 4.3 mm. Antenna 1 flagellum with 10 articles. Antenna 2 similar in shape to male, flagellum with 15 articles. Gnathopod 1 similar in size, and different in shape to gnathopod 2; similar to male gnathopod 1 in size and shape. Gnathopod 2 different from male gnathopod 2 in shape and smaller, propodus length more than three times maximum width, normally subchelate, palm transverse (figure 4 fG1, fG2).

Habitat. Freshwater, epigean, littoral.

Distribution. Represa Belvefere, Campus da UFV, Minas Gerais State; Campos, Rio de Janeiro State.

Remarks. The original description of this species was based on a single female specimen. The holotype is in good shape, but uropod 3 is missing. However, the general morphology, the characteristic length of antenna 2, and the comparison with the other species of the area justified a redescription of H. gracilicornis . The material analysed here from the Museu Nacional, Universidade Federal do Rio de Janeiro, was labelled as H. pernix , but it did not agree with Moreira’s (1903) or Pereira’s (1983, 1985) description of H. pernix . This species was considered by Barnard and Barnard (1983) to have a wide distribution, including Lake Titicaca and Chile. The specimens analysed and the present knowledge of the Chilean species do not support this view (Gonzalez and Watling, 2001). The distribution appears to be confined to the type locality and its area of influence. Hyalella gracilicornis differs from H. longistila in the shape of gnathopod 2 in the male, the relative size of antennae and the length of uropod 3. It also differs from H. warmingi in the shape of gnathopod 2 in both sexes and the structure of uropod 3. The figure given by Faxon (1876) depicts a small round eye, which is absent in the type material.