Hyalella longistila (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 : 2053-2060

Hyalella longistila (Faxon, 1876)

(figures 6–11)

Allorchestes longistilus Faxon, 1876: 375 , figure 37.

Hyalella longistila: Stebbing, 1906: 577–578 .

Hyalella longistilus: Barnard and Barnard, 1983: 708 .

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

Type material. Museum of Comparative Zoology , Harvard University .

Type locality. Swamp 3 miles from Campos, Rio de Janeiro, Brazil (21 ° 28 ∞ S, 41 ° 13 ∞ W).

Material examined. Only material from the type locality.

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 minute, reaching less than 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 twice or more maximum width (rectangular), hammer-shaped, inner face with five pappose setae, disto-posterior and disto-anterior borders without setose scales. Gnathopod 2 propodus rectangular, elongated, palm as long as posterior margin, slope oblique, anterior edge with wide truncated or rounded process. Peraeopods 3 and 4 merus and carpus posterior margin with four marginal clusters of long setae; propodus posterior margin with five to eight groups of setae. Uropod 3 peduncle with three strong distal setae; outer ramus longer than peduncle, basal width twice or less that of tip of ramus. Telson longer than wide, apically truncated, with two widely apart, long simple setae, symmetrically distributed, occasionally with shorter setae close to the main setae. Sternal gills on segments 2–7.

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

Description of male. Size 5.9 mm. Body surface smooth (figure 6). Epimeral plates 1–3 acuminate (figure 7 E). 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, medium-sized, oval, located behind insertion of antenna 1.

Antenna 1 half as long as body, shorter than antenna 2, much longer than peduncle of antenna 2; peduncle longer than head, articles 1 and 2 subequal in length, article 3 the shortest; flagellum with 13 articles, longer than peduncle, basal article not elongated (figure 8 A1).

Antenna 2 reaching more than half of body length; peduncle slender, longer than head, article 4 shorter than article 5, setal groups on article 4 and 5 scarce; flagellum with 18 articles, longer than article 5, basal article not elongated (figure 8 A2)

Basic amphipodan mandible (in the sense of Watling, 1993); incisor toothed; left lacinia mobilis with five teeth; setae row on left mandible with three main setae plus accessory setae, right mandible with two main setae plus accessory setae; molar large, cylindrical and triturative, accessory seta present. Labrum ventral margin slightly rounded to truncate (figure 8 U, Ml). Lower lip outer lobes rounded without notches or excavations, mandibular projection of outer lobes round.

Maxilla 1 palp uniarticulate, minute, reaching less than half of the distance between base of palp and tip of setae on outer plate, distal setae strong; inner plate slender, smaller than outer plate, with two strong and pappose apical setae; outer plate with nine stout and serrate setae. 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 8 X1l; figure 9 X2r).

Maxilliped inner plates apically truncated, with three connate setae, pappose setae apically and medially; outer plates larger than inner plates, apically rounded, apical, medial and facial setae simple; palp longer than outer plate, four articles; article 2 longer than wide, 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, subequal in length to third article, distal setae simple and as long or longer than nail, inner border with setae, distal nail present (figure 9 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 medial side, medial surface with one to three pappose setae, border pectinate and with several pappose setae; propodus length twice or more maximum width (rectangular), hammer-shaped, with no setae on anterior border, medial surface with five pappose setae, one to five small, triangular setae; distoposterior and disto-anterior borders without setose scales; palm slope transverse, margin convex, posterior distal corner with robust setae; dactylus claw-like, no endal setae, congruent with palm (figure 8 G1).

Gnathopod 2 subchelate; basis hind margin with one to three groups of setae; merus with less 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 rectangular, elongated, scales on disto-posterior border, palm as long as posterior margin, slope oblique; palmar margin irregular, bearing several strong short setae, few long setae and several medium-sized setae; anterior edge with a wide, truncated or rounded process, posterior distal corner with weak setae, and with cup for dactyl; dactylus claw-like, congruent with palm, no endal setae (figure 9 G2).

Peraeopods 3–7 simple. Peraeopods 3 and 4 merus and carpus posterior margin with four clusters of long setae; propodus posterior margin with five to eight groups of setae; dactylus less than half as long as propodus. Peraeopods 5–7 all similar in structure and slightly longer successively; dactylus less than half as long as propodus. Peraeopod 5 subequal to peraeopod 4, basis posterior lobe deeper than long, smaller than posterior lobe of peraeopod 7; merus with two marginal setae posteriorly, proximal and distal setae subequal. Peraeopod 6 longer than peraeopod 4, basis posterior lobe deeper than long, similar to posterior lobe of peraeopod 5, and smaller than posterior lobe of peraeopod 7. Peraeopod 7 subequal to peraeopod 6, basis posterior lobe longer than deep (figure 10 P3, P4, P5; figure 7 P6, P7).

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

Uropod 1 longer than uropod 2; peduncle longer than rami; rami subequal; inner ramus with two dorsal and five distal setae, of which two are longer; male without curved setae on medial side of ramus; outer ramus with more than five dorsal setae and four distal setae; peduncle setation present (figure 9 R 1).

Uropod 2 outer ramus shortened; inner ramus with two dorsal and four distal setae; outer ramus with three dorsal and three distal setae; peduncle setation present (figure 9 R 2).

Uropod 3 longer than urosomite 3, shorter than peduncle of uropod 1, longer than peduncle of uropod 2; peduncle slender (rectangular), slightly wider than ramus, with three strong distal and no marginal setae; inner ramus absent; outer ramus uniarticulate, longer than peduncle, proximally twice as wide or less than distally, with five simple apical slender setae, and one connate seta (figure 7 R 3).

Telson longer than wide, entire, apically truncated, with two widely spaced, long simple setae, symmetrically distributed on the apical margin, occasionally accompanied by shorter setae (figure 7 T).

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

Characters of female that differ from male. Size 6.2 mm. Antenna 1 flagellum with more than 10 articles. Antenna 2 flagellum with 11–14 articles. Gnathopod 1 similar in size, but 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 more than three times as long as maximum width, normally subchelate, palm transverse (figure 11 fG1, fG2).

Habitat. Freshwater, epigean, littoral.

Remarks. This species, poorly described by Faxon (1876), was for some time thought to be synonymous with H. warmingi or H. gracilicornis . However, detailed study of the type material indicates that this is a valid species. The characteristic elongation of many of the appendages makes it very distinct from the other species in the area. Uropod 3 is the longest in the genus, and the elongated propodus of gnathopod 2 in male and female is also characteristic. Here we designate, from the syntype series, the figured material as the lectotype (male, 5.9 mm) for the species. The material is deposited in the Museum of Comparative Zoology, Harvard University. The lectotype consists of a vial with the body and the appendages. The rest of the syntype series now becomes part of the paralectotype series.