Astyanax stenohalinus Messner, 1962

Almirón, Adriana E., Casciotta, Jorge R., Azpelicueta, María De Las M. & Loureiro, Marcelo, 2010, Redescription of Astyanax stenohalinus Messner, 1962 (Characiformes: Characidae), a poorly known species from Argentina and Uruguay, Zootaxa 2434, pp. 60-68 : 61-66

Astyanax stenohalinus Messner, 1962

( Figs. 1–5, Tab. 1 )

Emended diagnosis: Astyanax stenohalinus is distinguished from all other Astyanax species by the possession of hooks on all fins of males, teeth not expanded distally, two or three maxillary teeth, 37–39 perforated scales in lateral series, 25–30 branched anal fin rays, and anal-fin origin located before a vertical through last dorsal-fin ray insertions ( Fig. 1).

Description: Morphometrics of holotype and 14 specimens are presented in Table 1 . Body relatively deep, especially in females ( Fig. 2 ), with maximum body depth located immediately anterior to dorsal-fin origin. Dorsal profile of body curved from snout tip to dorsal-fin origin, with concavity over supraoccipital area ( Fig. 3 ); straight and slanted ventrally to caudal peduncle, straight along caudal peduncle to base of caudal-fin rays. Ventral profile of body similar to dorsal one.

Dorsal-fin origin almost equidistant from tip of snout and base of caudal-fin rays. Anal-fin origin placed below posterior half of dorsal fin. Tip of pectoral fin surpassing pelvic-fin origin even in small specimens; in males, pectoral tip reaching half of pelvic fin. Tip of pelvic fin surpassing anal-fin origin in all males; in females, pelvic fin reaching or scarcely surpassing that point.

Head with lower jaw as long as upper jaw. Premaxilla with short ascending process. Outer row of premaxillary teeth with 4–5, tricuspidate teeth, second or third tooth of outer row sometimes positioned out of line. Inner series with 5 teeth, symphysial tooth tetracuspidate, remaining teeth pentacuspidate to tricuspidate, with central cusp notably larger. Maxilla long and slender; anterodorsal maxillary process long; laminar process with 2–3 tricuspidate or unicuspidate teeth ( Fig. 4 ). Dentary with 3–4 large pentacuspidate teeth followed by a median-sized tooth pentacuspidate or tricuspidate, and 7–9 smaller tricuspidate to unicuspidate teeth ( Fig. 5 ).

Eye larger than snout, interorbital distance about one third of head. Third infraorbital not contacting latero-sensory canal of preopercle.

Dorsal fin with iii,9 rays; first dorsal-fin ray very small, easily visible in cleared and stained specimens. Distal margin of dorsal fin straight. Anal fin with v-vi, 25–30 (holotype: v,27) rays. Males with distal margin straight, and females with branched rays somewhat lengthened to form a small lobe. Caudal fin with i,17,i rays; upper lobe scarcely shorter and narrower than lower lobe. Pectoral-fin margin rounded; pectoral fin with i,12–14 (holotype: i,13). Pelvic-fin margin slightly rounded; pelvic fin with i,7 rays.

Hooks on all fins of males. Few small hooks on distal region of dorsal-fin rays; one pair on each segment, on posterior branch of ray. Hooks extended on fifth and sixth unbranched and until 20th branched anal-fin rays; one pair of hooks on each segment, placed in both ray branches; hooks strong, curved dorsally. Caudal fin with very small hooks placed on distal portion of middle rays, one hook on each segment. Pectoral fin with few small hooks, placed on distal portion of ray, especially on internal branch, and one per segment. Pelvic fin with large hooks on last unbranched and both branches of all branched rays; hooks slightly curved dorsally and inward.

Scales cycloid. Lateral series with 37–39 (holotype: 38) perforated scales. Scales between dorsal-fin origin and lateral line 6–7 (holotype: 7); scales between lateral line and pelvic-fin origin 6 (holotype: 7). Scales around caudal peduncle 15–16 (holotype: 16). Twelve to 16 scales forming an irregular row between tip of supraoccipital spine and dorsal-fin origin (holotype: 12). Fourteen to 18 scales (holotype: 14) situated along anal-fin base, covering base of all unbranched and first 15 to 20 branched anal-fin rays.

First branchial arch with 27 or 28 gill rakers (2–3 on hypobranchial, 14 on ceratobranchial, 1 on cartilage, and 10 on epibranchial). Caudal fin with 10 dorsal and 9 ventral procurrent rays. Thirty six vertebrae including complex centrum+Weber apparatus. Dorsal-fin pterygiophores between neural spines of vertebrae 10–11 and 17–18; anal-fin pterygiophores between hemal spines of vertebrae 18 and 29. Eleven pairs of ribs. Five or 6 supraneurals with distal tips bifids (2 C&S specimens examined).

Color in alcohol: Background light yellowish, dorsal region of body and head very slightly darker. A very narrow grey stripe extending along middorsal line. Narrow lateral band from humeral spot to dorsal-fin level, wider posteriorly, ending with large spot on caudal peduncle. Dark stripe continuing onto middle caudal-fin rays, especially dark at distal margin of rays. One vertically elongated humeral spot, posterior to vertical through pectoral-fin insertion, irregularly shaped. A second very faint humeral spot, positioned at vertical through middle pectoral fin. Most specimens with chromatophores concentrated along myosepta on lateral band. Dark chromatophores concentrated on scale margins of uppermost portion of body. Small chromatophores present on premaxilla, and around nares. Mouth surrounded by chromatophores. All fins, excluded caudal one, hyaline. Caudal fin with dark chromatophores concentrated on distal third of fin and uppermost and lowermost rays.

Color upon capture: Background silver, a wide whitish lateral band, reddish caudal fin with middle portion faint yellowish and a light gray humeral spot.

Distribution: Astyanax stenohalinus is known from Río Uruguay basin in Argentina and Uruguay; Río de la Plata basin in Argentina; Atlantic slope and Laguna Merín in Uruguay. The type locality is Laguna Mazangano, Río Negro basin, Uruguay ( Fig. 6 ). See geographical distribution in the map ( Fig. 7 ). Following Messner (1962), A. stenohalinus is also present in the río Vacacaí, Rio Grande do Sul, Brazil.

Figures

FIGURE 2. MHNM 582 - 1, paratype, female, 68.7 mm SL, collected with the holotype.

FIGURE 3. Astyanax stenohalinus, AI 259, 68.3 mm SL, Argentina, Río de la Plata basin, Buenos Aires, Punta Indio, Arroyo Casilda.

FIGURE 4. Astyanax stenohalinus. AI 132, left upper jaw in lateral view. Scale bar = 1 mm.

FIGURE 5. Astyanax stenohalinus. AI 132, right lower jaw in medial view. Scale bar = 1 mm.

FIGURE 6. Type locality, Departamento Cerro Largo, Laguna Mazangano (32 º 11 ’ S- 54 º 65 ’ W)

FIGURE 7. Geographical distribution of Astyanax stenohalinus. 1, río Timboy; 2, Segunda Laguna; 3, Arroyo Carpintería; 4, Laguna Mazangano (type locality); 5, Arroyo Conventos; 6, Bañado Medina; 7, Paso Ceniza; 8, Paso del Dragón; 9, Paso del Rey; 10, Arroyo de los Perros; 11, Arroyo Rolón; 12, Potrerillo de Santa Teresa; 13, Arroyo Las Conchas; 14, Arroyo Sauce; 15, Arroyo José Ignacio; and 16 arroyos Casilda and Villoldo.

Tables

TABLE 1. Morphometric data of holotype and 14 specimens of Astyanax stenohalinus.

	holotype	males	mean	females	mean
Standard length (mm)	68.3	42.1 – 62.5		46.1 – 71.4
Percents of standard length Predorsal distance	55.6	53.0 – 56.1	(53.9)	54.3 – 55.9	(55.2)
Preventral distance	43.2	42.3 – 45.6	(43.7)	43.2 – 47.9	(45.8)
Preanal distance Body depth	63.3 40.7	58.2 – 62.7 35.3 – 39.1	(59.8) (37.3)	60.6 – 65.3 39.7 – 42.7	(63.3) (41.1)
Dorsal-fin base	13.5	12.7 – 15.2	(13.7)	12.4 – 14.5	(13.8)
Anal-fin base Pectoral-fin length	34.1 25.8	35.0 – 37.5 24.5 – 26.6	(36.2) (25.9)	33.4 – 36.4 23.6 – 26.8	(35.0) (25.2)
Pelvic-fin length	19.9	18.9 – 20.6	(19.8)	16.5 – 19.9	(18.6)
Distance between pectoral and pelvic-fin origins Distance between pelvic and anal- fin origins	22.5 18.7	17.7 – 20.2 16.0 – 19.5	(19.8) (17.4)	20.6 – 22.5 17.1 – 20.1	(21.3) (18.3)
Head length	26.2	26.8 – 30.2	(27.8)	26.2 – 28.6	(27.2)
Peduncle depth Percents of head length	11.6	10.8 – 12.2	(11.5)	10.5 – 12.4	(11.4)
Peduncle length	40.2	33.9 – 40.0	(37.4)	34.4 – 40.4	(38.1)
Snout length Eye diameter	22.3 39.1	21.2 – 24.5 33.3 – 41.4	(22.3) (38.8)	20.1 – 25.2 37.7 – 40.6	(23.1) (38.9)
Interorbital length	34.6	28.6 – 34.8	(31.6)	30.4 – 34.6	(32.6)
Postorbital length Maxillary length	43.0 31.8	40.2 – 46.8 25.9 – 31.9	(44.4) (29.3)	42.4 – 45.5 28.8 – 31.8	(43.9) (31.1)