Astyanax obscurus ( Hensel, 1870 )

Bertaco, Vinicius A. & de Lucena, Carlos A. S., 2010, Redescription of Astyanax obscurus (Hensel, 1870) and A. laticeps (Cope, 1894) (Teleostei: Characidae): two valid freshwater species originally described from rivers of Southern Brazil, Neotropical Ichthyology 8 (1), pp. 7-20 : 8-11

Astyanax obscurus ( Hensel, 1870)

Figs. 1-4a

Tetragonopterus obscurus Hensel, 1870:86-87 . Type locality: Rio Cadea [rio Cadeia, tributary of rio Caí, rio Jacuí drainage] above large waterfall.

Astyanax obscurus . - Lima et al., 2003:11 (valid species). - Melo & Buckup, 2006:50 (morphometric and meristic data of the syntypes).

Diagnosis. Astyanax obscurus belongs to the A. scabripinnis species complex and is distinguished from most species of this complex (except A. burgerai , A. leonidas , A. ojiara and A. rivularis ), by the shape of the first humeral spot: vertically elongate with upper portion wider, located over third to fifth vertical series of scales and extending over 2 to 3 horizontal series of scales above of the lateral line, lower portion narrow (1 to 2 scales pigmented), and extending over 1 to 2 horizontal series of scales below the lateral line (vs. upper and lower portions of similar width or upper portion slightly wider). Astyanax obscurus differs from A. burgerai by the number of lateral line scales (37-39 vs. 31-34), number of scale rows between lateral line and pelvic fin origin (4-5 vs. 3), and absence of bony hooks in the anal fin of males; from A. leonidas by the number of lateral line scales 37-39, usually 38 (vs. 35-37, usually 35-36), number of total vertebrae (36-38 vs. 32-33), orbital diameter 25.7-34.6% (vs. 34.3-41.2% of HL), and absence of bony hooks in fins in males; from A. ojiara by maxillary tooth tricuspid (vs. heptacuspid), and absence of bony hooks in all fins and on axillary scale in males; from A. rivularis by the number of the lateral line scales (37-39 vs. 33-35), maxillary tooth tricuspid (vs. pentacuspid), and absence of bony hooks on anal and pelvic fin rays. Astyanax obscurus differs from three sympatric species of the A. scabripinnis complex by first humeral spot shape (horizontally elongate humeral spot with a narrow anteroventral downward extension in A. laticeps , and wedge shape uniform in A. brachypterygium and A. cremnobates ), and by the absence of bony hooks in pelvic and anal fin rays of males (vs. presence). The holotype, only specimen known of A. scabripinnis , is relatively well preserved, but has completely lost its color pattern, which preclude a comparison of humeral spot form with A. obscurus . Astyanax obscurus differs from A. scabripinnis by the number of predorsal scales (11-14 vs. 9), interorbital width (28.2-33.9 vs. 40.7% of HL), and by anal fin base length (20.4-24.0 vs. 30.2% of SL).

Description. Morphometric data summarized in Table 1. Body compressed and elongate, with greatest body depth anterior to dorsal-fin origin. Dorsal profile of head straight or slightly convex from posterior nostril to tip of supraoccipital spine. Dorsal body profile convex from tip of supraocciptal spine to base of last dorsal-fin ray; straight from latter point to adipose fin origin. Ventral profile of body slightly convex from vertical through posterior nostril to pectoral fin origin, nearly straight to anal-fin origin, and posterodorsally slanted along anal fin base. Caudal peduncle elongate, nearly straight to slightly concave in the dorsal and ventral margins.

Snout rounded from margin of upper lip to vertical through anterior nostrils. Head small. Mouth terminal, lower jaw slightly longer than upper jaw. Maxilla extending posteriorly to vertical through anterior margin of orbit, slightly curved, aligned at angle of approximately 45 degrees to longitudinal body axis. Maxilla slightly widened anteroposteriorly.

Two tooth rows in premaxilla; outer row with three to five (4-5*) pentacuspid teeth, central cusp longer; inner row with five teeth, gradually decreasing in length from the first to fourth, last distinctly smaller, with five to seven cusps; central cusp twice as long and broad as others cusps. Maxilla with one* (rarely two) teeth, one to three cusps, with central cusp longer. Three to four anteriormost dentary teeth larger, with five or seven cusps, followed by one medium-sized teeth with three to five cusps, and four or six teeth with one to three cusps. Central cusp in all teeth two to three times as long and broad as other cusps. All cusp tips slightly curved posteriorly towards inside mouth ( Fig. 3 ).

Dorsal-fin rays ii,9 (ii,9-10*, one specimen with ii,11, n = 28); first unbranched ray approximately half length of second ray. Distal margin of dorsal fin nearly straight to somewhat convex. Dorsal fin origin approximately at middle of SL. Adipose fin approximately at vertical through last anal-fin ray insertion. Anal-fin rays iii-v,16-21 (18-19*, one specimen with 15, mean = 18.3, n = 28). First unbranched ray normally only apparent in c&s specimens. Anal fin profile smoothly concave. Anal fin origin posterior to vertical through base of last dorsal-fin ray. Pectoral-fin rays i,11-13 (i,12-13*, mean = 11.7, n = 28). Pectoral-fin tip reaching one or two scales before vertical through pelvic-fin insertion. Pelvic-fin rays i,6-7 (i,7*, n = 28). Pelvic fin origin slightly anterior to vertical through dorsal-fin origin. Pelvic-fin tip trespasses the genital opening but not reach the anal-fin origin. Caudal fin forked, lobes similar in size, 19* principal rays. Dorsal procurrent rays 10- 11, and ventral procurrent rays 8-10 (n = 6).

Lateral line complete with 37-39 scales (37-39*, mean = 38, n = 28). Scale rows between dorsal-fin origin and lateral line 5- 6* (mean = 5.9, n = 28); 4-5* scale rows between lateral line and pelvic-fin origin (mean = 4.5, n = 28). Predorsal scales 11- 14 (11-12*, mean = 12.2, n = 28) arranged in regular series. Scale rows around caudal peduncle 14*-16 (mean = 14.3, n = 28). Axillary scale on pelvic fin origin extends posteriorly covering 2-3 scales. Scale sheath along anal fin base 6-9 scales, in single series, covering base of anteriormost rays.

Precaudal vertebrae 17-19; caudal vertebrae 18-19; total vertebrae 36-38 (n = 6). Supraneurals 5-6 (n = 6). Gill-rakers upper branch 6-8 (mean = 6.7, n = 30), lower branch 11-14 (mean = 12.7, n = 30).

Color in alcohol. Dorsal and dorsolateral portions of head and body dark brown. Dark chromatophores scattered on lateral portion of head, more densely concentrated on snout and anterior border of eye. Dorsal portion of body densely pigmented in larger specimens. Body with black, midlateral stripe extending from the second humeral spot to caudal fin base; faint dark pigmentation present on middle caudal-fin rays. Midlateral body stripe expanded dorsally and ventrally to caudal fin base, forming small caudal spot. Two humeral spots. Anterior one, conspicuous, vertically elongate with superior portion wider, located over third to fifth vertical series of scales, extending over 2 to 3 horizontal series of scales above lateral line; inferior portion of spot narrow (1 to 2 scales pigmented), extending over 1 to 2 horizontal series of scales below lateral line. Second humeral spot large, occasionally faint, not ventrally surpassing lateral line, extending over 3 horizontal series and 2 to 3 vertical series of scales ( Figs. 2 and 4a ). Region between spots pale. In specimens smaller than 28.0 mm SL superior portion of first humeral spot lightly larger than inferior portion, second humeral spot can be absent. Fins with dispersed dark chromatophores. Specimens a few days fixed in formalin (MCP 40000) with overall body coloration greenish and all fins red and yellowish pigmented.

Sexual dimorphism. Secondary sexual characters were not found on examined specimens. Testes were observed in one dissected c&s specimen (MCP 26125, 67.7 mm SL). Gill glands ( Burns & Weitzman, 1996) were not found on first gill arch in all specimens.

Distribution. Astyanax obscurus is known from the upper and middle rios Caí and Taquari-Antas, rio Jacuí drainage, Rio Grande do Sul, Brazil ( Fig. 5 ).

Ecological notes. The rio Cadeia at the locality where were caught the syntypes is a fast-flowing clear water river until 1.0 m deep and about 10-15 m wide. The locality is situated upstream of the natural waterfall with about 125 m highest named by Cascata do Herval, around 370 m a.s.l., 29 o 29’59”S 50 o 56’49”W; the bottom consists of mud, stones, and rocks and with moderately riparian vegetation. Topotypes of Astyanax obscurus were collected with A. laticeps , Bryconamericus iheringii , Crenicichla punctata , Hyphessobrycon luetkenii , Phalloceros caudimaculatus , and Rineloricaria cadeae .

Material examined. Syntypes. ZMB 7478 , 3 (3, 46.5-57.8 mm SL), rio Cadea [Cadeia] above large waterfall, [Santa Maria do Herval], Rio Grande do Sul, Brazil, 1863-1866 , R. F. Hensel. Non-types :

Brazil: Rio Grande do Sul: laguna dos Patos system: rio Jacuí drainage: MCP 11145, 28, 31.3-83.3 mm SL, arroio Dizimeiro on the road between São Francisco de Paula and Lageado Grande. MCP 15198, 5, 69.1-83.8 mm SL; MCP 15396, 13 (12, 65.0- 82.8 mm SL), tributary of arroio Feitoria, Dois Irmãos. MCP 17493, 10, (10, 41.7- 88.3 mm SL), tributary of arroio Castelhano, rio Taquari drainage, Venâncio Aires. MCP 20017, 3, 43.4-73.6 mm SL, arroio Feitoria, Sapiranga. MCP 20040, 13 (10, 38.2-47.0 mm SL), arroio Feitoria, Sapiranga. MCP 21331, 4, 65.4-74.2 mm SL, arroio Feitoria, Sapiranga. MCP 21542, 6 (6, 58.3-76.3 mm SL), stream about 7 km North of Barros Cassal, tributary of rio Fão, Barros Cassal. MCP 25686, 28, 29.0- 57.9 mm SL, stream about 7 km North of Barros Cassal, tributary of rio Fão, Barros Cassal. MCP 21567, 5 (5, 43.4-61.9 mm SL), stream on the road beetwen Barros Cassal and Gramado Xavier, Gramado Xavier. MCP 21730, 12 (8, 51.3-83.0 mm SL), stream between Dois Irmãos and Morro Reuter, locality of Travesso, Dois Irmãos. MCP 22304, 4, 31.5-82.8 mm SL, headwater of rio Lageado Grande, about 1 km of Várzea do Cedro, Lageado Grande. MCP 26125, 14 (13, 35.2-79.8 mm SL; 2 c&s, 65.5-67.7 mm SL), arroio Cadeia in the Jacaré beach, tributary of rio Caí, Santa Maria do Herval. MCP 39999, 24, 19.9-32.3 mm SL, rio Cadeia about 3 km from Amizade beach, Santa Maria do Herval. MCP 40000, 43 (10, 33.5- 60.4 mm SL; 4 c&s, 33.5-53.7 mm SL), rio Cadeia in the Amizade beach, Vila Amizade, Santa Maria do Herval. MCP 40001, 11 (6, 34.3-45.0 mm SL), rio Cadeia across from Jacaré beach, Vila Cristo Rei, Santa Maria do Herval.

Figures

Fig. 1. Astyanax obscurus, ZMB 7478, syntype, 57.8 mm SL, rio Cadeia above of the large waterfalls, Santa Maria do Herval, Rio Grande do Sul, Brazil.

Fig. 2. Astyanax obscurus, MCP 40000, 59.2 mm SL, rio Cadeia above the large waterfalls, Santa Maria do Herval, Rio Grande do Sul, Brazil.

Fig. 3. Astyanax obscurus, MCP 26125, 65.5 mm SL. SEM image of upper and lower jaws, right side. Scale bar = 2 mm.

Fig. 4. Lateral view of left side of anterior region showing humeral spots of (a) Astyanax obscurus, MCP 26125, 75.6 mm SL, and (b) Astyanax laticeps, MCP 26127, 71.1 mm SL.

Fig. 5. Map of southern Brazil and Uruguay, showing the distribution of examined material of Astyanax obscurus (circles), and Astyanax laticeps (squares). Some symbols represent more than one lot or locality.