Myxiops aphos, Zanata & Akama, 2004

Zanata, Angela M. & Akama, Alberto, 2004, Myxiops aphos, new characid genus and species (Characiformes: Characidae) from the rio Lençóis, Bahia, Brazil, Neotropical Ichthyology 2 (2), pp. 45-54 : 47-48

Myxiops aphos , new species

Figs. 1-2

Holotype. MZUSP 81026 , 56.0 mm SL; Brazil, Bahia, Lençóis, rio Lençóis, tributary of rio Paraguaçu , 3 km upstream of Lençóis city, above Cachoeira do Serrano , 12 o 34’S 41 o 22’W. A. Akama, 22 Feb 1995.

Paratypes. MZUSP 81025 , 22 specimens, 27.6-56.1 mm SL, 4 c&s, 26.4- 46.8 mm SL ; MCP 35007, 3 specimens, 41.2-48.3 mm SL; same data as holotype .

Diagnosis. As for the genus.

Description. Morphometric data presented in Table 1. Body relatively small, maximum observed standard length 56.1 mm, relatively elongate and transversely rounded, somewhat flattened posterior to terminus of dorsal fin base. Greatest body depth at dorsal-fin origin. Dorsal profile of head distinctly convex from margin of upper lip to region approximately at vertical through anterior nostril, nearly straight from that point to posterior tip of supraoccipital spine. Dorsal profile of body slightly convex from tip of supraoccipital spine to dorsal-fin origin, posteroventrally-inclined along dorsal-fin base, nearly straight from posterior terminus of dorsal-fin base to adipose fin, and slightly convex from posterior part of adipose-fin base to origin of dorsalmost procurrent caudal-fin ray. Ventral profile of head and body distinctly convex on anterior tip of dentary, gently convex from this point to anal-fin origin, straight and posterodorsally inclined along anal-fin base. Ventral profile of caudal peduncle concave.

Head obtusely rounded anteriorly in lateral profile. Contralateral frontals in contact anteriorly; frontal fontanel restricted to posterior portion of frontal bones. Parietal portion of fontanel extending between parietals posteriorly. Supraorbital absent. Jaws equal, mouth terminal, albeit somewhat ventrally positioned. Posterior tip of maxilla extending to vertical slightly anterior to or reaching the center of pupil. Gill-rakers 7+1+10 (2) or 6+1+11 (1). Branchiostegal rays 4 (4).

Premaxilla with 5 (26) teeth arranged in single regular tooth row. Teeth pedunculated, expanded, compressed distally, and bearing 5-7 cusps. Cusps compressed, aligned along the distal tooth margin. Central cusp largest. Maxilla with 2 (16) or 3*(10) compressed teeth, each bearing 5 cusps. Three central cusps largest and of approximately same size. Dentary teeth 10 (6), 11 (13), or 12*(7) arranged in single row with anterior 8-9 teeth largest and pentacuspid, followed posteriorly by 2 tricuspid teeth. Smallest cleared and stained specimen with four posteriormost teeth conical. Dentary teeth similar in shape to premaxillary ones, although slightly curved towards oral cavity. Central cusp largest.

Scales cycloid, circuli absent on exposed area of scales, radii well developed and extending to posterior margin of scales parallel or slightly divergent. Lateral line slightly decurved ventrally, completely pored from supracleithrum to base of caudal fin, with 36* (16), 37 (8) or 38 (2) perforated scales. Scale rows between dorsal-fin origin and lateral line 5 (26). Scale rows between lateral line and pelvic-fin origin 3* (22) or 4 (4). Scales along middorsal line between tip of supraoccipital process and origin of dorsal fin 12 (3), 13 (20) or 14* (3). Horizontal scale rows around caudal peduncle 14 (26).

Dorsal-fin rays ii,8,i (24) or ii,9* (3). Dorsal-fin origin situated at midlength of SL. Base of last dorsal-fin ray located at vertical through anal-fin origin. Adipose-fin origin located posterior to vertical through insertion of last anal-fin ray. Anal-fin rays iv,14 (2), iv,15 (10), iv,16 (7) or iv,17* (7). Analfin origin along vertical through insertion of posteriormost dorsal-fin ray. Pectoral-fin rays i,10 (11) or i,11* (16). Tip of depressed pectoral fin falling short of pelvic-fin insertion. Pelvic-fin rays i,7 (26). Pelvic-fin origin anterior to vertical through dorsal-fin origin. Caudal fin forked, lobes similar in size. Principal caudal-fin rays 10/9 (26). Vertebrae 36 (2). Supraneurals 5 (2) or 6 (1).

Color in alcohol. Examined specimens lack guanine on body or head. Ground color tan. Dark chromatophores concentrated on middorsal surface of head and body. Anterior portion of both dentary and maxilla with dense concentrations of dark chromatophores. Ventral portion of head with scattered, dark chromatophores. Humeral region with two dark blotches, anterior blotch more evident and bordered anteriorly and posteriorly by regions of paler coloration. Anterior blotch vertically elongate, wider dorsally, and situated immediately posterior to opercle. Posterior humeral blotch much less conspicuous, somewhat square-shaped, and formed by dark chromatophores concentrated over three or four scales immediately dorsal to anterior portion of midlateral stripe. Midlateral dark stripe extending from pale area posterior to humeral spot to end of caudal peduncle, more conspicuous and relatively wider posterior to vertical through dorsal-fin origin. Stripe on four middle caudal-fin rays extending to tips of rays. All rays with thin line of chromatophores along margins, giving dark contour to each ray. Adipose fin with scattered dark chromatophores. Smaller specimens with overall lighter ground body and fins coloration and with posterior humeral blotch less distinct than in larger individuals.

Sexual dimorphism. None of the specimens have hooks on the fins or any other sexually dimorphic features.

Geographic distribution and habitat notes. The species is known from the rio Lençóis, a tributary of rio Paraguaçu, which is a relatively small, eastern drainage in Bahia State, Brazil ( Fig. 3 ). The type-locality of Myxiops aphos is a shallow, headwater stream, with fast water current over a rocky bottom and margin, and with a small amount of riparian or submerged vegetation. Specimens were collected in small pools 50 cm deep. The rio Lençóis is a blackwater river such as is characteristic of rivers draining soils that are extremely poor in nutrients and highly acidic ( Harley, 1995). Copionodon pecten , a trichomycterid endemic to the rio Paraguaçu drainage ( de Pinna, 1992), and one species of loricariid, Neoplecostomus sp. , were collected together with Myxiops .

Etymology. The name aphos , from the Greek, meaning dark, obscure, in allusion to the dark color of the water of the rio Lençóis.

Stomach contents. The analysis of the stomach contents of two specimens revealed the presence of filamentous algae, terrestrial vegetal fragments, larvae of the Chironomidae , and unidentified insects fragments.

Figures

Fig. 1. Myxiops aphos, holotype, MZUSP 81026, 56.0 mm SL; Brazil, Bahia, rio Lençóis.

Fig. 2. Myxiops aphos, paratype, MZUSP 81025, male, 45.0 mm SL; Brazil, Bahia, rio Lençóis.

Fig. 3. Northeastern Brazil, showing the approximate position of the type locality of Myxiops aphos.