Melanorivulus melanopterus, Nielsen & Ohara, 2024

Nielsen, Dalton Tavares Bressane & Ohara, Willian Massaharu, 2024, A new restricted-range Melanorivulus (Cyprinodontiformes: Rivulidae) from the upper rio Guaporé, Chapada dos Parecis, Brazil, Zootaxa 5496 (3), pp. 332-342 : 333-338

Melanorivulus melanopterus , new species

( Fig.1 ; Table 1 )

Holotype. INPA 60664 , male, 30.2 mm SL, Brazil, Mato Grosso, Conquista do Oeste, headwaters of rio Galera, tributary of the right bank of the upper rio Guaporé , 14°30'4.98"S 59°25'26.03" W, 643 m a.s.l.; W.M. Ohara, D. Zdradek & V. Souza, 20 Feb 2020.

Paratypes. INPA 60665 , 2 males, 25.2–27.7 mm SL, 2 females, 24.3–27.5 mm SL; CPUFMT 8596, 3 males, 25.7–28.6 mm SL, 3 females, 21.8–24.3 mm SL ; ZUEC 18282 , 10 males, 19.9–29.7 mm SL, 9 females, 20.6–24.5 mm SL, 2 c&s, 18.0– 25.4 mm SL; same data as holotype .

Diagnosis. The new species is distinguished from all other species of Melanorivulus , except M. pictus , M. planaltinus (Costa & Brasil) and M. nelsoni Deprá, Silva & da Graça by the absence or extreme reduction of pelvic fin and pelvic girdle (vs. pelvic fin and pelvic girdle well-developed and always present). Males of Melanorivulus melanopterus can be distinguished from M. pictus , M. planaltinus and M. nelsoni by presence of a black band on whole caudal-fin margin, typically more conspicuous on dorsal and ventral portion (vs. absence of a black band on caudal-fin margin) and by conspicuous, wide dark brown bars, oblique anteriorly and wide chevron-like marks posteriorly to middle of flank (vs. bars narrow, inconspicuous or absent in M. pictus , M. planaltinus and M. nelsoni ).

Description. Morphometric data are presented in Table I. Largest male 30.2 mm SL, largest female 24.8 mm SL. Body slender, cylindrical and compressed, greatest body depth at level of pelvic-fin base. Dorsal profile slightly convex from snout to the first dorsal-fin ray, slightly concave from last dorsal-fin ray to the end of the caudal fin peduncle; ventral profile slightly convex from lower jaw to anal fin terminus, slightly concave along caudal peduncle. Snout short, rounded. Jaws short. Dorsal-fin rounded, anal fin slightly pointed, without filaments in both sexes; caudal fin rounded in males and female; pelvic fin and pelvic girdle absent. Pectoral fin rounded. Dorsal-fin origin at vertical through the base of the 6 th anal-fin ray, between neural spines of vertebrae 20–21, anal-fin origin between pleural ribs of vertebrae 17–18. Dorsal-fin rays 11(28); anal-fin rays 13 (28); caudal-fin rays 33(28); pectoral-fin rays 13–14 (28).

Scales small, cycloid. Frontal squamation F-patterned; F-scales scales not overlapping. Longitudinal series of scales 33–34(24); transverse series of scales 8(24); scale rows around caudal peduncle 16(24); no contact organs on scales and fin rays.

Cephalic neuromasts: supraorbital 3+3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1+11– 12+1, preorbital 2, otic 2, post-otic 1, supratemporal 1, median opercular 1, ventral opercular 2, preopercular 2+4, mandibular 3+1, lateral mandibular 1, paramandibular 1; two neuromasts on caudal-fin base. Six branchiostegal rays. Vertebrae 30(4).

Coloration in life. Males. Based in photographs taken in field and in aquarium of four males. Sides of body light green with a total of 9–10 wide irregular marks: 4–5 wide, oblique red bars anteriorly and 4–5 wide, red chevron-like marks posteriorly to middle of flank. Dorsum brown; venter whitish; sides of head light yellow, with irregularshaped post-orbital brown blotches from eyes to pectoral fin; jaw brown, iris brownish-red or yellow. Dorsal fin hyaline, with thin metallic green line parallel to the dorsum, distal area iridescent blue. Anal fin pale yellow, base with pale metallic blue and distal area with a brown line. Caudal fin yellow, with wide black pigmentation on distal margin overlaid by iridescent blue pigment. Pectoral fin hyaline yellow, with dark rays ( Fig. 1A–B ).

Females. Based in photographs taken in field and in aquarium of two females. Sides of body light brown, green pigmentation inconspicuous with a total of 9–10 wide irregular marks: 4–5 narrow, oblique red bars anteriorly and 4–5 narrow, red chevron-like marks posteriorly to middle of flank. Abdominal region whitish. Dorsum brown. Side of head light brown with irregular dark brown spots, upper and lower jaws brown; iris pale yellow with brown spots. Dorsal fin with dark pigmentation on distal margin, with irregular blue and green metallic spots. Pectoral fin hyaline. Pelvic fin hyaline with a narrow black stripe on distal margin. Caudal fin with narrow, poorly-defined black stripe on distal margin; middle portion of the fin with reticulate pattern. Black ocellated spot surrounded by white area on dorsal portions of caudal fin base ( Fig. 1C–D ).

Coloration in alcohol. Males. Dorsal portion of head and trunk brown, lighter ventrally. Lower lip densely pigmented with small melanophores. Ventral portion of head with white spotted pattern. Dorsal portion of opercle and humeral region with diffuse dark blotches. Sides of body brownish, with a total of 9–10 dark brown irregular marks; 4–5 oblique brown bars anteriorly and 4–5, brown chevron-like marks posteriorly to middle of flank. Pectoral fin hyaline. Distal margin of dorsal fin with a dark stripe present or absent in both sexes. Dorsal and caudal fins with conspicuous and clear spotted pattern only in females. Caudal fin with black band along the entire caudal-fin margin in males (, typically more conspicuous on dorsal and ventral portions ( Fig. 2A–B ).

Coloration in alcohol. Females. Sides of body color brownish, with a total of 9–10 wide dark brown irregular marks posteriorly to middle of flank. Dorsum brown; venter beige or white. Side of head on infraorbital and lower opercle regions usually with some dark blotches as in males, jaw brown, iris white. Dorsal fin with irregular thin black spots. Anal fin white with black bar in distal area. Black ocellated spot on dorsal portions of caudal fin base. Caudal fin white, with irregular thin black spots. Pectoral fin hyaline ( Fig. 2C–D ).

Ecological notes. The type-locality of Melanorivulus melanopterus is the headwater of the rio Galera, a small clear water stream, partially dammed, 2.0–8.0 m wide, 0.3–2.5 m deep, with moderate water current, submerged vegetation, grasses on the banks, and bottom composed by sand and mud ( Fig. 3 ). The stream run across cerrado vegetation at Chapada dos Parecis at an altitude of 643 m a.s.l. Melanorivulus melanopterus was captured near the margins, amid the marginal vegetation in slow-flowing water.

Distribution. Only known from the type locality, upper rio Galera on the Chapada dos Parecis, a tributary of the rio Guaporé, rio Madeira basin, Mato Grosso state, Brazil ( Fig. 4 ).

Etymology. The specific epithet melanopterus come from Greek melano (Gr. μέλανος), black, and pterus (Gr. πτερόν), fin, referring to black band on caudal fin. An adjective.

Figures

FIGURE 1. Coloration in life of Melanorivulus melanopterus: a) INPA 60664, holotype, male, 30.2 mm SL; b) male, 25.2 mm; c) female, 21.8 mm SL, from INPA 60665, paratype.

FIGURE 2. Melanorivulus melanopterus: a) holotype, male, 30.2 mm SL, INPA 60664; b) male, 27.7 mm SL, c), female, 21.8 mm SL, d) female, 20.8 mm SL, all from INPA 60665, paratypes.

FIGURE 3. Type locality of Melanorivulus melanopterus, upper rio Galera, a tributary of the right bank of the upper rio Guaporé, Mato Grosso, Brazil.

FIGURE 4. Map of central Brazil and adjacent Bolivia showing the geographical distribution of Melanorivulus melanopterus at the rio Guaporé basin (white circle) and M. modestus at the rio Tapajós basin (black circle).