Melanorivulus nelsoni, Deprá & Silva & Graça, 2017

Deprá, Gabriel De Carvalho, Silva, Hugmar Pains & Graça, Weferson Júnio Da, 2017, A new pelvic-less species of Melanorivulus Costa (Cyprinodontiformes: Cynolebiidae), with a discussion on the pelvic-fin absence in killifishes, Zootaxa 4300 (1), pp. 111-124 : 113-117

Melanorivulus nelsoni , new species

( Figs. 1 , 2 )

Holotype. MCP 50981, 39.1 mm SL, male, Brazil, State of Mato Grosso, municipality of Juscimeira, unnamed creek tributary to the córrego Beleza, tributary to the rio São Lourenço ( rio Paraguai basin), 16°10′57″S, 55°0′40″W; H. P. Silva, 29 Apr 2014.

Paratypes. All from Brazil, State of Mato Grosso, municipality of Juscimeira, rio São Lourenço basin, rio Paraguai basin. NUP 18967, 5, 20.1–33.1, córrego Beleza ; 16°10′58″S, 55°0′54″W; H. P. Silva; 18 Jan 2014 . NUP 18968, 4, 20.8–35.3 mm SL, same locality and collector as NUP 18967; 30 Apr 2014 . NUP 18969, 7 (1 c&s, 23.6 mm SL), 17.5–32.5 mm SL, same data as holotype . NUP 18970, 1, 29.0 mm SL, same locality and collector as holotype; 9 May 2016 .

Diagnosis. Melanorivulus nelsoni is distinguished from all congeners, except most specimens of M. planaltinus and some specimens of M. pictus , by the lack of the pelvic girdle and fin, vs. presence (see below). Melanorivulus nelsoni is distinguished from M. pictus and M. planaltinus by having a light yellowish-plumbeous background on the flank (vs. blue or greenish-blue); and by having 10 transverse series of scales (vs. 8). The number of transverse series of scales (10) also distinguishes M. nelsoni from all other congeners, except M. pindorama , M. pinima and M. rutilicaudus (remaining Melanorivulus species present 6–9 transverse series of scales).

Melanorivulus nelsoni is further distinguished from M. rutilicaudus by having the opercular and humeral regions marked with black in life ( Fig. 2 ) (vs. marked with dark red). It differs from M. atlanticus , M. jalapensis and the species belonging to the “ M. zygonectes species group” by the lack of a dark stripe from eye to caudal-fin base (vs. presence). It differs from M. bororo , M. decoratus , M. pindorama and M. violaceus , by lacking a series of dark spots along flank midline (vs. presence). It differs from the species belonging to the “ M. pinima species group” (i.e., M. illuminatus , M. kayapo , M. kunzei and M. pinima ), by having normally developed black pigmentation on the postorbital region of head and on the humeral region in both males and females (vs. vestigial, at least in males); and by presenting red marks on the flank forming oblique, chevron-like stripes (vs. forming longitudinal rows of spots at least in the anterior portion of the flank). From the miniature species Melanorivulus atlanticus , M. decoratus and M. jalapensis , M. nelsoni differs by having six branchiostegal rays (vs. five). Finally, Melanorivulus nelsoni differs from the species belonging to the “ M. dapazi species group”, by lacking a dark reddish brown stripe on the distal margin of the anal fin in males.

Description. Morphometric data in Table 1. Body slender, highest slightly anterior to anal fin. Dorsal profile slightly convex from tip of snout to base of first dorsal-fin ray; straight, posteroventrally slanted along dorsal-fin base; straight along caudal peduncle. Ventral profile slightly convex from tip of dentary to base of first anal-fin ray; straight to slightly concave along anal-fin base, posterodorsally slanted; slightly concave along caudal peduncle. Body elliptical anteriorly in cross-section, almost as wide as deep, maximum width at pectoral-fin base; strongly compressed from dorsal-fin base to end of caudal peduncle. Head slightly convex ventrally and laterally, flat dorsally. Flat region extending to dorsal-fin base. Mouth dorsally oriented, prognathous. Lips broad. Upper lip comprised between anterior nostrils. Anterior nostrils tubular, situated on dorsal rim of mouth, contacting lower jaw when mouth is closed, distant from eye by half orbit diameter. Posterior nostril as small pit partially covered by skin flap, anterodorsal to eye, closer from it than to anterior nostril. Eye without free margin. Urogenital papilla short in both sexes, not visible laterally.

All fins rounded. Pectoral-fin rays 14(1), 15(9), 16(6) (ii,10,ii in c&s specimen). Dorsal-fin rays 10(5), 11(6), 12(3) (ii,8,i in c&s specimen), anteriormost nine rays longer, subsequent rays gradually decreasing in size. Anal-fin rays 12(3), 13(9), 14(2) (iii,10,i in c&s specimen), anteriormost eleven rays longer, subsequent rays gradually decreasing in size. Caudal-fin rays 22(1), 23(3), 24(5), 25(3), 26(2), 27(1) (vii,16,vi in c&s specimen). Pelvic fin and girdle absent.

Scales cycloid, covering whole body, except lips, lateral ethmoid, and anterior portion of lower jaw. Pectoral, dorsal and anal fins lacking scales. Base of caudal-fin rays covered with scales somewhat smaller than caudalpeduncle scales. Frontal squamation D- (one specimen), E- (nine specimens), F- (two specimens), G-patterned (two specimens) or with two patterns, one on each side (E and F; one specimen). Two large scales anterior to scale H. Scales in longitudinal series 32(3), 33(2), 34(5), 35(1), 36(1). Scales in transversal series 10(15). Circumpeduncular scales 16(9), 17(7).

Cephalic neuromasts: supraorbital 3 + 3, parietal 1, anterior rostral 1, posterior rostral 1, infraorbital 1 + 9 + 1, preorbital 1, otic 1, post-otic 1, supratemporal 1, preopercular 2 + 1 + 2 + 2, mandibular 4, lateral mandibular 3. Opercular neuromasts apparently absent.

Dorsal arm of preopercle pointed, immediately ventral to articulation between opercle and hyomandibula.

Ventral process of angulo-articular long, pointed. Epibranchial 1 slightly curved distally. Vomerine teeth 3(1). Basihyal about 65% as broad as long. Outer gill-rakers on first branchial arch 1(1) in the dorsal portion, 9(1) in the ventral portion. Branchiostegal rays 2(1) on anterior portion of anterior ceratohial, slender; 2(1) on posterior portion of anterior ceratohial, broad; 2(1) on anterior portion of posterior ceratohial. Ribs 11(1), attached to 2nd to 12th vertebrae. Medial intermuscular bones simple, first one attached to first vertebra. Abdominal vertebrae 13(1); caudal vertebrae 18(1). Dorsal-fin pterygiophores 11(1). Base of first dorsal-fin ray inserting between neural spines of 18th and 19th vertebrae. Anal-fin pterygiophores 13(1). Base of first anal-fin ray inserting between haemal spines of 15th and 16th vertebrae. Hypurals 2(1).

Female paratypes (n = 4) Male paratypes (n = 13) Color in alcohol. Based on both sexes, except when noted. Dorsal portion of head and trunk pale brown, lighter ventrally. Three narrow, irregular, oblique dark brown stripes on postorbital portion of head and humeral region. Anterior stripe very short, from posterodorsal margin of eye, running posterodorsally to include only two scales; indistinct in some specimens. Median stripe from posteroventral margin of eye, running posterodorsally to two scale rows short from dorsal midline; usually interrupted, especially in larger specimens, or irregular, or fused to other dark marks. Posterior stripe extending from anterior margin of opercle to humeral region. Mid-dorsal stripe on supra-occipital, extending three scales posteriorly, more conspicuous in smaller specimens. Scales on cheek and opercular series with melanophores scattered on distal margins, darker in cheek scales, more diffuse in scales over opercular series, producing spotted pattern. Patches of melanophores forming series of densely pigmented spots around ventral margin of eye. Skin flaps surrounding neuromasts, especially those in the dorsal portion of the snout, densely pigmented with melanophores in adults, but not in smaller specimens. Remainder of trunk with melanophores not forming discernable patterns. Smaller specimens with brownish stripes all over flank, formed by thin lines of pigment on distal margin of scales. Stripes usually forming chevron-like marks with angle on midline of flank, pointing anteriorly, separated from each other by two or three scales. Occasionally, chevron-like marks separated by one scale, seemingly forming double stripes or reticulate patterns. Some marks with Y-shaped ends dorsally or ventrally (see ‘Color in life’, Fig. 2 ). Pectoral fin hyaline. Dorsal fin in males with about 7 or 8 nearly vertical dark brown stripes, anterior ones almost indistinguishable, interspaces white, wider. Stripes frequently forming reticulate pattern, interspaces appearing as round spots. Anal fin dusky or patterned as dorsal fin, but more diffusely and only at interradial membranes of last three rays; remainder of fin always dusky. Caudal fin with about 5 to 8 vertical stripes, increasing in number and irregularity with standard length; stripes narrow with broader interspaces, as in other vertical fins, almost indistinguishable on ventral half of caudal-fin and close to dorsal margin of fin. Females with similar patterns on fins, but more contrasting; both stripes and interspaces wider than in males; vertical fins with diffuse dark grey margin; vertically oval black spot on base of dorsal caudal-fin lobe, Color in life. Lower lip pale orange to reddish. Posterior region of opercle bright yellow. Top of head and predorsal area with golden hue. Flanks yellowish plumbeous. Abdominal region white to beige. Chevron marks red to orangish red, occasionally broken into spots both dorsally and ventrally. Pectoral fin hyaline to light yellow. Dorsal and anal fins of males light yellow, occasionally light bluish posteriorly, near base. Dorsal- and anal-fin live coloration of females not recorded. Caudal-fin in males light yellow, with dark brown stripes. In females, light orangish yellow with black stripes.

Geographic distribution. Melanorivulus nelsoni is so far only known from the córrego Beleza, which empties in the rio São Lourenço, a tributary of the rio Paraguai, state of Mato Grosso, Brazil ( Fig. 3 ).

Habitat and ecological notes. The stretch of the córrego Beleza (16°10′58″S, 55°0′54″W) where Melanorivulus nelsoni was collected exhibits a rocky substrate with sandy patches, silt and organic matter. The stream presents a few rapids. Some specimens (NUP 18967 and NUP 18968) were collected in a shallow pool (about 0.6 m deep) immediately upstream of a small rapid ( Fig. 4 a). Signs of trampling by cattle were detected in this area. The type-locality is a first order tributary of the córrego Beleza (16°10′57″S, 55°0′40″W), surrounded by well-preserved riparian vegetation, where Melanorivulus nelsoni specimens were collected in a lentic stretch, with bottom composed by silt, rocks, logs and leaf matter ( Fig. 4 b). Species collected along with Melanorivulus nelsoni in the córrego Beleza include Aequidens plagiozonatus Kullander , Characidium aff. zebra Eigenmann , Crenicichla lepidota Heckel and the introduced Oreochromis sp.

Etymology. We name Melanorivulus nelsoni in honor of Joseph Nelson (1937–2011), who published on the absence of pelvic-fin in bony fishes ( Nelson, 1989), and also in recognition for his pivotal contributions to the knowledge of fish diversity as a whole.

Figures

FIGURE 1. Melanorivulus nelsoni, new species: (a) holotype, MCP 50981, 39.1 mm SL, male, Brazil, State of Mato Grosso, municipality of Juscimeira; (b) paratype, NUP 18969, 23.4 mm SL, female, collected with holotype.

FIGURE 2. Live coloration of Melanorivulus nelsoni, new species: (a) paratype, NUP 18970, 29.0 mm SL, male; (b) paratype, NUP 18967, 27.5 mm SL, female; (c) paratype, NUP 18968, 35.3 mm SL, male.

FIGURE 3. Map of South America showing the distribution of Melanorivulus nelsoni, new species (star).

FIGURE 4. Collection sites of Melanorivulus nelsoni, new species, in Brazil, Mato Grosso, rio São Lourenço basin: (a) pool at córrego Beleza; (b) tributary of the córrego Beleza (type locality).