Rhyacoglanis seminiger, Shibatta & Vari, 2017

Shibatta, Oscar Akio & Vari, Richard P., 2017, A new genus of Neotropical rheophilic catfishes, with four new species (Teleostei: Siluriformes: Pseudopimelodidae), Neotropical Ichthyology 15 (2), No. e 160132, pp. 1-30 : 17-22

Rhyacoglanis seminiger , new species

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Figs. 11-12; Tab. 5

Holotype. LIRP 12466 , 74.2 mm SL, Brazil, Mato Grosso, Sapezal, rio Tapajós basin: rio Juruena , Pequena Central Hidrelétrica Cidezal , 13º22’39”S 59º00’57”W, 3 Jun 2010, R. J. Hilário.

Paratypes. Brazil. Mato Grosso. LIRP 8042 , 9 , 48.3- 74.8 mm SL, collected with holotype . MZUEL 14123 , 2 , 60.4-64.8 mm SL, collected with holotype . MZUSP 82085 , 3 , 44.4-70.4 mm SL, Campo Novo do Parecis, rio Tapajós basin, rio do Sangue , tributary of the rio Juruena , 13º18’56”S 57º35’42”W, 18 Feb 2003, K. de Silimon . MZUSP 118019 , 1 , 72.4 mm SL, São José do Rio Claro, rio Tapajós basin, rio Claro , tributary of rio Arinos , 13º30’15”S 56º37’6.70”W, 14 Aug 2015, O . T. Oyakawa et al .

Neotropical Ichthyology, 15(2): e160132, 2017 O. A. Shibatta & R. P. Vari

Diagnosis. Rhyacoglanis seminiger differs from all congeners in having the dark subdorsal and subadipose bands seamlessly conjoined (vs. completely separate or joined only dorsally and ventrally by narrow horizontal connecting extensions). Rhyacoglanis seminiger differs from R. annulatus in the uniformly dark caudal-peduncle band (vs. band with an unpigmented central section) and total vertebrae (32-33 vs. 34-35). Rhyacoglanis seminiger can be differentiated from R. epiblepsis by the pointed caudal-fin lobes (vs. rounded lobes), lack of intensely dark body spotting (vs. such spotting) and weak serrations limited to the base of the anterior margin of the pectoral-fin spine (vs. well developed serrations along the entire anterior margin). Rhyacoglanis seminiger additionally differs from R. pulcher in having 32-33 vertebrae (vs. 34-35).

Description. proportional measurements are given in Tab. 5. Body depressed from snout to dorsal-fin origin, progressively compressed from that point to caudal-fin base. Dorsal profile of head and body slightly convex and posterodorsally nearly straight from snout tip to dorsal-fin origin; dorsal-fin base slightly posteroventrally nearly straight; profile nearly straight from terminus of dorsal-fin base to adipose-fin origin. Ventral profile of head and body convex overall from lower-jaw margin to anus; approximately straight from anus to posterior limit of anal-fin base. Caudal peduncle profile concave dorsally and ventrally.

Head depressed; slightly longer than wide.Anterior margin convex in dorsal view. Head with unculiferous tubercles mainly dorsally. Mouth terminal and wide; width more than one-half of HL. Upper and lower jaws of equal length. Lips thick and well developed, more so on lateral surface of lower jaw and proximate to rictus. Premaxillary tooth plate posterolaterally pointed. Anterior nostril positioned immediately posterior to vertical through rictus. Eye small, superior, covered by skin, positioned slightly posterior of anterior one-third of HL. Opercular membrane well developed; margin reaching pectoral-fin origin. Maxillary-barbel base enlarged. Tip of adpressed barbel reaching pectoral-fin origin but falling short of opercular margin. Tip of adpressed inner mental barbel slightly surpassing outer mental-barbel base but falling short of barbel tip. Tip of adpressed outer mental barbel reaching margin of opercular membrane.

Dorsal fin trapezoidal overall with distal margin rounded; longest ray shorter than length of dorsal-fin base. Dorsal-fin origin immediately posterior of anterior one-third of body but anterior to midpoint of SL. Tip of adpressed dorsal fin falling short of midpoint between dorsal-fin insertion and adipose-fin origin. First dorsal-fin ray (spinelet) small, rigid, and forming dorsal-fin locking-mechanism. Second ray a spine with anterior margin smooth and posterior with retrorse serrations. Dorsal-fin rays I,6* rays (10). Adipose fin long; its base length equal to, or slightly shorter than, length of dorsal-fin base; posterior extremity free and rounded. Pectoral fin triangular overall; posterior margin rounded. Tip of adpressed pectoral fin falling short of pelvic-fin origin. First pectoral-fin ray strong, rigid and forming spine with retrorse very small serrations anteriorly. Posterior serrations extending along entire spine; largest distally. Anterior serrations distinctly smaller that posterior serrations; limited to basal one-half of spine ( Fig. 3e). Pectoral-fin rays I,6*(10). Tip of pectoral-fin spine notched. Pelvic fin triangular overall with posterior margin rounded. Pelvic-fin origin at vertical through base of penultimate dorsalfin ray. Tip of adpressed pelvic-fin reaching anal-fin origin and surpassing vertical through adipose-fin origin. Pelvic-fin rays i,5*(10). Anal fin rounded distally; its base shorter than adipose-fin base. Anal-fin rays iv,5*(6), iii,6(1), or iv,6(3). Caudal fin forked with pointed lobes; lower lobe slightly longer than upper lobe. Principal caudal-fin rays i,6,7,i*(1) or i,6,8,i(9). Dorsal procurrent rays 13(1), 14(3), 15*(3) or 16(2); ventral procurrent rays 11(1), 12*(6), or 13(2).

Posterior cleithral process well developed, pointed and reaching vertical through dorsal-fin origin. Axillary pore present. Lateral line complete. Total vertebrae 32(2) or 33*(8). Ribs 8*(9), 9(1). Gill rakers 1,1,3*(5), 1,1,4(3), 1,1,5(1), or 1,1,6(1).

Color in alcohol. Ground color light brown with dorsal portion of head slightly darker and region over the adductor mandibulae muscle less intensely pigmented. Subtle transverse dark stripe sometimes present from nape to tip of post-cleithral process; when present stripe sometimes darker on nape. Dark subdorsal and subadipose bands conjoined into wide band extending from vertical through dorsal-fin origin nearly to vertical through posterior one third of adipose-fin base. Lightly pigmented horizontal blotch sometime present within band in region between verticals through dorsalfin base terminus and beginning of adipose-fin base. Light colored blotch present at adipose-fin origin. Dark caudalpeduncle band vertically elongate with undulate anterior border and biconcave posterior margin. Some specimens with scattered dark dots over lightly colored regions of head and body. Contralateral dark bands conjoined ventrally anterior of pelvic fin, between pelvic and anal fins, and along caudal peduncle. Dorsal fin darkly pigmented other than for narrow, lightly colored band along anterodorsal margin; sometimes with lightly colored region on basal one-third.Adipose fin with dark central region. Caudal-fin lobes with dark stripes or small dots on distal one-half; dark pigmentation restricted to dorsal and ventral portions of upper and lower lobes, respectively. Anal fin with dark basal blotch anteriorly and dark stripe over posterior one-third; patches fused in some individuals. Pectoral fin with broad dark stripe extending over two-thirds of spine and covering progressively more of fin rays medially. Pelvic fin dark basally, followed by unpigmented transverse band and large dark band over most of rest of fin other than for unpigmented marginal stripe.

Geographical distribution. Rhyacoglanis seminiger is only known from the rio Tapajós basin, Mato Grosso, Brazil. The species was collected in rapids of the rio Juruena and the rio Arinos, in the foothills of the Serra do Parecis ( Fig. 4).

Ecological notes. The upper rio do Sangue, where some of the paratypes were collected, has very fast-flowing clear waters over a substrate composed nearly exclusively of quartz sand and small rocks, with the river margins covered by riparian forest. The Serra do Parecis foothills hold potential for small hydroelectric power plants that could adversely impact rheophilic species such as R. seminiger . Indeed, the rio Juruena specimens originate at one such site (Pequena Central Hidrelétrica Cidezal).

Etymology. The specific name, seminiger , is an adjective from the Latin semi = half, and niger = black, in reference to the dramatically contrasting light vs. dark coloration pattern.

suggests discrete populations with limited gene flow, or phenotypic plasticity to environmental differences. In the absence of samples from intervening regions, we tentatively consider these to be intraspecifically variable, or conspecifics. If additional samples demonstrate consistent differences, then the rio do Sangue populations may merit recogniton as a new species.

Comparative morphometry. The first three principal components axes account for 92.8% of the total variance, with the first axis accounting for 84.2%. All variable first axis loadings were positive and presumably of approximately equal magnitude, indicating it reflects the size component. The second and third principal components include positive and negative variable loadings, and help index morphological variation among individuals that helps to differentiate species ( Fig. 13). Rhyacoglanis pulcher is easily discriminated along the third component, which indexes its greater anus to analfin distance, dorsal-fin spine length, posterior nostril distance, pectoral-fin spine length, and caudal-peduncle length (all positive loadings; Tab. 6). Rhyacoglanis paranensis and R. seminiger differ from R. epiblepsis and R. annulatus by their greater dorsal-fin spine length, postcleithral process length, body depth and pectoral-fin spine length (all of which load positively on the second axis) and the lesser anus to anal-fin distance, distance between the posterior nostril and the orbit and adipose-fin length (negative loadings in the second axis). Some specimens of R. paranensis differ from R. seminiger by having longer dorsal and pectoral-fin spines, but these species are overall morphometrically very similar. Rhyacoglanis annulatus and R. epiblepsis ( Fig. 13) also share similar measurements.

Conservation status. Few specimens of R. seminiger were obtained from collections, suggesting that the species may be naturally rare. The geographic distribution of R. seminiger in the rio Tapajós basin is probably wide, as the known localities cover a broad geographic region of approximately 90,000 km 2. The construction of hydroelectric power plants potentially threaten R. seminiger , and may result in fragmentation of its distribution by destruction of its preferred habitat in rapids. However, considering the extent of occurrence exceeding 20,000 km 2, and lack of evidence indicating current population decline or fluctuations, we recommend assigning an IUCN (2016) status for R. seminiger of Least Concern (LC).

Remarks. Pigmentation features, in particular the fusion of the subdorsal and subadipose bands allows the unequivocal identification of Rhyacoglanis seminiger . Some differences exist, however, in dorsal fin and body band coloration between the rio do Sangue (some individuals with light blotches; Fig. 12) and rio Juruena samples (nearly completely dark). This variation along with the disjunct geographic distribution Unculiferous tubercles in species of Rhyacoglanis . Unculi are unicellular keratinous projections, specialized skin structures found across the main groups of Ostariophysi ( Roberts, 1982) and observed in all pseudopimelodids. All species of Rhyacoglanis have small epidermal unculiferous tubercles. These unculiferous tubercles have a mammiliform shape, are rounded to elliptical, covered by unculi of irregular penta- to heptagonal-shapes, have a prominent irregular shaped apex ( Fig. 14a). The length of their longest axis ranges from 60 to 120 μm (mean = 82.3±18.5 μm; N = 22). The unculiferous tubercles are usually more developed on the dorsal and lateral surfaces of the head and body with a highest density on the head, particularly the barbels, lips and around the orbit. On the dorsal, pectoral, pelvic and anal fins the unculiferous tubercles are limited to the rays. No unculiferous tubercles were detected ventrally on the abdomen. Unculiferous tubercles were observed in smaller specimens (16.0 mm SL, MZUSP 22505) indicating their presence is not related to sexual maturity. Unculiferous tubercles in R. annulatus and R. seminiger are larger and more conspicuous than in R. paranensis , R. epiblepsis and R. pulcher . Each unculus in R. paranensis and R. seminiger has several rounded projections with the one in the middle slightly larger ( Fig. 14b).

Unculiferous tubercles tend to be most developed in substrate-dwelling Cypriniformes and Siluriformes inhabiting swift water habitats ( Roberts, 1982). Tubercles likely decrease skin surface turbulence and, thus, reduce drag on the fish in a way comparable to denticles in shark skin ( Oeffner, Lauder, 2012) and contribute to adaptation of Rhyacoglanis to its rheophilic ecology.