Batrochoglanis melanurus, Oscar Akio Shibatta & Carla Simone Pavanelli, 2005

Oscar Akio Shibatta & Carla Simone Pavanelli, 2005, Description of a new Batrochoglanis species (Siluriformes, Pseudopimelodidae) from the rio Paraguai basin, State of Mato Grosso, Brazil., Zootaxa 1092, pp. 21-30 : 23-27

Batrochoglanis melanurus new species

Figs. 1 and 2

Holotype. MZUSP 87240 (136.7 mm SL); corrego Cancela, affluent of rio Cuiaba , rio Paraguai basin ; 14º42,501 S / 56º15,850 W; Nobres ; State of Mato Grosso; Brazil; 19 April 2000; col. Nupélia.

Paratypes (all from the same locality and collector as the holotype). NUP 3430 (92.8 mm SL); 14 December 2000. MZUEL 3669 (60.8 mm SL); 18 November 20002. MZUEL 3670 (51.7 mm SL); 18 November 20002. MZUEL 3671 (49.1 mm SL); 21 May 2003.

Diagnosis. The new species can be differentiated from all of its congeners by having a homogeneous dark-brown caudal-fin coloration pattern. In addition, it can be distinguished from B. acanthochiroides ( Guentert) by having a deeper head (39.5 to 46.2 versus 26.0 to 38.7), a wider pectoral girdle (30.7 to 31.9 versus 28.4 to 30.4), and a longer adipose-fin base (16.0 to 21.5 versus 12.5 to 15.5); from B. raninus (Valenciennes) by a deeper head (39.5 to 46.2 versus 29.0 to 39.4); and from B. transmontanus (Regan) by having a longer adipose-fin base (16.0 to 21.5 versus 11.5 to 14.3), and a longer pelvic-fin length (19.8 to 22.4 versus 14.0 to 16.3).

Description. Morphometric data are presented in Table 1. Body depressed from snout to dorsal-fin origin, and posteriorly compressed. Profile gently convex from snout tip to dorsal-fin origin, slightly inclined upwards. Dorsal-fin base profile more pronouncedly convex, due to well developed musculature. Profile from dorsal-fin end to adipose-fin end almost straight (not considering adipose fin), inclined downwards. Dorsal profile more straight in young specimens (Fig. 2). Ventral profile from tip of lower jaw to anal-fin end slightly convex, almost straight. More straight on isthmus region. Profile pronounced convex in young specimens from pectoral girdle to pelvics, and from this point to anal-fin origin (Fig. 2). Procurrent rays present just after adipose and anal fins. Head large, its length about 3 times in SL; its width greater than its depth. Mouth terminal, with lower jaw a little shorter than superior. Adductor mandibulae muscles and rictal fold at the corners of the mouth well developed, giving thickset appearance to head and lower jaw. Opercular membrane large and well developed. Eye relatively small, in upper lateral position and covered by skin. Tubular anterior nostril over lip. Maxillary barbel surpassing distal opercle edge, sometimes reaching vertical through dorsal-fin origin in young specimens. Outer mental barbel reaching pectoral-fin base. Anterior cranial fontanelle extending slightly beyond posterior orbit border. Dorsal fin trapezoidal, with posterior border rounded and origin situated before midpoint of standard length; not reaching adipose-fin origin when adpressed; first lepidotrichium (“spinelet”) small and rigid, forming dorsal-fin locking mechanism; second ray elongated, forming spine; six branched rays. Adipose fin slightly elongated, with posterior border angular and free. Pectoral fin triangular, not reaching pelvic-fin origin when adpressed; first ray rigid and strongly serrate on both sides; six branched rays. Pelvic fin rounded, originating just after vertical through dorsalfin end, reaching anal-fin origin when adpressed; one unbranched and five branched rays. Anal fin rounded, its base length smaller than adipose-fin base; three to five unbranched rays and six or seven branched ones. Caudal fin almost rounded, with rounded lobes, with upper lobe longest in smaller specimens; 13 branched rays. Lateral-line pores present until just beyond adipose-fin end. Eight to eleven total gill rakers. Axillary pore absent.

Color in alcohol. Coloration pattern variable with ontogenetic development. Largest specimen, the holotype, dark-brown with blotches light-brown irregularly disposed on dorsal, pectoral, pelvic and anal fins; adipose-fin anterior and posterior borders, nape, adductor mandibulae muscles region, and median region of the flank lighter than other parts of the trunk; caudal fin entirely dark-brown. Smaller specimens (three paratypes) with coloration pattern of body and pectoral and pelvic fins dark-blotched. Regions of head, just after nape, below dorsal and adipose fins, and caudal-fin base darkened by concentration of dark dots. Dorsal and anal fins dark with median hyaline band. Smaller specimens have a dark-colored tail similar to the holotype.

Distribution. The new species is known only from the type-locality, córrego Cancela, affluent of rio Cuiabá, rio Paraguai basin, State of Mato Grosso, Brazil (Fig. 3).

Etymology. The species name melanurus, from Greek melan, black, and ouras, tail, referring to the dark coloration of the caudal fin. An adjective.

Discussion

Whereas Lundberg et al. (1991) demonstrated the monophyly of the family (subfamily in that paper), and Shibatta (2003b) determined the phylogenetic relationships among the genera, the taxonomy of pseudopimelodid species remains obscure. Three color patterns of the caudal fin in Batrochoglanis species are known. The first pattern, in what is henceforth called the “ raninus Group ”, containing B. raninus , B. transmontanus and B. acanthochiroides , is a light caudal fin, with a dark band on the posterior third. The second pattern, shown only in B. villosus , is a light caudal fin, with dark dots irregularly distributed. The third pattern, found only in B. melanurus , has the caudal fin completely dark. The coloration pattern is useful for separating species, but apparently does not allow inferences about phylogenetic relationships to be safely made, since this character is widespread in this family. A light caudal fin with a dark band is also displayed by species of Pseudopimelodus , and Microglanis , and the completely dark caudal fin probably arose independently in Batrochoglanis , Cephalosilurus , and Microglanis .

Otherwise, the variation in coloration of B. melanurus , from on ontogenetic perspective, suggests that the presence of dark blotches on the body is a plesiomorphic condition, and that the homogeneously dark coloration pattern of the body is an apomorphic condition. The same blotches occur in B. villosus , but with the formation of a homogeneous spotted pattern. The fact that the pattern with dark blotches is found in species of Pseudopimelodus , Microglanis , and Cephalosilurus reinforces this hypothesis. However, adults of B. raninus and B. transmontanus present a black body rather than a dark brown one, while in B. acanthochiroides it is spotted. The morphometric similarity between B. melanurus and B. villosus is perhaps the best clue to infer possible phylogenetic relationships. These two species would therefore compose a group, sister group of the “ raninus Group ”.

The absence of the axillary pore in B. melanurus also occurs in the other species of the genus, as well as in Microglanis species. Shibatta (2003a) presumes that this character is synapomorphic for these two genera.

The type locality of B. melanurus is situated in the upper rio Paraguai basin, near the rio Arinos one of the headwaters of the rio Tapajós, a large tributary of the Amazon basin. Several specimens of B. villosus from the rio Tapajós basin were examined (see appendix); however, none presented a dark caudal fin. Even if there previously occurred an episode of ichthyofaunal transfer by headwater capture, the elapsed time since would be enough for the new population to establish a character that differentiates it from the others (assuming a sister-species relationship between B. melanurus and B. villosus ). It is possible that other new species, sympatric to B. melanurus and related with Amazonian forms, will be discovered in the region.