Centromochlus meridionalis , a new catfish species from the southern Amazonian limits , Mato Grosso State , Brazil ( Siluriformes : Auchenipteridae )

Centromochlus actually comprises eleven species, being the most problematic genus among the Centromochlinae, including morphologically heterogeneous taxa. The Centromochlus species have a wide distributional area on northern South America. Centromochlus meridionalis, new species, is described from headwaters of rio Teles Pires, contributor of rio Tapajós, Mato Grosso State, Brazil, and represents one of the southernmost records of a centromochlin catfish for Meridional Amazon rivers. Centromochlus meridionalis is promptly distinguished from its congeners by the small orbital diameter (relative to head length), and also by the combination of absence of first nuchal plate, anterior margin of dorsal-fin spine smooth, six branched rays in anal fin, seven pairs of ribs and 34 vertebrae. They are small sized catfishes with adults between 33 to 61 mm in standard length. The modified male anal fin is conspicuous, with the third unbranched ray enlarged, about twice the width of first unbranched ray. The new species inhabits a region strongly endangered by environmental changes due to expansion of agropecuary activities on Brazilian Amazon, which include this species in an uncertain situation regarding the conservation status of its natural population.

The rio Teles Pires, together with rio Juruena, are tributaries of the clear-water rio Tapajós.The rio Tapajós basin crosses the Brazilian states of Mato Grosso and Pará, extending over more than 900 km since headwaters to its mouth into the rio Amazonas, at Santarém, Pará.The aquatic ecorregion Tapajós-Juruena includes the upper reaches of rio Tapajós, upstream from the confluence of rios Juruena and Teles Pires, to the middle stretch, where it receives the rio Jamanxin (Barthem et al., 2003).The rio Tapajós headwaters are inhabited by a characteristic fish fauna, mostly endemic, represented by small catfishes, characins and electric fishes (Netto-Ferreira et al., 2009;Zanata et al., 2009;Campos-da-Paz, 2002;Vari et al., 2012).
During recent fieldwork in the headwater streams of rio Teles Pires, in the Municipality of Claudia, Mato Grosso State, a small distinctly colored auchenipterid catfish was found, with distinctive blunt snout and body shape, which is described herein and represents the southernmost record of a Centromochlus species in the Brazilian Amazon.

Material and Methods
Osteological features were examined in cleared and stained (c&s) specimens prepared according to the procedures of Taylor & Van Dyke (1985).Prior to clearing and staining, specimens were dissected when possible to determine gut contents, sexual maturity of gonads, and to check for myological characteristics.Osteological data from some species poorly represented in ichthyological collections were obtained from radiographs (noted as "R" in the Examined Material section).Nomenclatures of osteological elements are based on Fink & Fink (1981), Arratia (2003), and on suggestions by Britto (2002:13) and Birindelli (2010:29).Most names are explained in The Zebrafish Information Network (ZFIN).Muscle names follow Sarmento- Soares & Porto (2006).Drawings were rendered from camera lucida or digital photographs preferably of cleared and stained specimens.
Straight-line measurements were made with a digital caliper, and recorded in tenths of a millimeter, based on Sarmento-Soares & Martins-Pinheiro (2008).
Counts of fin rays and bony elements were obtained from alcohol-preserved and c&s specimens.Vertebral counts included all rib-bearing centra plus five anterior, complex centrum elements without ribs, and also including the compound caudal centrum (PU1+U1) as the last element.Count of branchiostegal rays were done only in c&s specimens."Centromochlus"musaicus and "Centromochlus" simplex were ranked as incertae sedis species in Centromochlinae by Sarmento-Soares & Martins-Pinheiro (2008).These two species are herein assigned with the generic epithet under quotation marks (" ").
Institutional abbreviations are as follows: American Rostral border of cranium with mesethmoid longer than broad; premaxilla underneath with synchondral articulation; elliptical cranial fontanel, with irregular narrow opening between mesethmoid and frontals (Fig. 3).Nasal ossified as short tubular bone canal, lying between mesethmoid cornua and lateral ethmoid, not sutured to mesethmoid.Autopalatine as a rod, oriented almost parallel to longitudinal axis of body; maxilla very small, less than half the size of autopalatine; vomer short, with arrow-shaped lateral process.Jaws of equal size; premaxilla and dentary narrow with three or four rows of robust conical teeth.First nuchal plate absent; second nuchal plate slightly concave along lateral margins; third nuchal plate thin, projected laterally, with prominent tip.Epioccipital process very small.Hyomandibula broad, projected anteriorly, connected to both quadrate and metapterygoid through cartilage and deeply dentate suture.Metapterygoid conical, as a wide lamina, joined to quadrate via dentate suture (Fig. 4).Quadrate trapezoidal, with broad base, connected to preopercle, hyomandibula and metapterygoid; long preopercle ventral margins sutured to both quadrate and hyomandibula; suprapreopercle present as short canal bone; opercle laminate, ornamented and broadly subtriangular.
First and second ceratobranchials supporting single row of rakers; third and fourth ceratobranchials with two rows of rakers; fifth ceratobranchial supporting single row of rakers, expanded postero-medially to support lower pharyngeal toothplate with short conical teeth.Four epibranchials, all largely ossified except for cartilaginous ends, supporting one or two rakers each, close to articulation with ceratobranchials.Dorsal fin I,5, dorsal spine smooth anteriorly, posterior margin with minute serrations becoming progressively small towards fin base.Pectoral fin I,5, pectoral spine with 10-16 serrations along entire anterior margin, proximal ones retrorse, distal ones antrorse; 9-10 retrorse serrations along posterior margin; serrations on anterior margin smaller than posterior.Pelvic-fin i,5, margin rounded.Adipose fin small, origin at vertical through anal-fin base.Anal fin iii,6-7; analfin pterygiophores in eight rod-like proximal radials and seven cartilaginous distal radials.Caudal fin deeply forked, lobes with rounded tips, 8+9 principal rays, 17 upper and 17 lower procurrent rays.
Sexual dimorphism.Based on examination of gonads, C. meridionalis attains sexual maturity around 30-35 mm SL.In females a genital papilla is prominent, with a small fleshy tissue around opening.The genital papilla of mature males is visible as an emergent deferent duct (Fig. 5, dd).The anal fin of mature males is strongly modified with all proximal radials basally fused to each other, forming a single ossification.Third unbranched ray elongated and thickened, ending in a rounded tip, together with the slim first branched ray (Fig. 5 Epibranchials 1 and 2 rod-like; epibranchial 3 with posterior uncinate process in articulation to epibranchial 4; epibranchial 4 with laminar extension; reduced accessory cartilage, located on angle between cartilaginous ends of epibranchial 4 and ceratobranchial 4. Pharyngobranchial 1 absent; pharyngobranchial 2 short, cartilaginous, somewhat ellipsoid, placed between anteromedial cartilaginous tips of epibranchials 1 and 2; pharyngobranchial 3 elongate, ossified, with expanded posterior border; pharyngobranchial 4 ossified.Upper pharyngeal tooth plate with conical teeth, supported by pharyngobranchial 3 and 4, and also epibranchials 3 and 4.
Infraorbital 1 with ventro-lateral process restricted to anterior border of eye.Subsequent three infraorbitals thin and canal-like, in complete infraorbital series.Lateral line on body straight, inconspicuous, with ossified canal bones only anteriorly, unbranched at caudal fin.Etymology.The specific name makes reference to the record of a Centromochlus species in southern Brazilian Amazon, a region referred to as "Meridional Amazon".Other Centromochlus species were recorded for southern Amazon, such as C. schultzi from upper Xingu and C. perugiae, from Rondônia and herein registered for southwestern Mato Grosso State.These two species, however, have a wide distributional range, respectively along central brazilian plateau and also western Amazon and upper Paraguay.On the other hand, Centromochlus meridionalis is the single species in the genus originally described from Meridional Amazon, and with distribution apparently restricted to this region.

Discussion
Reports on the ichthyofaunistic composition of the rio Tapajós are scarce (e.g., Fisch-Muller et al., 2005) and the ichthyofauna of the upper reaches of this basin is still poorly known.Some auchenipterids have been recorded for the Tapajós, including species such as Tocantinsia piresi, Ageneiosus dentatus, Ageneiosus inermis, two new Ageneiosus species, Trachycorystes porosus (Lima & Ribeiro, 2011;Smerman, 2007;Ribeiro, 2011).Within the centromochlines, Tatia intermedia, Color in alcohol.Color dark brown with wavy longitudinal pale bands on dorsal shield and mid-dorsal portions of body; dorsal surface of head and dorsal fin largely dark brown; sides of body with dark brown dots, becoming sparse towards belly.Paired and anal fins pale brown with hyaline tips.Caudal fin hyaline with irregular blackish brown spots.
Live coloration.Body color dark brown mottled in black, in a reticulated pattern, on dorsal shield and mid-dorsal portions of body.Mid-ventral portions of body with scattered brown chromatophores.Fins almost hyaline, where principally the rays are mottled with pale brown spots towards base.Ventral surfaces white somewhat translucent with little scattered brown chromatophores (Fig. 6).Overall body color strongly reminiscent of that of species of Trachelyopterus, possibly due to life style associated to submersed litter banks.
Distribution.Centromochlus meridionalis was recorded so far only from headwater streams of the upper reaches of rio Tapajós, at the rio Teles Pires, Mato Grosso State (Fig. 7).Regarding global biogeographic regionalization of freshwater systems, the new species occurs in the Tapajós-Juruena ecoregion (sensu Abell et al., 2008).
Ecological notes.Centromochlus meridionalis was captured in 1 st and 2 nd order streams, with 1.22 to 3.16 m in width and 0.17 to 0.72 m in depth, characterized by clear water and slow current that varies from 0.15 to 0.36 cm/s, over sand bottom with litter, and riparian surrounding vegetation (Fig. 8).The fishes were captured under trunks and principally inset somewhat compressed submerged litter banks.It is a micro generalist carnivore that eat small fish (Moenkhausia phaeonota, Characidae), shrimps, aquatic insect larvae and nymphs, fragments of terrestrial arthropods (ants, spiders), seeds and particulate organic matter  Tatia neivai in headwaters of the rio Tapajós is in accordance with the suggestion of Lima et al. (2007) of a stream capture between headwaters of the Paraguay and Tapajós basins.The presence of such shared species across the divide of these two drainages reinforces biogeographical relationships between the upper rio Tapajós and neighbouring river drainage basins.Some species of Centromochlus, such as C. heckelii, inhabit medium-to large-sized rivers, where they occupy the upper layer of deep lotic water bodies at dusk and during the night in search of food (mostly arthropods swept by the currents).The new species described herein differs markedly from its congener by occupying small and shallow headwater streams, a kind of habitat predominantly occupied by species of Tatia.In this sense, the combination of a short and stout body, a darkly mottled color pattern, and the use of submerged litter banks for shelter in small headwater forest streams represent unusual characteristics and a considerable extension to the known habits of Centromochlus catfishes.
The new species is herein considered a member of Centromochlus due to the elongated maxilla, extending into the maxillary barbel (a condition also present in Gelanoglanis species), an elongate ventrolateral process of infraorbital 1, forming anterior border of orbit and by a longitudinal crest at the parasphenoid and orbitosphenoid for adductor arcus palatini muscle attachment (Soares-Porto, 1998;Sarmento-Soares & Porto, 2005).Centromochlus meridionalis lacks features that diagnose the other centromochlin genera, such as Tatia, Glanidium, and Gelanoglanis.In comparison to Tatia, Centromochlus meridionalis lacks the hyomandibula elongated anterodorsally not contacting the narrow metapterygoid (vs.hyomandibula in contact to metapterygoid via dentate suture  The history of the Amazonian aquatic systems and the comprehension about the emergence of such a diversified freshwater fish fauna is still challenging.4).Relative to Glanidium, the new species lacks the first nuchal plate (vs.presence in all recognized Glanidium species) (Sarmento-Soares & Martins-Pinheiro, 2013).In comparison to Gelanoglanis, the new Centromochlus meridionalis differs by the two pairs of mental barbels (vs.single pair in Gelanoglanis), vomer present (vs.absent) and by other diagnostic features stated in Soares-Porto et al. (1999).Centromochlus is under long term revisionary studies (LMSS), and investigation regarding its monophyly it out of the scope of present paper.
The new Centromochlus meridionalis is the first record of the genus at the almost unexplored upper Tapajós reaches.Centromochlus perugiae, from upper Amazon, was recently recorded for the rio Madeira drainage and upper rio Paraguay, and represents the morphologically nearest congener to C. meridionalis.Both Centromochlus perugiae and C. meridionalis have pectoral fin spine anterior margin serrations smaller and more numerous than those on posterior margin; a modified anal fin of mature males with distinctly enlarged third unbranched ray, and also a similar color pattern.Centromochlus perugiae have polygonal blotches over head and flanks, forming a reticulated pattern (vs.pale bands on dorsal shield and mid-dorsal portions of body in C. meridionalis).Although such similarities, C. meridionalis is promptly distinguished by its small eye and arrangement of cranial roof bones.Centromochlus perugiae and Centromochlus meridionalis are possibly closely related, exhibiting similar morphological features and a complimentary distribution patterns, over headwaters of neighboring Amazonian river systems.
The headwaters of the rio Tapajós basin, formed by the rio Juruena and rio Teles Pires, in western Mato Grosso State, have been more intensively sampled for fish only in recent years (e.g., Netto-Ferreira et al., 2009;Zanata et al., 2009).Neighboring headwaters include those of the rio Paraguai to the south and the rio Guaporé to the west.However, there has been little discussion of the biogeographic relationships between the Tapajós and neighboring basins (Carvalho & Albert, 2011).A relationship between upper rio Tapajós and upper rio Paraguai areas was inferred by Shibatta & Pavanelli (2005) based on the presence of sister taxa in those basins, evidenced by the distributional patterns of Batrochoglanis melanurus and B. villosus, respectively.The documented cases of shared fish species between upper rio Tapajós and upper rio Paraguai basins occur in the rio Juruena drainage (Carvalho & Bertaco 2006;Hubert & Reno, 2006;Lima et al., 2007).Lundberg et al. (1998) reported headwater-capture events between the upper rio Madeira and rio Paraguai basins, and our results are consistent with this hypothesis (the purported close relation between C. perugiae and C.
, uiii, b1).First unbranched anal-fin ray thickened and short.Second unbranched ray elongated, with an intermediate size between the neighboring first and third rays.Third unbranched ray longest, twice the width of first branched ray, bearing 13-15 segments (Fig. 5, uiii, b1).Posterior branched rays progressively shorter; last ray the smallest (Fig. 5, b6).No tegumentary keel preceding the first unbranched anal-fin ray.No modifications observed in the maxillary barbel and in the dorsal spine of males, as is usual in some Auchenipteridae, where transformed males have stiff, ossified maxillary barbels, and an elongated dorsal-fin spine (recent reports in Ferraris & Vari, 1999; Reis & Borges, 2006; Ribeiro & Py-Daniel, 2010).
Fig. 7. Map of rio Teles Pires drainage and rio Tapajós headwaters illustrating collecting sites of Centromochlus meridionalis.Red lozenge in square indicates type locality.Yellow circles refer to paratypes localities.Some symbols may represent more than one collecting locality.

Paratypes. Brazil, Mato Grosso State, rio Teles Pires basin: MNRJ
. existimatus by pectoral-fin spine 20-25% of SL (vs.29-42%) and 6 branched anal-fin rays (vs. 4 or 5).The new species is also distinguished from all congeners, except C. perugiae and C. romani, by having male modified anal fin with enlarged third unbranched ray, about twice thicker than first unbranched ray.Description.Measured adult specimens 32.6-61.6 mm SL; morphometric data in Table1.Body stout when compared to other centromochlines.Head large, robust, slightly depressed; outline of head in dorsal view elliptic, broader than long.Trunk from dorsalfin base to caudal peduncle gradually compressed.Lateral profile of head from snout tip to opercular margin slightly convex until pectoral-fin insertion.Ventral profile of head and abdomen almost flat.Ventral profile of body gently curved, concave behind anal-fin C. existimatus, C. altae, and C. perugiae by absence of anterior nuchal plate (vs.presence).It is distinguished from C. concolor, C. reticulatus, C. macracanthus, C. punctatus, and C. schultzi, by having smooth anterior margin of dorsal spine (vs.with serrae).From C. romani by a trapezoid quadrate, with metapterygoid in contact with hyomandibula (vs.enlarged quadrate, interposed between metapterygoid and hyomandibula).Further distinguished from C. heckelii and Corigin.Head integument thin, cranial roof visible; well-developed adipose eye lid; eye latero-dorsally located in anterior portion of head; mouth terminal, upper lip extended postero-laterally as well-developed fleshy rictal fold; snout margin rounded in dorsal view; anterior nostril tubular, located on anterior border of snout;posterior nostril somewhat larger, rounded, limited by small skin flap; transverse distance between anterior nostrils proportionally shorter than distance between posterior ones.Maxillary barbel short, extending posteriorly close to membranous border of opercle; mental barbel short, tip extending to pectoral-fin base, arranged in arc along ventral surface of jaw; inner mental barbel about two-thirds of length of outer mentals.Posterior process of cleithrum short, almost reaching vertical through insertion of dorsal fin spine.

Table 1 .
Morphometric data for Centromochlus meridionalis n. sp.SD = Standard deviation; N = Number of specimens examined.