Oligosarcus perdido, Alexandre C. Ribeiro, Marcel R. Cavallaro & Otávio Froehlich, 2007

Alexandre C. Ribeiro, Marcel R. Cavallaro & Otávio Froehlich, 2007, Oligosarcus perdido (Characiformes, Characidae), a new species of freshwater fish from Serra da Bodoquena, upper Rio Paraguai basin, Brazil., Zootaxa 1560, pp. 43-53 : 44-52

Oligosarcus perdido , new species

Fig. 1

Holotype: MZUSP 94691 , 97.8 mm SL, bridge over the Rio Perdido, Harmonia farm, municipality area of Porto Murtinho , State of Mato Grosso do Sul, Brazil (21°17’07”S 56°41’45”W), Froehlich, O., Cavallaro, M. R., Pomini, E. & I. S. Escote, 22 October 2005.

Paratypes: LIRP 5890 , 2, 85.8-91.1 mm SL, collected with holotype. The following tree lots were collected at type locality: LIRP 5891 , 1, 80,1 mm SL, Froehlich, O., 09 September 2005; LIRP 5894 , 2, 41,5- 43,2 mm SL, Froehlich, O., Cavallaro, M. R., Vilela, M. J. A., Almeida, N. V. A., Forster, O. C. & Vargas, R. D., December 2005; LIRP 5896 , 5, 39,9-57,7 mm SL, 2 (c&s), 57,7 and 53,4 mm SL, Froehlich, O., Cavallaro, M. R., Vilela, M. J. A., Almeida, N. V. A., Forster, O. C. & Vargas, R. D., December 2005; LIRP 5893 , 1, 25,3mm SL, Rio Perdido, upper to its underground section (21°05’42”S / 56°48’25”W), municipality area of Bonito , MS, Brazil, Froehlich, O., Cavallaro, M. R., Vilela, M. J. A., Almeida, N. V. A., Forster, O. C. & Vargas, R. D., December 2005.

Non type material: LIRP 5474 , 1, 33.0 mm SL, bridge over Corrego Seputa , a tributary of Rio Perdido at road MS-382 (21°03’52”S / 56°44’27”), municipality of Bonito , State of Mato Grosso do Sul, Brazil, Castro et al., 30 November 2004.

Diagnosis. The new species can be distinguished by the number of lateral-line scales (61-63) from O. argenteus (44-48), O. bolivianus (49-55), O. brevioris (47-55), O. longirostris (47-51), O. macrolepis (44- 46), O. menesezi (40-48), O. oligolepis (71-81), O. paranensis (47-54), O. pintoi (36-40), O. planaltinae (38-40), O. robustus , (75-85) O. schindleri (45-54), and O. solitarius (48-55). It differs in number of scales around the caudal peduncle (15-20) from O. jenynsi (21-23) and O. hepsetus (23-28). It differs from O. acutirostris by the absence of the enlarged foramen on the premaxila that accommodates the largest dentary tooth when mouth is closed.

Description. Morphometrics and meristics in table 1. Body fusiform, completely covered by cycloid scales. Greatest body depth approximately at mid-point between supraoccipital and dorsal-fin origin. Smaller body depth at caudal peduncle. Dorsal profile of head straight from snout tip to posterior terminus of supraoccipital. Slightly convex to almost straight from that point to dorsal-fin origin, between dorsal and adipose fin and slightly concave from adipose fin to caudal-fin base. Ventral profile of body slightly convex from the anteriormost region of dentary tip to pelvic-fin origin, straight from this point to anal-fin origin, slightly convex along anal-fin base, and concave at caudal peduncle (Fig.1).

Head triangular in lateral view with laterally placed eyes. Mouth terminal with flat anterior profile. Premaxilla overlapping dentary when mouth is closed. Nares located immediately anteriorly to eyes.

Adpressed pectoral fin reaching to or extending slightly beyond the origin of pelvic fin in larger specimens and distinctly short in specimens smaller then 40 mm SL. Adpressed pelvic fin extending posteriorly to anus, but not reaching origin of anal-fin. Dorsal fin inserted at vertical located posteriorly to pelvic-fin origin, just anterior to anus. Origin of anal-fin at vertical located at posterior terminus of dorsal-fin base.

Orbital ring consisting of six infraorbitals with third infraorbital much bigger than remaining elements (Fig.2A). Anterior orbital margins formed by lateral ethmoid; superior orbital margin formed by frontal; supraorbital absent. Antorbital lying immediately anterior to lateral ethmoid.

Mesethmoid pointed anteriorly, contacting frontals immediately posterior to the posterior tip of nasals. Well-developed frontal and parietal fontanelles.

Premaxilla boomerang-shaped, having a single row of conical teeth (Fig.2B). Some teeth with vestigial cusps. First and sixth tooth bigger than remaining premaxillary teeth. Anterior border of the elongate nasal bone extending over posterior border of premaxilla.

Maxilla narrow anteriorly and wide posteriorly (Fig.2B). Tip of anterior maxillary process bulbous. Maxillary with conical to slightly tricuspid teeth with about same size throughout.

Dentary narrow anteriorly and wide posteriorly (Fig.2C). Dentary with four larger anteriormost conical teeth, followed by much smaller posterior teeth, which decrease in size gradually and varying in shape from slightly tricuspid to conical. Anguloarticular forked, with a long forward extension lying against the medial surface of dentary and a short extension, projected upward and slightly forward, which delineates the posterior margin of mandible. Retroarticular small and trapezoid-shaped, located over posterior corner of anguloarticular.

Palatine small, cartilaginous anteriorly, reaching the second ectopterigoid tooth posteriorly (Fig.2C).

Ectopterygoid with slightly tricuspid to conical teeth (Fig.2C). Ectopterygoid dentition extending posteriorly to slightly anterior to a vertical at last dentary tooth. Posterior terminus of ectopterygoid overlapping antero-superior process of quadrate.

Mesopterigoid extending from a vertical through third ectoperigoid tooth to immediately anterior to metapterigoid channel (Fig.2C). Major mesopterigoid depth at its posterior portion.

Metapterygoid strangulated at mid-point in the region of the metapterigoid canal (Fig.2C). Posterodorsal margin of metapterigoid projecting over anterior hyomandibular margin along its whole extension. Metapterygoid articulation with quadrate immediately ventral to posterior notch of metapterygoid.

Anterior, upward projecting extension of quadrate wider then the posterior, horizontal extension. Symplectic elongated. Broad cartilaginous contacts between suspensorial elements. (Fig.2C).

Hyomandibular wide dorsally, narrow ventrally, with a cartilaginous area of contact with skull along its antero-dorsal corner (Fig.2C). Hyomandibular articulates with skull via both sphenotic (anteriorly) and pterotic (posteriorly).

Preopercle “L” shaped, wider at ventral region (Fig.2C). Preopercular canal ossified at dorsal aspect of preopercle.

Interopercle ellipsoid, slightly wider posteriorly. Opercle longer on dorso-ventral axis, with straight anterior outline, and posterior outline slightly convex ventrally and concave dorsally (Fig.2C).

Hyoid arch supporting four branchiostegal rays; three at anterior ceratohyal and one at posterior ceratohyal(Fig.3A). Dorsal hypohyal slightly smaller than ventral hypohyal. Anterior ceratohyal very narrow at mid-point and much wider posteriorly than anteriorly. Interhyal connected at posterodorsal corner of posterior ceratohyal (Fig.3A). Hyoid arch elements connected to each other by broad cartilages (Fig.3A). Branchial arch floor composed of four basibranchials and interconnecting cartilages (Fig.3B). Three large hypobranchials between three anteriormost basibranchials and ceratobranchials. Fourth and fifth ceratobranchials connected to basibranchials via broad cartilages. Well-developed tooth plates on fifth ceratobranchial (Fig.3C). Upper pharyngeal teeth below and anterior to the fourth epibranchial (Fig.3D). Three suspensory pharyngeals attaching branchial arches to skull (Fig.3D).

Coracoid, cleithrum, supracleithrum and posttemporal aligned in a “C”-shaped outline (Fig.4A). Postcleithrum1 located immediately posterior to the overlapping area between supracleithrum and cleithrum. Postcleithrum2 and 3 at posteroventral corner of cleithrum. Broad cartilaginous contact between cleithrum, scapula, coracoid and mesocoracoid (Fig.4B). Scapular and coracoid foramen well-developed.

Seven supraneurals anterior to dorsal-fin insertion. Dorsal-fin insertion at a vertical between pelvic and anal-fins, between neural spines of 13th to 21st vertebrae. Pelvic-fin insertion about half of distance between pectoral and anal-fin bases, between the third and seventh pleural rib. Adpressed pelvic fin not reaching analfin base. Anal fin inserted between haemal spines of the 20th and 32nd vertebrae.

Upper caudal-fin procurrent rays inserted posterior to the 32nd vertebra. Lower procurrent rays inserted porterior to 33rd vertebra. Dorsal caudal-fin lobe with two epurals, one unoreural and four hypurals (3-6). Ventral caudal-fin lobe with two hypurals (1-2). Urostyle forked, with anterior process longer than posterior one (Fig.4C).

Color in alcohol. Background color yellowish to tan. Conspicuous triangular-shaped humeral spot. Flank crossed by a dark lateral stripe extending posteriorly from humeral spot, where it is more diffuse, to caudal-fin membrane. Lateral stripe enlarged at the caudal-peduncle. A dark dorsal stripe extending from tip of supraoccipital to caudal-fin base. Scales of dorsal flanks dark pigmented below lateral strip, increasingly more pigmented towards dorsum. Flank below lateral strip almost unpigmented. Fins mostly hyaline. Dark-pigmented caudal-fin membrane between the five central-most caudal-fin rays. Membrane of dorsal and pelvic fins with dark chromatophores along anterior margins of fin rays.

Distribution. Know only from the Rio Perdido, a tributary of Rio Apa, in the upper Paraguai basin (Fig. 5) and one of its tributaries, the Seputá stream.

Ecological notes. The upper part of the Rio Perdido is situated on a carbonate plateau of about 68 km long located at the southern margin of the Brazilian Pantanal Wetland denominated Serra da Bodoquena (Bodoquena Ridge). This river drains a carstic region and has a 2 to 3 km long underground section. In the plateau, the river is dammed by calcareous tufa deposits, which form 1 to 6 meter tall sequences of waterfalls. The dammed section can be as deep as 12 meters, with vertical rock banks and very slow flow. The substrate has many logs, branches and whole trees lying on whitish calcareous clay. During the rainy season, the augmented flow disturbs the clay and the water transparency, which is usually great, is significantly reduced.

Specimens were collected in stretches with sluggish to still waters. Underwater observations were made while collecting, for a total of some 20 hours of scuba diving by two of the authors (MRC and OF) and two helpers. The species seems to be present in low densities, with only a few adult individuals observed at any time. Active adult individuals were observed from dusk to around 21:00, from near the surface to a depth of 2 meters, and always alone. After this time they could be seen resting along the vertical rocky banks. During daylight hours they remain hidden and were not seen. Young specimens, at least up to 50 mm SL, were observed in activity during the day on two occasions, in small groups (5-6), mingled with schools of Jupiaba acanthogaster (Characiformes: Characidae).

Etymology. The specific epithet “perdido” is Portuguese for “lost”. This is the name of the river basin where the new species was collected. It is a noun in apposition.