Characidium summus

description

Description. Morphometric data of holotype and paratypes presented in Table 1. Body fusiform and moderately compressed. Greatest body depth at vertical through dorsal-fin origin. Dorsal profile convex from upper lip to vertical through nares, slightly convex to nearly straight from this point to end of occipital process, slightly convex to nearly straight from this point to origin of dorsal-fin base, slightly convex along dorsal-fin base, and nearly straight from end of dorsal-fin base to origin of anteriormost dorsal procurrent caudal-fin ray. Vent ral profile straight to slightly convex from anterior portion of head to pelvic-fin origin, straight from latter point to origin of anal fin, straight and posterodorsally slanted along anal-fin base and straight along caudal peduncle. Snout triangular-shaped in lateral view. Mouth subterminal, slightly more dorsal than ventral edge of orbit. Distal tip of maxilla surpassing anterior margin of orbit. Orbit rounded, horizontal length longer than snout length. Cheek thin, its depth around a third to a quarter of orbit diameter. Nares separated; anterior naris with raised margins; posterior naris considerably closer to orbit than to anterio

A new species of Characidium Reinhardt (Ostariophysi: Characiformes: Crenuchidae) from headwaters of rio Pacaás Novos, rio Madeira basin, Rondônia, Brazil

diagnosis

Diagnosis. Characidium summus is a unique species among congeners by lacking an adipose fin and having a complete lateral line. It can be further distinguished from congeners, including C. vestigipinne Buckup & Hahn, that have complete lateral line and adipose fin absent, vestigial or reduced by the absence of dark bars or spots on head, body or fins, except for a dark blotch immediately on the rear of opercle, a narrow midlateral dark stripe (less than half scale deep), and a dark basicaudal spot.

A new species of Characidium Reinhardt (Ostariophysi: Characiformes: Crenuchidae) from headwaters of rio Pacaás Novos, rio Madeira basin, Rondônia, Brazil

discussion

Remarks. Characidium summus does not fit into any group of Characidium previously proposed in the literature (e. g., Buckup & Reis, 1997; Buckup & Hahn, 2000; Graça & Pavanelli, 2008; Netto-Ferreira et al., 2013). The unique potential synapomorphy shared with clades within Characidium proposed by Buckup (1993 b) is the scaleless isthmus, one of the features of the Clade C 1. The absence of adipose fin is shared with C. mirim Netto-Ferreira, Birindelli & Buckup, C. nupelia da Graça, Pavanelli & Buckup, C. stigmosum Melo & Buckup, C. xavante da Graça, Pavanelli & Buckup, and C. vestigipinne. However, according to the hypothesis of Netto- Ferreira et al. (2013), these species belongs to the Clade C 4, supported by two synapomorphies not observed in Characidium summus: absence of the parietal branch of the supraorbital canal and absence of the inner row of teeth on dentary. Furthermore, the new species does not share the incompleteness of lateral line or color patterns of C. mirim, C. nupelia, C. stigmosum, and C. xavante. The combination of adipose fin absent and lateral line complete occurs occasionally in specimens of C. vestigipinne. According to the original description, in C. ves

A new species of Characidium Reinhardt (Ostariophysi: Characiformes: Crenuchidae) from headwaters of rio Pacaás Novos, rio Madeira basin, Rondônia, Brazil

distribution

Distribution. The new species is only known from upper rio Pacaás Novos, a tributary of rio Mamoré, rio Madeira basin, Rondônia State, Brazil (Fig. 4). Habitat and ecological notes. Characidium summus occurs in a headwater of the rio Pacaás Novos, at 920 meters above sea level, situated on the Serra dos Pacaás Novos, close to the highest peak of the Rondônia State, named Pico Tracoá, higher than 1100 m. The type locality of C. summus is within the Conservation Unit Parque Nacional de Pacaás Novos, situated in the central portion of Rondônia State. The rio Pacaás Novos is a black water river, with moderate water current and predominantly rocky bottom. Specimens of C. summus were sampled in small puddles (1 – 4 m wide and 1.5 m deep) and laterally in the river backwaters, with water temperature of 22 ºC, pH 3.58, dissolved oxygen 7 mg / L, satured oxygen 16 %, and conductivity 10 µm / cm. Only juveniles of Erythrinus erythrinus (Bloch & Schneider) were sampled syntopically with the new species. Both species have color pattern and general shape of the body remarkably similar (Fig. 1 b).

A new species of Characidium Reinhardt (Ostariophysi: Characiformes: Crenuchidae) from headwaters of rio Pacaás Novos, rio Madeira basin, Rondônia, Brazil