Sternarchorhynchus caboclo, DE SANTANA & NOGUEIRA, 2006

Santana, Carlos David De & Vari, Richard P., 2010, Electric fishes of the genus Sternarchorhynchus (Teleostei, Ostariophysi, Gymnotiformes); phylogenetic and revisionary studies, Zoological Journal of the Linnean Society 159 (1), pp. 223-371 : 284-285

STERNARCHORHYNCHUS CABOCLO DE SANTANA & NOGUEIRA ( FIGS 28 , 30 ; TABLE 4)

Sternarchorhynchus oxyrhynchus , not of Müller & Troschel, Ferreira, dos Santos & Jégu, 1988: 344 [in listing of species from Rio Mucajaí in vicinity of Ilha Paredão, Roraima, Brazil].

Sternarchorhynchus caboclo de Santana & Nogueira, 2006: 89 : figs 1b, 2b, c [type locality: Brazil, Roraima, Rio Mucajaí , below Cachoeira Paredão ].

Diagnosis: Sternarchorhynchus caboclo is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the absence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body anterior of the origin of the electroreceptive filament, the lateral line that extends posteriorly to the base of the caudal fin, the dusky anal fin, the possession of four branchiostegal rays, six premaxillary teeth, 13–15 branched pectoral-fin rays, 153–174 total anal-fin rays, ten to 12 scales above the lateral line at the midbody, the greatest body depth (11.0–14.7% of LEA), the length of the base of the anal fin (79.3–86.4% of LEA), the mouth length (5.6–12.4% of HL), the snout length (44.3–61.3% of HL), the head depth at the nape (45.4–60.3% of HL), the internarial distance (3.0– 6.4% of HL), the eye diameter (4.1–10.5% of HL), the pectoral-fin length (31.3–46.9% of HL), the interocular distance (6.1–19.7% of HL), and the caudal-fin length (17.2–38.9% of caudal length).

Description: Morphometric data for examined specimens in Table 4.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and ranging from nearly straight to slightly curved ventrally. Mouth terminal and relatively small, with rictus located slightly anterior to, or along, vertical through anterior naris. Anus and urogenital papilla located ventral to head, with their position apparently somewhat ontogenetically variable in available sample. Anus and urogenital papilla in smaller individuals located along vertical slightly less than one orbital diameter posterior of rear margin of eye and very slightly anterior of vertical through anterior margin of eye in larger individuals. Combined opening for anus and urogenital papilla longitudinally ovoid in all specimens.

Premaxilla with eight to ten teeth (N = 8) apparent in whole specimens. Dentary with two tooth rows; outer row with seven to eight teeth and inner row with three to four teeth (N = 7).

Branchiostegal rays four; with first two rays narrow and elongate and fourth and fifth rays large and broad and posteriorly forked ( de Santana & Nogueira, 2006: fig. 3). Precaudal vertebrae 16 (13–14 anterior; two to three transitional; N = 18).

Pectoral-fin rays ii + 13–15 [ii + 13] (N = 58). Analfin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 20–40 [23] (N = 58). Total anal-fin rays 153–174 [168] (N = 58). Scales above lateral line at midbody 10–12 [10] (N = 58). Scales present along mid-dorsal line to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located relatively far posteriorly, approximately at 67% of TL. Filament extending posteriorly to point two to four scales beyond vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate caudal fin. Caudal-fin rays 12–15 [12] (N = 33).

Coloration in alcohol: Overall coloration dark brown. Head dark overall with stripe of slightly darker pigmentation extending anteriorly from orbit to, or nearly to, tip of snout. Slightly more lightly coloured region present on lateral surface of anterior onethird to one-half of snout. Ventral margin of snout dark. Mid-dorsal region of head dark. Body dark brown overall with mid-dorsal region uniformly dark. Pectoral fin dusky with rays covered with small dark chromatophores. Anal fin dusky with rays overlain by small, dark chromatophores. Basal portions of caudal fin dusky, but with distal portions of fin hyaline.

Distribution: Sternarchorhynchus caboclo is only known from the Rio Mucajaí basin of Roraima, Brazil ( Fig. 28 ).

Ecology: The types of S. caboclo were collected above and below a waterfall along the Rio Mucajaí. According to Ferreira et al. (1988: 341) the river in that region is characterized by steep banks and with dense forest along the shore. Large amounts of aquatic plans of the family Podostemonacease were present in the area, with the water being acidic (pH 5.8) and well oxygenated (91% saturation).

Secondary sexual dimorphism: de Santana & Nogueira (2006: 90: fig. 2b, c) noted apparent differences in the angle of the snout and depth of the posterior portion of the snout in males versus females of S. caboclo . A definitive statement on the generality of this difference requires examination of more extensive samples of the species.

Remarks: Sternarchorhynchus caboclo and S. severii were collected at the type location. The two species are readily distinguishable by the number of branchiostegal rays (four versus five, respectively), pigmentation of the mid-dorsal region of the head and body (head and body dark versus with narrow, lightly coloured stripe extending posteriorly to the origin of the electroreceptive filament, respectively), the pectoral-fin length (31.3–46.9 versus 48.7–66.6% of HL, respectively), and the depth at the eye (23.3–31.1 versus 31.7–36.0% of HL, respectively).

Material examined

BRAZIL. Roraima: Rio Branco basin, Rio Mucajaí , below Cachoeira Paredão 2 (approximately 2°57′N, 61°27′W), INPA 22894, 1 (225.0, holotype, male); INPA 10594, 60 (46.3–226, 2 CS specimens, paratypes). Rio Branco basin, Rio Mucajaí , above Cachoeira Paredão 2 (approximately 2°57′N, 61°27′W), INPA 10578, 1 (178, paratype). Rio Branco basin, Rio Mucajaí, Estrada between Fé and Esperança, INPA 10592, 2 (156–160, paratypes). Rio Branco basin, side channel of Rio Mucajaí, INPA 10541, 1 (36). Rio Branco basin, Rio Mucajaí , approximately 2 km from Cachoeira Paredão 2, mouth of tributary igarapé (approximately 2°57′N, 61°27′W); INPA 10580, 1 (193). Rio Branco basin, mouth of Igarapé Traíra, INPA 10590, 1 (200).

Figures

Figure 28. Map of central and northern South America showing geographical distribution of Sternarchorhynchus axelrodi sp. nov. (1 = holotype locality), Sternarchorhynchus britskii, Sternarchorhynchus caboclo, Sternarchorhynchus chaoi sp. nov. (2 = holotype locality), and Sternarchorhynchus cramptoni sp. nov. (3 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Figure 30. Sternarchorhynchus caboclo, holotype, male, 225 mm total length, INPA 22894; Brazil, Roraima, Rio Mucajaí.