Sternarchorhynchus severii, 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 : 342-344

STERNARCHORHYNCHUS SEVERII DE SANTANA & NOGUEIRA ( FIGS 68 , 70 ; TABLE 12)

Sternarchorhynchus oxyrhynchus , not of Müller & Troschel, Ferreira et al., 1988: 344 [in listing of species from Rio Mucajaí ].

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

Diagnosis: Sternarchorhynchus severii is distinguished from congeners in the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through the anterior nares, the presence of a definite series of scales along the mid-dorsal region of the body, the lateral line that extends posteriorly to the base of the caudal fin, the dorsal filament that extends to a point two scales beyond the vertical through the posterior terminus of the base of the anal fin, the presence 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 and sometimes extending posteriorly beyond that point, the dusky anal fin without a distinct dark band along the distal portions, the possession of nine to ten premaxillary teeth, seven to eight teeth on the outer row of the dentary, two to three teeth on the inner row of the dentary, 24–32 anterior unbranched anal-fin rays, 161–175 total anal-fin rays, 12–14 caudal-fin rays, the greatest body depth (12.7–14.0% of LEA), the caudal length (7.7–11.3% of LEA), the preanal distance (12.0–16.7% of LEA), the distance from the anus to the anal-fin insertion (7.8–19.9% of HL), the prepectoral-fin distance (16.4–21.2% of LEA), the caudal length (7.7–11.3% of LEA), the snout length (44.7–53.2% of HL), the head depth at the nape (53.8–72.0% of HL), the height of the branchial opening (15.4–21.7% of HL), the postocular length (45.2–56.8% of HL), the internarial distance (4.0–6.0% of HL), the pectoral-fin length (48.7–66.6% of HL), and the tail depth (14.5–24.1% of caudal length).

Description: Morphometric data for examined specimens in Table 12.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and slightly curved ventrally distally. Mouth terminal and relatively small, with rictus located slightly anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head. Those structures situated within region delimited anteriorly by vertical less than one orbital diameter anterior of eye and posteriorly by vertical less than one orbital diameter posterior of eye. Combined opening for anus and urogenital papilla circular to longitudinally ovoid.

Premaxilla with nine to ten teeth (N = 6) apparent in whole specimens. Dentary with two tooth rows; outer row with seven to eight teeth and inner row with two to three teeth (N = 6).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15–16 (12–14 anterior; two to four transitional; N = 6).

Pectoral-fin rays ii + 12–13 [ii + 12] (N = 13). Analfin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 24–32 [26] (N = 12). Total anal-fin rays 161–175 [164] (N = 12). Scales above lateral line at midbody seven to 12 [9] (N = 13). Scales clearly present along mid-dorsal line of body to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located approximately at 65% of TL. Filament extending posteriorly to point two scales anterior of vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, moderate, posteriorly rounded caudal fin. Caudal-fin rays 12–15 [15] (N = 11).

Coloration in alcohol: Overall coloration brown. Head brown overall laterally other than for slightly lighter region on midlateral surface of snout. Fleshy pad on anterior of dentary less intensely unpigmented than adjoining regions. Darker pigmentation on dorsolateral portion of snout forms lateral margin of anterior region of lightly coloured, mid-dorsal stripe on head. Lightly coloured, mid-dorsal area on head slightly laterally expanded above orbit and along anterior portion of postocular region of head and variably continuous posteriorly with narrow, mid-dorsal lightly pigmented stripe on body that extends posteriorly to, or beyond, origin of electroreceptive filament. Pectoral fin dusky with rays overlain by dark chromatophores. Anal fin dusky with rays overlain by dark chromatophores. Caudal fin very dark basally, but with hyaline region along rounded posterior margin.

Distribution: Sternarchorhynchus severii is only known from the type locality in the Rio Mucajaí , below Cachoeira Paredão ( Fig. 68 ).

Secondary sexual dimorphism: No sexual dimorphism was observed in the limited available material of S. severii .

Ecology: According to Ferreira et al. (1988: 341), the area where the samples of S. severii were collected is characterized by steep banks and a shoreline bordered by dense forest. Large amounts of aquatic plans of the family Podostemonacease were present in the river, with the water being acidic (pH 5.8) and well oxygenated (91% saturation).

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

Material examined

BRAZIL. Roraima: Rio Mucajaí , below falls of Cachoeira Paredão (approximately 2°57′N, 61°27′W); INPA 22893, 1 (161, male, holotype); INPA 22898, 13 (paratypes; 67–169, 2 cleared and stained) .

Figures

Figure 68. Map of central and northern South America showing geographical distribution of Sternarchorhynchus roseni, Sternarchorhynchus schwassmanni sp. nov. (1 = holotype locality), Sternarchorhynchus severii, and Sternarchorhynchus starksi sp. nov. (2 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Figure 70. Sternarchorhynchus severii, holotype, male, 161 mm total length; Brazil, Roraima, Rio Mucajaí.