Sternarchorhynchus schwassmanni, Santana & Vari, 2010

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 : 341-342

publication ID 10.1111/j.1096-3642.2009.00588.x


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scientific name

Sternarchorhynchus schwassmanni



( FIGS 68 View Figure 68 , 69 View Figure 69 ; TABLE 12)

Diagnosis: Sternarchorhynchus schwassmanni is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through the anterior nares, in having the scales along the mid-dorsal portion of the body anterior to the origin of the electroreceptive filament sparse and covered by skin to varying degrees, the presence of a more lightly coloured, narrow band of mid-dorsal pigmentation on the head and mid-dorsal region of the body extending posteriorly to the origin of the electroreceptive filament and sometimes beyond that point, the extension of the dorsal filament to a point one or two scales posterior of the vertical through the posterior terminus of the base of the anal fin, the possession of ten teeth in the outer tooth row on the dentary, 15 precaudal vertebrae, 25–26 anterior unbranched anal-fin rays, nine to ten scales above the lateral line at the midbody, 164–166 total anal-fin rays, the greatest body depth (12.4–12.7% of LEA), the length of the base of the anal fin (80.2–83.9% of LEA), the prepectoral-fin distance (21.8–22.2% of LEA), the head depth at the nape (49.5–49.6% of HL), the snout length (56.8–57.3% of SL), the mouth length (5.7– 6.3% of HL), the interocular width (9.1–9.7% of HL), the eye diameter (5.3–6.4% of HL), the distance from the posterior naris to the eye (44.2–45.4% of HL), the postocular distance (42.4–43.3% of HL), the height of the branchial opening (14.8–15.2% of HL), and the pectoral-fin length (35.5–38.2% of HL).

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 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 along, or slightly anterior of, vertical through eye. Combined opening for anus and urogenital papilla ovoid.

Premaxilla with six to eight teeth (N = 2) apparent in whole specimens. Dentary with two tooth rows; outer row with ten teeth and inner row with two to three teeth (N = 2).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15 (12 anterior; three transitional; N = 2).

Pectoral-fin rays ii + 12–13 [ii + 13] (N = 2). Anal-fin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 25–26 [25] (N = 2). Total anal-fin rays 164–166 [166] (N = 2). Scales above lateral line at midbody nine to ten [9] (N = 2). Scales present but sparse and covered to varying degrees by skin along mid-dorsal line to origin of midsaggital electroreceptive filament. Origin of electroreceptive filament located approximately at 65% of TL. Filament extending posteriorly to point one to 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 16 (N = 1).

Coloration in alcohol: Overall coloration brown. Head brown overall other than for slightly lighter region on midlateral surface of snout. Fleshy pad on anterior of dentary largely unpigmented. Darker pigmentation on snout forms lateral margin of lightly coloured, mid-dorsal stripe on snout. Mid-dorsal stripe continues posteriorly and expands very slightly laterally above postocular portion of head. Mid-dorsal stripe on head continuous posteriorly with narrow, lightly coloured, mid-dorsal stripe that extends posteriorly from rear of head to, or beyond, origin of electroreceptive filament. Posterior portion of tail darker than remainder of body. Pectoral fin dusky with rays overlain by dark chromatophores. Anal fin dusky with rays overlain by dark chromatophores. Caudal fin dark basally, but with hyaline posterior margin.

Distribution: Sternarchorhynchus schwassmanni is only known from the type locality in the Rio Araguaia of eastern Brazil ( Fig. 68 View Figure 68 ).

Etymology: The species name, schwassmanni , is in honour of Horst O. Schwassmann, University of Florida, for his contributions to the knowledge of Neotropical electric knifefishes.

Secondary sexual dimorphism: No sexual dimorphism was apparent in the limited examined sample of S. schwassmanni .

Remarks: One of the striking attributes of S. schwassmanni is its relatively small size at maturity. The 98.5 mm TL holotype is a female with developed white eggs and the 115.8 mm TL paratype is a male with developed testes.

Material examined

Holotype: – BRAZIL. Pará : Rio Araguaia , small stream between São João do Araguaia (5°23′S, 48°46′W) and São Bento (5°28′S, 48°20′W), collected by R. Stawikowski, 12.xi.1990; MZUSP 95633 (98.5, female, formerly MHNG 2566.093, in part). GoogleMaps

Paratype: – BRAZIL. Pará : Rio Araguaia , small stream between São João do Araguaia (5°23′S, 48°46′W) and São Bento (5°28′S, 48°20′W), collected with holotype by R. Stawikowski, 12.xi.1990; MHNG 2566.093, 1 (115.8, male) GoogleMaps .