Sternarchorhynchus gnomus, DE SANTANA & TAPHORN, 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 : 299-301

STERNARCHORHYNCHUS GNOMUS DE SANTANA & TAPHORN ( FIGS 35 , 40 ; TABLE 7)

Sternarchorhynchus gnomus de Santana & Taphorn, 2006: 2 , figs 1, 2 [type locality: Venezuela, Bolivar, Río Caroni , at confluence with Río Claro , 7°54′30″N, 63°02′50″W].

Diagnosis: Sternarchorhynchus gnomus is distinguished from congeners by the following combination

Number of specimens indicated in parentheses.

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 lateral line that extends to the base of the caudal fin, 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 or with the stripe very obscure, the hyaline anal fin lacking a dark band along its distal margin, the possession of six to eight premaxillary teeth, one row of dentary dentition with seven to eight teeth, 12 branched pectoral-fin rays, 15 precaudal vertebrae, 21–25 anterior unbranched anal-fin rays, 156–177 total anal-fin rays, six to seven scales above the lateral line at the midbody, ten to 12 caudal fin-rays, the greatest body depth (9.3–11.4% of LEA), the distance from the snout to the anus (11.8–17.4% of LEA), the preanal distance (14.1–17.7% of LEA), the prepectoral-fin distance (20.0–23.2% of LEA), the caudal length (10.1–17.4% of LEA), the head length (20.1–24.3% of LEA), the snout length (57.6–65.4% of HL), the head depth at the nape (39.7–44.9% of HL), the postocular distance (36.9–41.6% of HL), the pectoral-fin length (34.8–41.0% of HL), and the tail depth (8.3–13.8% of caudal length).

Description: Morphometric data for examined specimens in Table 7.

Lateral line extending posteriorly to approximately three to four scales anterior of base of caudal fin, but absent fin. Snout elongate, compressed and slightly curved ventrally along anterior portion. Mouth terminal. Rictus located distinctly anterior of vertical through anterior naris. Anus and urogenital papilla located ventral to head. Openings located at vertical approximately two eyes diameter posterior of margin of orbit in largest specimens. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with six teeth (N = 1) in cleared and stained specimen, seven to eight apparent in whole specimens (N = 4). Dentary with one tooth row with seven functional and five replacement teeth in cleared and stained specimen (N = 1), six to eight functional plus three replacement teeth apparent in whole specimens (N = 4).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15 (13 anterior; two transitional; N = 4).

Pectoral-fin rays ii + 12 [ii + 12] (N = 10). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 21–25 (N = 5). Total anal-fin rays 159–177 [175] (N = 9). Scales above lateral line at midbody six to seven [7] (N = 10). Scales present along mid-dorsal region of body to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately 60% of TL. Filament extending posteriorly to vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate, pointed caudal fin. Caudalfin rays ten to 12 (N = 4).

Coloration in alcohol: Overall coloration of head and body dark brown. Slightly darker bands of pigmentation extending from anterior of orbit anteriorly along dorsolateral portion of snout and along ventral portions of snout. Two dark bands border more lightly pigmented region on midlateral surface of anterior half of snout. Mid-dorsal region of head and body with narrow, more lightly coloured, irregularly interrupted stripe extending to origin of midsaggital electroreceptive filament. Pectoral and anal fins hyaline. Caudal fin dark with distal regions hyaline.

Distribution: Sternarchorhynchus gnomus is known from the lower portions of the Río Caroni basin, Venezuela ( Fig. 35 ). The type locality is now submerged by the waters of the Caruachi Reservoir ( de Santana & Taphorn, 2006: 1).

Ecology: The type locality of S. gnomus was a side channel of the Río Caroni near its confluence with the Río Claro. At that site, the water was clear with a visibility of approximately 2 m and the pH ranging from 6.6–7.6. Stomach contents of the species included chironomids, ephemeropterans, and trichopterans.

Secondary sexual dimorphism: S exually mature males of S. gnomus appear to have the ventral curvature of the snout more pronounced than do females of comparable sizes ( de Santana & Taphorn, 2006: fig. 2).

Material examined

VENEZUELA. Bolivar: Río Caroní , at its confluence with Río Claro (7°54′30″N 63°02′50″W); ANSP 182798, 1 (108.8; paratype); INPA 182798, 3 (one cleared and stained, 145.0–152.0; paratypes); MCNG 53963, 1 (133.5; holotype, female); MCNG 18435, 5 (76.5–152.0; paratypes).

Figures

Figure 35. Map of central and northern South America showing geographical distribution of Sternarchorhynchus curumim, Sternarchorhynchus curvirostris, Sternarchorhynchus freemani sp. nov. (1 = holotype locality), Sternarchorhynchus galibi sp. nov. (2 = holotype locality), and Sternarchorhynchus gnomus (some symbols represent more than one locality and/or lot of specimens).

Figure 40. Sternarchorhynchus gnomus, holotype, female, 133.5 mm total length, MCNG 53963; Venezuela, Bolivar, Río Caroní.