Sternarchorhynchus stewarti, 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 : 346-349

STERNARCHORHYNCHUS STEWARTI SP. NOV.

( FIGS 72 , 73 ; TABLE 13)

Sternarchorhynchus curvirostris , not of Boulenger, Stewart, Barriga & Ibarra, 1987: 37 [ Ecuador, Río Napo basin].

Diagnosis: Sternarchorhynchus stewarti 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, 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 posteriorly to a vertical approximately eight to ten scales posterior of 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 extending posterior to the origin of the electroreceptive filament and sometimes beyond that point, the hyaline to slightly dusky anal fin without a distinct band of dark pigmentation either distally or basally, the possession of seven to eight teeth on the premaxilla, six to seven teeth on the outer tooth row of the dentary, three to four teeth on the inner tooth row of the dentary, 16 precaudal vertebrae, 13–15 branched pectoral-fin rays, 26–34 anterior unbranched anal-fin rays, 180– 191 total anal-fin rays, eight to nine scales above the lateral line at the midbody, 15–17 caudal-fin rays, the greatest body depth (11.4–13.7% of LEA), the distance from the anus to the anal-fin insertion (7.8–13.2% of HL), the preanal distance (12.0–13.9% of LEA), the prepectoral-fin distance (18.3–21.8% of LEA), the caudal length (6.1–7.1% of LEA), the head length (18.3–21.1% of LEA), the distance from the posterior naris to the snout (8.1–9.3% of HL), the mouth length (6.1–7.5% of HL), the snout length (61.7–65.7% of HL), the internarial distance (2.3–3.3% of HL), the head width (22.5–25.5% of HL), the head depth at the nape (50.4–59.0% of HL), the height of the branchial opening (11.8–14.5% of HL), and the tail depth (19.8– 25.7% of caudal length).

Description: Morphometric data for examined specimens in Table 13.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and slightly to distinctly curved ventrally. Mouth terminal and relatively small, with rictus located slightly anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head, with position apparently ontogenetically somewhat variable. Both structures positioned along vertical running approximately two to three orbital diameters posterior of rear margin of eye in smaller specimens and at, or up to one orbital diameter anterior of, vertical running through anterior margin of eye in larger individuals. Opening for anus and urogenital papilla ovoid, but with fleshy puckered margin.

Premaxilla with seven to eight teeth (N = 15) apparent in whole specimens. Dentary with two tooth rows; outer row with six to seven teeth and inner row with three to four teeth (N = 10).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16 (12–13 anterior; three to four transitional; N = 25).

Pectoral-fin rays ii + 13–15 [ii + 14] (N = 27). Analfin origin located anterior to vertical through margin of opercle. Anterior unbranched anal-fin rays 26–34 [27] (N = 20). Total anal-fin rays 179–191 [179] (N = 20). Scales above lateral line at midbody eight to nine [9] (N = 25). Scales present along mid-dorsal region of body to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located approximately at 64% of TL. Filament extending posteriorly to vertical running through posterior terminus of base of anal fin. Tail compressed and short, ending in small, moderate, fan-shaped caudal fin. Caudal-fin rays 15–17 [16] (N = 21).

Coloration in alcohol: Overall coloration ranging from tan to dark brown. Snout with distal portions pale even in darkly pigmented specimens. Overall lightly pigmented specimens with narrow, stripe of slightly darker pigmentation extending anteriorly from orbit for distance one-half to two-thirds length of snout. Dark pigmentation on snout masked to varying degrees in overall more darkly pigmented individuals. Darker specimens with second band of dark pigmentation running along ventral portion of snout. Two bands of dark coloration separated by very narrow, more lightly pigmented stripe along lateral surface of snout. Dorsolateral region of dark pigmentation on head forms lateral margin of narrow, somewhat obscure and more lightly coloured mid-dorsal stripe. Stripe somewhat laterally expanded over postocular region and continuous posteriorly with more lightly coloured mid-dorsal stripe on body that extends varying degrees posteriorly, sometimes to origin of electroreceptive filament. Lightly coloured stripes on head and body more obvious in darker specimens. Pectoral and anal fins in lightly pigmented specimens hyaline. Pectoral fin in darker specimens dusky with dark chromatophores concentrated over fin rays. Anal fin in dark specimens slightly dusky, with dark chromatophores overlying fin rays but without any indication of distinct dark band along the distal margin. Caudal fin with basal portions ranging from dusky to distinctly dark and with distal region hyaline.

Distribution: Sternarchorhynchus stewarti is known from the rivers of south-eastern Peru and eastern Ecuador ( Fig. 73 ).

Secondary sexual dimorphism: No sexual dimorphism was apparent in the available samples of S. stewarti .

Etymology: The species name, stewarti , is in honour of Donald Stewart, College of Environmental Science of Forestry, State University of New York, who collected the specimens that first brought this species to our attention, and who has made many contributions to our knowledge of the fishes of the western portions of the Amazon basin.

Remarks: Examination of the specimens that served, at least in part, as the basis for the citation of S. curvirostris from the Río Napo basin ( Stewart et al., 1987: 37) have shown them to be S. stewarti . Sternarchorhynchus stewarti is very similar to S. cramptoni , a species distributed in the Amazon basin from Iquitos to the eastern portions of that drainage system. Although the two species demonstrate overlap in a number of meristic and morphometric features, even the overall very dark individuals of S. stewarti lack the distinct band of dark pigmentation along the distal portions of the anal fin that is present in all examined material of S. cramptoni including overall lightly coloured individuals. These species also differ to a large extent in the tail depth and distance from the posterior nares to the snout.

Material examined

Holotype: – PERU. Amazonas : Río Marañon , log riffle, 1.57 km east-north-east of Juan Velasco Santa Maria de Nieva (4°50′S, 77°51′W), collected by N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, K. A. Capps, D. P. German, D. Osorio, 3.viii.2006; MUSM 30409 (182).

Paratypes: – PERU. Amazonas: Río Marañon , log riffle, 1.57 km east-north-east of Juan Velasco Santa Maria de Nieva (4°50′S, 77°51′W), collected with holotype; MUSM 30410, 2 (105–132); AUM 46266, 2; (164–180); UF 168165, 2 (96–126); CU 93464, 1 (74); USNM 391571, 2 (155–242); ANSP 187150, 1 (170). Río Marañon , log riffle, 1.57 km east-north-east of Juan Velasco Santa Maria de Nieva (4°50′S, 77°51′W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, K. A. Capps, D. P. German, D. Osorio, 8.viii.2006; AUM 46272, 4 (186–260); UF 168166, 3 (121–164), CU 93465, 3 (136–157); USNM 391572, 4 (133–155); ANSP 187151, 3 (116–183). Río Marañon , 6.3 km north-east of Juan Velasco, Santa Maria de Nieva (4°50′S, 77°51′W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, D. Osorio, 4.viii.2006; AUM 46267, 1 (71); UF 168167,1 (95). Río Marañon , 1.5 km east-north-east of Juan Velasco, Santa Maria de Nieva (4°50′S, 77°51′W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, D. Osorio, 4.viii.2006; AUM 46268, 1 (111); UF 168168, 1 (128); CU 93466, 1 (202). Río Nieva , 7.4 km south-south-west of Juan Velasco, Santa Maria de Nieva (4°39′36″S, 77°53′24″W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, D. Osorio, 5.viii.2006; AUM 46269, 3 (63– 120); UF 168169, 2 (160–177); CU 93467, 2 (183–215); USNM 391573, 2 (189–207); ANSP 187152, 2 (124– 200). Río Marañon , pongo above Borja, 35.5 km northeast of Juan Velasco, Santa Maria de Nieva (4°26′36″S, 77°34′54″W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, D. Osorio, 6.viii.2006; AUM 46270, 1 (251); UF 168170, 1 (253); CU 93468, 1 (226); AUM 46271, 1 (223). Río Marañon , 12 km north of Imacita (4°56′54″S, 78°20′24″W), N. K. Lujan, D. C. Werneke, D. C. Taphorn, A. S. Flecker, B. Rengifo, D. Osorio, 10.viii.2006; AUM 46273, 1 (110); UF 168171, 1 (154); CU 93469, 1(138).

Nontype specimens: – ECUADOR. Napo: Río Payamino , 23.3 km upstream from mouth in Río Napo, sandy beach on right bank (0°26′54″S, 77°06′12″W); FMNH 103364, 2 (214–222). Río Aguarico , few kilometres upstream from mouth of Río Eno (0°11′S, 76°30′W); FMNH 100616, 1 (154).

PERU. Amazonas: Río Marañon , vicinity of Santa Maria de Nieva (Santa Maria de Nieva at 4°27′36″S, 77°34′53′W), collected by D. J. Stewart, 16.iv.1980; LACM 41741–7, 8 (195–229). Loreto: Río Marañon , along south side of Río Marañon , opposite Nauta (4°30′39″S, 73°34′5′W); INHS 52735, 1 (94).

Figures

Figure 72. Sternarchorhynchus stewarti sp. nov., holotype, 182 mm total length, MUSM 30409; Peru, Amazonas, Río Marañon.

Figure 73. Map of central and northern South America showing geographical distribution of Sternarchorhynchus stewarti sp. nov. (1 = holotype locality), Sternarchorhynchus taphorni sp. nov. (2 = holotype locality), Sternarchorhynchus yepezi sp. nov. (3 = holotype locality; 4 = collecting location of possibly conspecific sample from central Amazon; under Remarks concerning that specimen and second specimen from aquarium trade possibly collected in the region of Iquitos, Peru), and Sternarchorhynchus villasboasi sp. nov. (5 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).