Sternarchorhynchus mendesi, 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 : 320-324

STERNARCHORHYNCHUS MENDESI SP. NOV.

( FIGS 50 , 56 ; TABLE 10)

Diagnosis: Sternarchorhynchus mendesi 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, having the scales along the mid-dorsal portion of the body anterior to the origin of the electroreceptive filament sparse and covered by skin, the presence of a more lightly coloured narrow band of mid-dorsal pigmentation on the head and middorsal region of the body anterior of the origin of the electroreceptive filament and sometimes posterior of that point, the extension of the mid-dorsal filament to a point approximately two to three scales beyond the vertical through the posterior terminus of the anal fin, the lateral line that extends to the base of the caudal fin, the possession of ten to 11 teeth in the outer row of the dentary, 29–37 anterior unbranched anal-fin rays, 174–182 total anal-fin rays, six to seven scales above the lateral line at the midbody, 12–17 caudal-fin rays, the length of the base of the anal fin (80.8–85.9% of LEA), the caudal length (8.8–11.1% of LEA), the length of the pectoral fin (33.9–44.2% of HL), the postocular distance (38.8–40.8% of HL), the distance from the posterior naris to the eye (46.6– 55.3% of HL), the height of the branchial opening (10.2–14.1% of HL), and the tail depth (15.1–20.7% of caudal length).

Description: Morphometric data for examined specimens in Table 10.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and straight overall, but slightly curved ventrally along anterior portion. Mouth slightly anterodorsally directed, with fleshy process at anterior limit of dentary. Rictus located along vertical ranging from slightly anterior of, to slightly posterior of, anterior naris. Anus and urogenital papilla located ventral to head, with position ontogenetically variable. Openings located along vertical approximately three orbital diameters posterior of eye in smaller specimens and maximum of one eye diameter anterior of orbit in largest specimens. Combined opening for anus and urogenital papilla ranging from circular to longitudinally ovoid.

Premaxilla with six to nine teeth (N = 4) apparent in whole specimens. Dentary with two tooth rows; outer row with ten to 11 teeth and inner row with three teeth (N = 4).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 15–16 (12 anterior; three to four transitional; N = 8).

Pectoral-fin rays ii + 10–14 [ii + 14] (N = 8). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 29–37 [33] (N = 8). Total anal-fin rays 174–182 [176] (N = 8). Scales above lateral line at midbody six to nine [9] (N = 8). Scales along mid-dorsal region of body sparse and covered by thick skin. Origin of midsaggital electroreceptive filament located on posterior half of body situated approximately at 55% of TL. Filament extending posteriorly to within area delimited anterior by vertical through posterior terminus of base of anal fin and posteriorly by vertical two to three scales beyond that landmark. Tail compressed and short, ending in small, caudal fin with posteroventrally angled oblique margin. Caudal-fin rays 12–17 [12] (N = 5).

Coloration in alcohol: Overall coloration of head and body of specimens brown to dark brown. Snout dark overall on lateral surface with little or no indication of somewhat lighter central band present along at least anterior one-third of snout in some congeners. Dark pigmentation on lateral surface of snout forms lateral margin of narrow, lightly coloured stripe running along mid-dorsal region of snout. Lightly coloured mid-dorsal stripe continues onto, and expands laterally on, postocular portion of head. Expanded lightly coloured mid-dorsal region on rear of head continuous posteriorly with narrow, lightly coloured mid-dorsal Number of specimens indicated in parentheses. stripe on body. Mid-dorsal stripe on body very obvious and extending from rear of head posteriorly to beyond origin of electroreceptive filament to varying degrees. Pectoral fin dusky with small, dark chromatophores overlying fin rays. Anal fin dusky with small, dark chromatophores overlying fin rays. Caudal fin dark overall with distal regions hyaline. One small examined specimen apparently albino.

Distribution: Sternarchorhynchus mendesi is only known from the Río San Juan and Río Guanipa basins of north-eastern Venezuela, both of which drain into the Golfo de Paria ( Fig. 50 ).

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

Etymology: The species name, mendesi , is in honour of George Nilson Mendes of the Universidade Federal de Pernambuco, in recognition of his assistance to the first author during the early phases of his studies of gymnotiforms.

Material examined

Holotype: – VENEZUELA. Monagas: Department of Maturín, Morichal Largo, Río Guanipa , 5 km south of Aguasay (Aguasay at 9°25′N, 63°44′W), Golfo de Paria drainage, collected by D. C. Taphorn, L. Page et al., 2.vii.1998; MCNG 29129, (155).

Paratypes: – VENEZUELA. Monagas: Department of Maturín, Morichal Largo, Río Guanipa , 5 km south of Aguasay (Aguasay at 9°25′N, 63°44′W), Golfo de Paria drainage, collected by D. C. Taphorn, L. Page et al., with holotype, 2.vii.1998; MCNG 55850, 1 (118 CS; formerly MCNG 29129, in part). Río Guanipa , bridge on Highway 5, Golfo de Paria drainage (9°22′06″N 63°46′47″W), collected by L. Page et al., 7.i.1994; INHS 31374, 3 (77.7–157). Río de Oro, Ríos Guarapiche –San Juan basins, Golfo de Paria drainage, L. Page et al., 7.i.1994, INHS 31453, 1 (192). Department of Maturín, Río de Oro , south-west of Jusepín (Jusepín at 9°45′N, 63°31′W), Golfo de Paria drainage, collected by D. C. Taphorn, L. Page et al., 2.vii.1998; MCNG 29194, 1 (157). Department of Maturín, Río Amaná , north-west of Santa Bárbara (Santa Bárbara at 9°37′N, 63°37′W), Golfo de Paria drainage, collected by D. C. Taphorn, L. Page et al., 2.vii.1998; MCNG 29166, 1 (104).

STERNARCHORHYNCHUS MESENSIS CAMPOS- dA- PAZ

( FIGS 57 , 58 ; TABLE 10)

Sternarchorhynchus mesensis Campos-da-Paz, 2000: 531, figs 5, 6 [type locality: Brazil, Goiás, Río Tocantins (upper Río Tocantins region), at Serra de Mesa dam, municípios Minaçu/Uruaçu (‘Minaçu/ Cavalcante’), approximately 13°50′S, 18°19′W]. – Albert, 2003: 501 [in listing of members of genus]. – Triques, 2007: 125 [ Brazil, Rio Tocantins basin].

Diagnosis: Sternarchorhynchus mesensis 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 first and second branchiostegal rays of approximately the same size, the presence of a definite series of scales along the mid-dorsal region of the body, the hyaline anal fin, the lateral line extending posteriorly to the base of the caudal fin, the possession of five branchiostegal rays, 11–14 premaxillary teeth, two rows of teeth on the dentary with ten to 11 teeth in the outer row, 171–189 total anal-fin rays, 13–18 anterior unbranched anal-fin rays, 16–17 precaudal vertebrae, 11–13 scales above the lateral line at the midbody, the greatest body depth (10.0–12.3% of LEA), the preanal distance (14.2–17.0% of LEA), the prepectoral-fin distance (21.4–23.8% of LEA), the caudal length (9.0–14.3% of LEA), the distance from the anus to the insertion of the anal fin (7.5–15.4% of HL), the caudal length (9.0–14.3% of LEA), the head length (20.5–23.6% of LEA), the distance from the posterior nares to the eye (49.0–56.5% of HL), the eye diameter (3.6–4.7% of HL), the height of the branchial opening (8.6–13.1% of LEA), the head depth at the nape (39.1–45.2% of HL), the interocular distance (4.7–6.0% of HL), the distance from the posterior naris to the snout (4.3–5.6% of HL), the postocular length (36.4–41.0% of HL), the length of the pectoral fin (27.0–35.3% of HL), and the tail depth (5.5–11.4% of caudal length).

Description: Morphometric data for examined specimens in Table 10.

Lateral line extending posteriorly to base of caudal fin, but absent on fin. Snout elongate, compressed and curved ventrally. Mouth terminal with fleshy process at anterior limit of dentary. Rictus located along vertical slightly anterior of anterior naris. Anus and urogenital papilla located ventral to head, with position ontogenetically variable. Structures located approximately at vertical one orbital diameter posterior of eye in small specimens, and within area delimited posteriorly by vertical running through eye and anteriorly by vertical approximately one orbital diameter anterior of eye in larger specimens. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with 11–14 teeth (N = 5) apparent in whole specimens and nine to ten teeth (N = 3) in cleared and stained material. Dentary with two tooth rows; outer row with eight teeth and inner row with three teeth (N = 5).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16–17 (13–14 anterior; three transitional; N = 5).

Pectoral-fin rays ii + 12–13 [ii + 13] (N = 9). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 23–28 (N = 10). Total analfin rays 171–189 [181] (N = 8). Scales above lateral line at midbody 11–13 [13] (N = 10). Scales present along mid-dorsal region to origin of electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately at 60% of TL. Filament extending posteriorly to vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, caudal fin. Caudal-fin rays 13–15 [14] (N = 8).

Coloration in alcohol: Overall ground coloration ranging from light tan to brown with pigmentation darker on dorsal portions of head and body. Snout and remainder of head with narrow, lightly coloured stripe extending to rear of head. Stripe extending in some individuals onto mid-dorsal region. Stripe on head and anterior portion of body indistinct in some specimens. Anal, caudal, and pectoral fins hyaline overall, but with some dark chromatophores along margins of rays, but lacking distinct bands of dark pigmentation along distal margins of fins.

Distribution: Sternarchorhynchus mesensis is known from sites in the upper Rio Tocantins system ( Fig. 58 ) and has been collected in caves of the São Domingos karst region (see Ecology below).

Ecology: Several examined lots of S. mesensis originated in caves within the São Domingos karst region in the upper portions of the Rio Tocantins basin. The region is home to numerous epigean fishes ( Bichuette & Trajano, 2003, 2004), some of which are limited to the cave systems whereas some others occur in both subterranean and surface drainages. Sternarchorhynchus mesensis demonstrates the latter pattern, occurring in both settings, with several population samples originating in caves (MZUSP 88577, MZUSP 24993, MZUSP 62652, MZUSP 44116) and others (MZUSP 58623, MZUSP 24497) from surface waters in the vicinity of those caves. Sternarchorhynchus mesensis appears to be well adapted to epigean habitats as indicated by the overall condition and sexual maturity of the individuals from cave habitats. Given the continuity between surface and epigean waters in the São Domingos region ( Bichuette & Trajano, 2004), it is impossible to determine whether the cave populations live their entire life cycles and reproduce in that epigean settings.

Secondary sexual dimorphism: No sexual dimorphism was reported for the species by Campos-da-Paz (2000) or is apparent in examined specimens.

Remarks: The samples of S. mesensis from epigean and surface locations around São Domingos differ somewhat from those from the main river channels of the Rio Tocantins. Variation in the available samples does not unequivocally separate the São Domingos samples from individuals of S. mesensis from the type region. Samples from intervening regions are necessary to resolve the question of whether these differences represent geographical variation or a possible second form.

Sternarchorhynchus mesensis was collected at the same locality as the type series of S. axelrodi . The two species differ in various details of pigmentation along with the meristic and morphometric features detailed under Remarks for S. axelrodi .

Material examined

BRAZIL. Goiás: Rio Tocantins, upper Rio Tocantins region, municípios Minaçu and Uruaçu, at Serra da Mesa dam (approximately 13°50′S, 48°19′W); MNRJ 11611, 2 of 4 (cleared and stained paratypes), MNRJ 11613, 4 of 7 (115–127, paratypes); MZUSP 57497, 1 (170). Serra da Mesa, Município de Minaçu. MZUSP 58623, 1 (220), re-emergence of Rio Angélica /Bezerra, Parque Estadual da Terra Ronca, São Domingos; MZUSP 54100, 1 (180). Lagos of Ribeirão do Bezerra, São Domingos; MZUSP 24497, 2 (145–212). Rio Angélica, Caverna Angélica , near its point of re-emergence, below cachoeira; MZUSP 88577, 1 (81). Rio São Mateus, Gruta Matilde, Município de São Domingos; MZUSP 24993, 1 (170). Caverna São Mateus, Imbira, Matilde II; MZUSP 62652, 1 (143). Rio São Mateus, Gruta São Mateus, Imbira, São Domingos; MZUSP 44116, 1 (188).

Figures

Figure 50. Map of central and northern South America showing geographical distribution of Sternarchorhynchus jaimei (1 = holotype locality), Sternarchorhynchus kokraimoro sp. nov. (2 = holotype locality), Sternarchorhynchus mareikeae sp. nov. (3 = holotype locality), Sternarchorhynchus marreroi sp. nov. (4 = holotype locality), and Sternarchorhynchus mendesi sp. nov. (5 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Figure 56. Sternarchorhynchus mendesi sp. nov., holotype, presumably female, 155 mm total length, MCNG 29129; Venezuela, Monagas, Río Guanipa.

Figure 57. Sternarchorhynchus mesensis, paratype, 127 mm total length, MNRJ 11613; Brazil, upper Rio Tocantins, municípios de Minaçu and Uruaçu, at Serra de Mesa dam.

Figure 58. Map of central and northern South America showing geographical distribution of Sternarchorhynchus mesensis, Sternarchorhynchus montanus sp. nov. (1 = holotype locality), and Sternarchorhynchus mormyrus (some symbols represent more than one locality and/or lot of specimens).