Sternarchorhynchus

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 : 277-284

SPECIES OF STERNARCHORHYNCHUS OCCURRING IN THE UPPER RIO PARANÁ BASIN

Only a single Sternarchorhynchus species , S. britskii , is known from the upper Rio Paraná basin or indeed across the Río de La Plata system. Sternarchorhynchus britskii can be distinguished by all congeners by the features listed in the Diagnosis for that species.

SPECIES ACCOUNTS

In order to facilitate the location of individual species accounts and cross references between them, the species descriptions are arranged alphabetically within Sternarchorhynchus rather than by region of occurrence or phylogenetic affinity. Species accounts are followed by a discussion (Possible Additional Undescribed Species) of population samples that represent likely undescribed species of Sternarchorhynchus that are not formally described at this time. darker dorsally, but without any distinct pigmentation pattern other than for dark mid-dorsal coloration in those regions that lack scales.

Pectoral fin dark, more so distally and sometimes nearly completely very darkly pigmented. Anal fin dark overall with distal pigmentation distinctly darker and forming irregular band. Caudal fin largely unpigmented, but with some individuals having variable dark pigmentation approximately midway along its length.

Distribution: Most examined specimens of S. axelrodi originated at one location in the upper portions of the Rio Tocantins basin; however, a single lot of the species collected in the lower portions of that river system at Tucuruí demonstrates that the species is broadly distributed along the Rio Tocantins ( Fig. 28 ). Filament extending posteriorly to point four scales anterior of vertical through posterior terminus of base of anal fin. Tail compressed and short, ending in small, elongate, pointed caudal fin. Caudal-fin rays 16–20 [19] (N = 10).

Coloration in alcohol: Overall ground coloration brown. Head dark overall with more concentrated band of dark chromatophores present along dorsal portion of snout in region from vertical through posterior naris to slightly posterior of eye; dark pigmentation in some individuals extending anteriorly nearly to tip of snout. Body brown overall, with pigmentation Ecology: The type locality of S. axelrodi was reported as a clear water system with the bottom formed by small stones and rocks ( Vari & Harold, 2001: 164). dos Santos et al. (2004: 103; species cited therein as S. mormyrus ) report that the species grows to 600 mm TL and feeds on insects and other aquatic invertebrates.

Secondary sexual dimorphism: Sternarchorhynchus axelrodi demonstrates pronounced secondary sexual dimorphism in the overall form of the snout. Large males with mature testes have the snout slightly curved ventrally, whereas mature females with welldeveloped ovaries have a more pronounced ventral curvature of the snout (compare Figs 26 , 27 ). No secondary sexual dimorphism of the dentary and lower jaw dentition comparable to that present in some congeners was apparent in the available samples of the species.

Anal-fin ray counts arranged in groups of four starting with listed number.

Etymology: The species name, axelrodi , is in honour of Dr Herbert Axelrod, whose generous support of ichthyological research assisted with the completion of this project.

Remarks: Sternarchorhynchus axelrodi was collected at the same locality as the type series of S. mesensis . The two species differ in various features including the total number of anal-fin rays (200–217 versus 171–189, respectively), caudal-fin rays (16–20 versus 13–15, respectively), coloration of the mid-dorsal portion of the body anterior to the origin of the electroreceptive filament (with distinct, narrow, lightly coloured stripe versus dark or sometimes with indistinct lightly coloured region, respectively), the greatest body depth (13.5–17.6 versus 10.0–12.3% of LEA, respectively), the caudal length (4.9–6.7 versus 9.0–14.3% of LEA, respectively), and other features.

Material examined

Holotype: – BRAZIL. Goiás: Município de Minaçu / Cavalcante, Rio Tocantins, at future site of Usina Hidroeléctrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected by D. F. Moraes, C. N. Ricci, O. Roppa, vii.1988; MNRJ 31279 (444).

Paratypes: – BRAZIL. Goiás: Município de Minaçu / Cavalcante , Rio Tocantins, at future site of Usina Hidroelétrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected with holotype by D. F. Moraes, C. N. Ricci, O. Roppa, vii.1988; MNRJ 12168, 8 (433–530), USNM 391570, 1 (475) .

Nontype specimens: – BRAZIL. Goiás: Município de Minaçu /Cavalcante, Rio Tocantins, at future site of Usina Hidroeléctrica Serra da Mesa (approximately 13°44′S, 48°08′W), collected with holotype; MNRJ 31244,1 (not measured); MNRJ 31245, 1 (not measured); MNRJ 31246, 1 (not measured). Pará: INPA 27484, 2 (300–360); Rio Tocantins, Tucuruí, Mercado de Cametá .

STERNARCHORHYNCHUS BRITSKII CAMPOS- dA- PAZ

( FIGS 28 , 29 ; TABLE 4)

Sternarchorhynchus sp. Britski, 1972: 91 [first citation of Sternarchorhynchus from Rio Paraná basin, Brazil].

Sternarchorhynchus curvirostris , not of Boulenger, Triques, 1993: 91 [details of osteology].

H, holotype; range includes nontype specimens. Number of specimens indicated in parentheses.

Sternarchorhynchus britskii Campos-da-Paz, 2000: 529, fig. 9 [type locality: Brazil, Mato Grosso do Sul, Rio Paraná at Ilha Solteira dam (right margin), approximately 20°30′S 51°00′W]. – Albert, 2003: 500 [in listing of members of genus]. – Campos-da-Paz, 2005: 399 [additional locality information; threatened status]. – Triques, 2007: 125 [threatened status]. – da Graça & Pavanelli, 2007: 193, unnumbered fig. [ Brazil, upper Rio Paraná; threatened status]. – Crampton, 2007: 289 [ Brazil, Rio Paraná].

Diagnosis: Sternarchorhynchus britskii is distinguished from congeners by the following combination of characters: a short gape that terminates posteriorly at, or slightly short of, the vertical through anterior nares, the hyaline anal fin, 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 possession of five branchiostegal rays, seven teeth on the premaxilla, eight teeth in the outer row of the dentary, two teeth in the inner row of dentary, 15 precaudal vertebrae, 16–22 anterior unbranched analfin rays, 166–178 total anal-fin rays, 11–15 branched pectoral-fin rays, 13–15 caudal-fin rays, 16–17 precaudal vertebrae, 11–13 scales above the lateral line at the midbody, the greatest body depth (12.1–15.7% of LEA), the length of the base of the anal fin (86.4–93.0% of LEA), the prepectoral-fin distance (19.4–23.6% of LEA), the preanal distance (12.2–15.8% of LEA), the distance from the anus to the anal-fin insertion (14.4– 24.8% of HL), the head length (18.9–23.9% of LEA), the snout length (59.8–65.7% of HL), the eye diameter (2.9–4.6% of HL), the distance from the posterior naris to the eye (53.9–57.9% of HL), the head depth at the nape (44.9–57.7% of HL), the caudal length (5.4–8.5% of HL), the mouth length (5.7–7.8% of HL), the postocular distance (40.0–45.9% of HL), the pectoral-fin length (35.5–46.4% of HL), the caudal-fin length (33.3– 45.8% of caudal length), and the tail depth (18.9–25.9% of caudal length).

Diagnosis: Morphometric data for examined specimens in Table 4.

Lateral line extending posteriorly to point between two scales short of base of caudal fin, but absent on fin. Snout elongate, compressed and curved ventrally anteriorly. Sexual dimorphism in form of lower jaw and associated dentition found in many congeners not apparent in examined samples. Anus and urogenital papilla located ventral to head. Openings shift anteriorly ontogenetically, with larger specimens having those structures situated along vertical three orbital diameters anterior of vertical through anterior margin of eye. Combined opening for anus and urogenital papilla longitudinally ovoid.

Premaxilla with seven teeth apparent in whole specimens (N = 1). Dentary with two tooth rows posteriorly and one sometimes somewhat irregular row anteriorly in whole specimens. Teeth in outer row eight and in inner row two (N = 1). Mouth small and terminal, rictus situated anterior of vertical through anterior naris.

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

Pectoral-fin rays ii + 11–13 [ii + 13] (N = 13). Analfin origin located anterior to opercle. Anterior unbranched anal-fin rays 16–22 [19] (N = 13). Total anal-fin rays 166–178 [173] (N = 13). Scales above lateral line at midbody 11–13 [13] (N = 13). Scales present along mid-dorsal line in region anterior to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately at 70% of TL. Filament extending to within region delimited by verticals through point one scale short of vertical through posterior terminus of base of anal fin or point slightly beyond terminus of the base of that fin. Tail compressed and short, ending in small, elongate, slightly pointed caudal fin. Caudal-fin rays 13–15 [14] (N = 13).

Coloration in alcohol: Overall ground coloration ranging from light brown up to grey to brown. Head with anteriorly attenuating band of more concentrated dark chromatophores along dorsolateral region of snout in region from eye to vertical through posterior naris. Dark pigmentation on snout forms lateral border of narrow, lightly coloured, mid-dorsal stripe extending from snout to rear of head and widening slightly laterally along postocular portion of head (stripe indistinct, particularly in postocular region in some specimens). Remainder of head light brown other than for lightly coloured region forming anteriorly convex arch overlying preopercle in some individuals. Body pigmentation darker dorsally, but with irregular and to varying degrees obscure, narrow, more lightly coloured mid-dorsal region. Stripe sometimes continuous anteriorly with mid-dorsal stripe on head and extending posteriorly in some specimens to origin of electroreceptive filament. Pectoral fin with dark chromatophores distributed over rays and otherwise hyaline. Anal fin lightly coloured overall, with small dark chromatophores overlying rays. Caudal fin with dusky central basal spot in some individuals but otherwise hyaline over rays.

Distribution: Sternarchorhynchus britskii is only known from the upper Río Paraná basin in the states of Mato Grosso do Sul, Minas Gerais, and São Paulo, Brazil ( Fig. 28 ).

Secondary sexual dimorphism: Sternarchorhynchus britskii is not known to demonstrate sexual dimorphism.

Remarks: Britski (1972: 91) reported that S. britskii (identified therein as Sternarchorhynchus sp. ) was rare in sampled habitats with the type series having been collected in a cofferdam during the building of the Ilha da Solteira hydroelectric dam across the upper Rio Paraná. More recently, Campos-da-Paz (2005: 399) proposed that the insect larvae utilized by S. britskii may be associated with a specific habitat in flowing deep river channels. The extensive modifications of the upper Rio Paraná associated with the construction of major impoundments along the length of the main river channels throughout the basin and the resultant destruction of flowing deep river habitats favoured by S. britskii has undoubtedly adversely affected many populations of the species. As a consequence, S. britskii is now listed as threatened ( Campos-da-Paz, 2005; Triques, 2007).

Material examined

BRAZIL. Mato Grosso do Sul: Rio Paraná, at Ilha Solteira dam, cofferdam at right margin of river (approximately 20°30′S, 51°00′W), MZUSP 24468 52923, 1 (262; holotype); MZUSP 24464, 8 [163–254; six specimens from this lot designated as paratypes by Campos-da-Paz (2000), but not possible to now determine which specimens were so designated]; MBUCV 13398, 1 (244; formerly MZUSP 24464, in part; nontype specimen collected with holotype). Rio Paraná, Ilha Solteira (approximately 20°30′S, 51°00′W), MZUSP 23097, 3 (125–240). Rio Paraná, ROM 37960 , 1 (210). São Paulo: Rio Grande, Represa de Água Vermelha along boundary between state of São Paulo and Minas Gerais, MZUSP 39956, 7 (128– 232). Fernandópolis (approximately 20°15′36″S, 50°13′59″W), MZUSP 50188, 1 (265) .

Figures

Figure 26. Sternarchorhynchus axelrodi sp. nov., holotype, male, 444 mm total length, MNRJ 31279; Brazil, Goiás, Rio Tocantins.

Figure 27. Sternarchorhynchus axelrodi sp. nov., paratype, female, 493 mm total length, MNRJ 12168; Brazil, Goiás, Rio Tocantins; showing more pronounced ventral curvature of snout in mature females of species.

Figure 28. Map of central and northern South America showing geographical distribution of Sternarchorhynchus axelrodi sp. nov. (1 = holotype locality), Sternarchorhynchus britskii, Sternarchorhynchus caboclo, Sternarchorhynchus chaoi sp. nov. (2 = holotype locality), and Sternarchorhynchus cramptoni sp. nov. (3 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).

Figure 29. Sternarchorhynchus britskii, holotype, 272 mm total length, MZUSP 52923; Brazil, Mato Grosso do Sul, Rio Paraná.