Sternarchorhynchus oxyrhynchus, (MULLER & TROSCHEL)

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 : 330-333

STERNARCHORHYNCHUS OXYRHYNCHUS (MÜLLER & TROSCHEL) ( FIGS 62 , 63 ; TABLE 11)

Sternarchus oxyrhynchus Müller & Troschel, 1848: 640 [type locality: Guyana, Essequibo River; locality information problematic, Remarks]. – Müller & Troschel, 1849: 16, pl. 2, figs 1, 2 [expanded description of species with accompanying illustration]. – Günther, 1870: 4 [ British Guiana; based on original description]. – Eigenmann & Eigenmann, 1891: 62 [assignment to Sternarchorhynchus ].

Sternarchorhynchus mulleri Castelnau, 1855: 95 [type locality: Guyana, Essequibo River; based on Sternarchus oxyrhynchus Müller & Troschel, 1848 ]. – Mago-Leccia, 1994: 36 [as junior synonym of S. oxyrhynchus ]. – Campos-da-Paz, 2000: 521 [as junior objective synonym of S. oxyrhynchus ].

Sternarchorhynchus oxyrhynchus, Ellis, 1913: 140 , 174 [meristic and morphometric data for holotype reported; diet; Sternarchus mormyrus and S. curvirostris placed into synonymy of S. oxyrhynchus ; not cited records of species from localities in Guyana]. – Fowler, 1951: 431 [in part; not records based on synonymy of S. mormyrus and S. curvirostris into S. oxyrhynchus ; not cited occurrence in the Amazon basin and Guianas]. – Fernández-Yépez, 1967: 18 [ Sternarchus mormyrus and S. curvirostris resurrected from synonymy of S. oxyrhynchus ]. – Mago- Leccia, 1994: 36, fig. 53 [as valid species in listing of members of genus]. – Taphorn et al., 1997: 80 [ Venezuela]. – Albert & Campos-da-Paz, 1998: 423 [phylogenetic relationships]. – Campos-da-Paz, 2000: 528, fig. 1 [recognized as valid species in key to species of Sternarchorhynchus ; head and anterior portion of body of holotype illustrated]. – Albert, 2001: 13 [in study of phylogenetic relationships]. – Albert, 2003: 501 [in listing of members of Sternarchorhynchus ]. – Lasso et al., 2004b: 142 [ Río Orinoco basin in Colombia and Venezuela]. – Lasso et al., 2004b: 181 [ Venezuela; Río Orinoco basin ]. – Crampton, 2007: 289 [widespread in Orinoco basin ; not cited occurrence in river systems outside that basin].

Diagnosis: Sternarchorhynchus oxyrhynchus is distinguished from all congeners other than S. goeldii by possessing the following combination of characters: the gape of the mouth is aligned with main axis of snout, with the gape elongate and extending posteriorly distinctly beyond the vertical through the posterior naris. Sternarchorhynchus oxyrhynchus differs from S. goeldii in the number of total vertebrae (97– 108 versus 92–94, respectively), the greatest body depth (6.3–9.2 versus 8.2–10.8 of TL, respectively), and to a notable degree in head depth at the nape (26.8–37.7 versus 35.0–44.8 of HL).

Description: Morphometric data for examined specimens in Table 11.

Lateral line extending posteriorly at least to base of caudal fin and continuing for short distance onto basal portions of fin in some specimens. Snout elongate, compressed and very slightly curved ventrally distally, but with snout having overall anteroventral orientation. Anus and urogenital papilla located ventral to head, with position demonstrating little intraspecific variation and located about two to four orbital diameters posterior of vertical through posterior margin of eye.

Premaxilla with ten functional teeth and six replacement teeth in cleared and stained specimen (N = 1; 280 mm TL). Whole specimens with ten to 13 functional teeth on premaxilla. No obvious sexual dimorphism present in form of dentary and associated dentition in examined samples. Dentary with two rows of teeth along at least anterior portion of bone. Outer row more elongate, with ten to 12 teeth in cleared and stained specimens (N = 2; 210– 240 mm TL) and with teeth in region situated short distance posterior of symphysis shifted somewhat laterally and pointed slightly dorsolaterally. Outer tooth row with six associated replacement teeth in that specimen. Mouth terminal. Gape of mouth aligned with main axis of snout, elongate and extending posteriorly distinctly beyond vertical through posterior naris ( Fig. 1A ). Lower jaw with terminal fleshy pad that extends beyond vertical through anterior limit of snout. Pad orientated dorsally and overlaps tip of snout in closed mouth.

Branchiostegal rays five; with first and second ray narrow and elongate, third to fifth rays becoming increasingly wider and with fourth and fifth rays with triangular ventral margins. Precaudal vertebrae 16–17 (13 anterior; three to four transitional; N = 12). Total vertebrae 97–108 [108].

Pectoral-fin rays ii + 12–15 [ii + 12] (N = 16). Analfin origin located anterior to vertical through opercle. Anterior unbranched anal-fin rays 24–26 [23] (N = 7). Total anal-fin rays 212–242 [215] (N = 12) [215 anal-fin rays reported by Müller & Troschel (1848: 640) in original description]. Scales above lateral line at midbody three to eight [3] (N = 12). Scales absent along mid-dorsal line from rear of head to electroreceptive filament origin. Origin of midsaggital electroreceptive filament located approximately at 54% of TL. Filament extending posteriorly beyond vertical through posterior terminus of base of anal fin for distance of approximately seven scales. Tail compressed and long, ending in small, elongate, pointed caudal fin. Caudal-fin rays 15–18 (N = 14).

Coloration in alcohol: Overall ground coloration tan to light brown. Head and body with small dark chromatophores relatively densely scattered over surfaces. Dark pigmentation somewhat more developed dorsally. Some specimens with posteriormost portion of body and all of tail distinctly darker than remain- der of body.

Pectoral fin coloration ranging from nearly completely hyaline to overall darkly pigmented, but with dark pigmentation most developed distally. Very lightly pigmented specimens with dark pigmentation limited to distal most portions of fin. Distal pigmentation on fin increasingly pronounced ontogenetically in both extent and intensity. Anal fin with variably developed dark pigmentation distally. Overall more lightly pigmented specimens often with dark pigmentation limited to posterior half of anal fin and/or forming narrow band along distal fin margin. Specimens with dark overall coloration with extent of dark pigmentation increasing both in terms of extension anteriorly and also in degree to which pigmentation extends basally from margin of fin. Distal pigmentation in some darker individuals extending forward nearly to anteriormost rays of fin and forming irregular dark band along entire fin margin. Caudal fin dark even in overall lightly pigmented individuals.

Distribution: Sternarchorhynchus oxyrhynchus is only known from the Río Orinoco basin ( Fig. 63 ; see under Remarks concerning purported occurrence of species in the Essequibo River, Guyana).

Remarks: Sternarchorhynchus was proposed by Castelnau (1855) on the basis of a species, S. mulleri , described in the same publication (this species erroneously cited by some authors as S. muelleri or S. mulleri ; Campos-da-Paz, 2000: 521). Mago-Leccia (1994: 36) noted that S. mulleri was based on S. oxyrhynchus and as such was a junior synonym of the latter species. That conclusion was reiterated by Campos-da-Paz (2000: 521), who specifically pointed out that S. mulleri was a junior objective synonym of S. oxyrhynchus under the rules of the International Code of Zoological Nomenclature.

Sternarchorhynchus oxyrhynchus and S. goeldii have a distinctive mouth form with the extent of the gape extending further posteriorly than in all other congeners and terminating distinctly posterior of the vertical through the posterior naris. The remaining species of Sternarchorhynchus have a shorter gape that fails to extend nearly as far posteriorly. The mouth form of S. oxyrhynchus was clearly illustrated by Müller & Troschel (1849: 16, pl. 2, fig. 2), albeit without the posterior naris indicated. The 108 total vertebrae in the type specimen of Sternarchus oxyrhynchus (the Sternarchorhynchus oxyrhynchus of this study; ZMB 4086) fall at the upper end of the range for that feature (97–108 vertebrae) amongst the radiographed specimens of S. oxyrhynchus , but that count lies distant from the 98–99 vertebrae present in S. goeldii , the only other species within Sternarchorhynchus characterized by this distinctive elongate gape.

All specimens examined in the course of this study that agree with the original description of S. oxyrhynchus and the type specimen of the species in terms of overall appearance, mouth form, and meristics originated within the Río Orinoco basin . In their original description of Sternarchus oxyrhynchus (the Sternarchorhynchus oxyrhynchus of this study), Müller & Troschel (1848: 640) reported that the type locality was the Essequibo River of British Guiana (= Guyana) with the collector being ‘Schomb.’ (= Schomburgk). Müller & Troschel reiterated that locality in their more encompassing description of the species that was accompanied by illustrations ( Müller & Troschel, 1849: 16, pl. 2, figs 1, 2). None of the specimens of Sternarchorhynchus that originated in the Essequibo River basin examined during this study agree with the original description of S. oxyrhynchus . The only species of Sternarchorhynchus we examined that originated in the Essequibo River system is S. freemani , a species distinguished from S. oxyrhynchus in numerous features including the presence of an unpigmented stripe along the dorsal midline from the snout to at least the origin of the electroreceptive filament (versus the lack of such a stripe in S. oxyrhynchus ), the presence of scales along the mid-dorsal portion of the body to the origin of the electroreceptive filament (versus the absence of scales along at least part of that region in S. oxyrhynchus ), and the number of anal-fin rays [170–177 in S. freemani versus 212–242 in S. oxyrhynchus , with 215 reported by Müller & Troschel (1848: 640) in the original description].

Schomburgk collected in the Río Orinoco basin in 1838–1839 in association with some of his collecting efforts in British Guiana ( Kullander & Stawikowski, 1997: 113; Riviere, 1998: 2). Given that all examined samples of S. oxyrhynchus were collected in the eastern portions of the Río Orinoco basin , this raises the possibility that some of the material reported by Müller & Troschel (1848: 1849) as having originated in the Essequibo River might represent specimens collected in the Río Orinoco basin with incorrect associated locality information.

The original description of Sternarchus oxyrhynchus ( Müller & Troschel, 1848: 640) cited a range of size of examined specimens (16–18 Zoll) presumably indicative of least two specimens at hand. The subsequent Müller & Troschel paper that discussed the species in greater detail ( Müller & Troschel, 1849: 16) lists, however, only a single length (18 Zoll), which was indicative of a unique specimen. Eigenmann (in Ellis, 1913: 142) similarly reported a unique ‘type’ as No. 4086 in the ‘Berlin Mus.’ (= ZMB) and only that specimen is now present in the ZMB holdings ( Eschmeyer, 1998; P. Bartsch, ZMB, pers. comm.). The original description possibly incorrectly listed a range of sizes despite a unique type specimen or alternatively a second syntype was lost subsequent to the original description. Given the brief timeframe between the two Müller & Troschel publications (1848, 1849), the option of an lapsus in the original description seems the more likely possibility.

In his study of the Gymnotidae (the Gymnotiformes of this study), Ellis (1913: 142) considered S. curvirostris and S. mormyrus to be junior synonyms of S. oxyrhynchus and ascribed the apparent differences between the nominal species as ‘being in part due to the size of the fish’. The concept of a monotypic Sternarchorhynchus was followed by subsequent authors (e.g., Eigenmann & Allen, 1942; Fowler, 1951) until Fernández-Yépez (1967: 18) resurrected S. mormyrus and S. curvirostris from the synonymy of S. oxyrhynchus based on differences in external features, albeit on the basis of misidentified specimens in the case of the purported S. curvirostris and probably also for S. mormyrus (see Remarks under that species). The results of our study, nonetheless, confirm the distinctiveness of those two nominal species with respect to S. oxyrhynchus .

Eigenmann (1912: 438) and Ellis (1913: 141) reported S. oxyrhynchus from Amatuk and Warraputa in the Essequibo River system of Guyana. As detailed above, none of the samples of Sternarchorhynchus that we examined from the river systems of the Guianas proved to be S. oxyrhynchus . Examination of the material that served, at least in part, as the basis of the Eigenmann and Ellis records (CAS 72248 formerly CM 1807; CAS 72246, formerly IU 12590) have shown that they are S. freemani , a species described as new in this study.

Material examined

GUYANA. Essequibo River , no specified locality (both locality information and possible number of specimens in original description problematic; see under Remarks; photograph and radiograph of type examined, with meristic and morphometric data provided by P. Bartsch, ZMB); ZMB 4086, 1 (469; type of Sternarchus oxyrhynchus Müller & Troschel ) .

VENEZUELA. Bolivar: Río Orinoco , approximately 50 km above mouth of Río Cuchivero (7°40′N, 65°57′W); ANSP 162670, 4 (268–290). Río Orinoco , near mouth of Río Caura (7°38′N, 64°52′W); ANSP 163044, 1 (283). Delta Amacuro : Río Orinoco , shallow river, north side of river across from Isla Tres Caños, 131.8 nautical miles (= 243.8 km) from sea buoy (8°39′48″N, 62°01′W); USNM 228787, 8 (1 CS, 207– 314). Río Orinoco , deep river channel, at Isla Tres Caño, 130 nautical miles (= 240.5 km) upstream from sea buoy (8°40′N, 61°59′W); USNM 228788, 1 (348); MBUCV 10886, 2 (227–297). Río Orinoco , north shore, in front of Isla Tres Caños (approximately 8°39′48″N, 62°01′W); MBUCV 11853, 1 (305); MBUCV 10691, 1 (250). MBUCV 12079, 4 (235– 275).

Figures

Figure 1. Lateral view of anterior portion of snout of A, Sternarchorhynchus oxyrhynchus, ANSP 162670; and B, Sternarchorhynchus caboclo, INPA 10594, showing differing degree of development of mouth and fold of skin at rear of rictus.

Figure 62. Sternarchorhynchus oxyrhynchus, male, 348 mm total length, USNM 228788; Venezuela, Delta de Amacuro, Río Orinoco.

Figure 63. Map of central and northern South America showing geographical distribution of Sternarchorhynchus oxyrhynchus (1 – Essequibo River, reported type locality of species, but see Remarks in species account concerning possible erroneous locality information), and Sternarchorhynchus retzeri sp. nov. (2 = holotype locality) (some symbols represent more than one locality and/or lot of specimens).