Sternarchorhynchus freemani, 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 : 296-298

STERNARCHORHYNCHUS FREEMANI SP. NOV.

( FIGS 35 , 38 ; TABLE 6)

Sternarchorhynchus oxyrhynchus , not of Müller & Troschel, Eigenmann, 1912: 438 [in part, specimens from Guyana, Warraputa, and Amatuk]. – Ellis, 1913: 141, 174 [in part, specimens from Guyana, Warraputa, and Amatuk; diet].

Diagnosis: Sternarchorhynchus freemani 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 absence of scales along the mid-dorsal region of the body as far posteriorly as the vertical through posterior limit of the pectoral fin, the presence of a more lightly coloured narrow band of middorsal pigmentation on the head and mid-dorsal region of the body extending as far posteriorly as the origin of the electroreceptive filament and sometimes beyond that point, the presence of four to five premaxillary teeth, 16–17 precaudal vertebrae, 26–28 anterior unbranched anal-fin rays, 170–177 total anal-fin rays, eight to nine scales above the lateral line at the midbody, the termination of the electroreceptive filament at the vertical running through a point four scales posterior of the posterior limit of the base of the anal fin, the termination of the lateral line at a point four scales anterior of the base of the caudal fin, the greatest body depth (11.0–11.9% of LEA), the distance from the anus to the anal-fin insertion (13.0–17.4% of HL), the caudal length (15.1– 16.9% of LEA), the head width (18.8–20.4% of HL), the distance from the posterior naris to the snout (6.5–7.5% of HL), the interocular width (5.2–6.2% of HL), the postocular distance (36.1–38.7% of HL), the height of the branchial opening (9.5–10.8% of HL), the head depth at the eye (25.6–27.7% of HL), and the tail depth (8.0–8.9% of caudal length).

Description: Morphometric data for examined specimens in Table 6.

Lateral line extending posteriorly to point four scales from base of caudal fin and absent on remain- der of tail and on caudal fin. Snout elongate, compressed and slightly curved ventrally along its anterior portion. Mouth terminal. Rictus located slightly anterior to vertical through anterior naris. Anus and urogenital papilla located ventral to head, either situated approximately at vertical through eye or along vertical one eye diameter anterior to margin of eye. Combined opening for anus and urogenital papilla longitudinally ovoid in all specimens.

Premaxilla with four to five teeth (N = 2) apparent in whole specimens. Dentary with two tooth rows; outer row with eight teeth and inner row with one to two teeth (N = 2).

Branchiostegal rays five; with first to third rays narrow and elongate and fourth and fifth rays large and broad. Precaudal vertebrae 16–17 (12–14 anterior; two to four transitional; N = 4).

Pectoral-fin rays ii + 11–12 [ii + 11] (N = 4). Anal-fin origin located anterior to opercle. Anterior unbranched anal-fin rays 22–28 [22] (N = 4). Total anal-fin rays 170–177 [176] (N = 4). Scales above lateral line at midbody eight to nine [8] (N = 4). Scales absent along mid-dorsal region of body from rear of head posteriorly to point approximately along vertical through posterior limit of pectoral fin. Scaleless region followed by area with sparse mid-dorsal scales and then fully scaled region extending to origin of midsaggital electroreceptive filament. Origin of midsaggital electroreceptive filament located on posterior half of body, approximately 60% of TL. Filament extending posteriorly four scales beyond vertical through posterior terminus of base of anal fin. Tail compressed and moderate, ending in small, elongate, pointed caudal fin. Caudal-fin rays 13–16 [13] (N = 4).

Coloration in alcohol: Overall coloration of head and body ranging from tan to brown. More lightly pigmented specimen with narrow band of darker pigmentation extending anteriorly from eye nearly to tip of snout and with second darker band extending along ventral portions of snout. Two bands of darker pigmentation border more lightly pigmented region extending along lateral surface of snout. Dark bands and intervening more lightly coloured area on snout less apparent in overall darkly pigmented individuals. Dorsal dark band forms lateral margin of narrow, lightly coloured stripe running along mid-dorsal region of snout; mid-dorsal stripe terminates above eye, or slightly posterior of that point. Mid-dorsal stripe on anterior portion of head separated from mid-dorsal stripe on body by dark pigmentation on dorsal surface of head in postocular region. Body with mid-dorsal more lightly pigmented stripe extending from rear of head to origin of electroreceptive filament or to varying degrees posterior of that point. Pectoral fin in overall more lightly coloured specimens hyaline but somewhat dusky with small dark chromatophores overlying fin rays in darker individuals. Anal fin ranging from hyaline to dusky, with darker individuals having small, dark chromatophores overlying fin rays. Caudal fin dark, more so in overall darker individuals, with distal regions somewhat more lightly coloured.

Distribution: Sternarchorhynchus freemani is only known from locations in the lower portions of the Essequibo River basin in Guyana ( Fig. 35 ).

Ecology: Ellis (1913: 174) reported that S. freemani (specimens identified as S. oxyrhynchus by that author) feeds on various types of aquatic larvae and annelids.

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

Etymology: The species name, freemani , is in honour of Bryon J. Freeman of the University of Georgia who provided the senior author with invaluable assistance at the Georgia Museum of Natural History.

Remarks: Eigenmann (1912: 438) and Ellis, 1913: 141) reported S. oxyrhynchus from Amatuk and Warraputa in the Essequibo River system of Guyana. More recently Watkins et al. (2004) cited that species from streams of the Iwokrama forest along the left bank of the Essequibo. As discussed under Remarks for S. oxyrhynchus , none of the examined samples of Sternarchorhynchus from Guyana proved to be S. oxyrhynchus . Examination of the material that served at least in part as the basis of the Eigenmann and Ellis citations of S. oxyrhynchus (CAS 72248 formerly CM 1807; CAS 72246, formerly IU 12590) have shown that they are S. freemani , a new species described in this study.

Müller & Troschel (1848) reported that the material on which they based their description of S. oxyrhynchus originated in the Essequibo River, Guyana. As mentioned within the Remarks section under S. oxyrhynchus , none of the samples of that species we examined originated in the Essequibo River or any other river in the Guianas. Rather, all examined specimens of S. oxyrhynchus were captured in the Río Orinoco basin . Samples of Sternarchorhynchus that originated in the Essequibo River are all S. freemani , a species distinguished from S. oxyrhynchus by numerous features. One other species of Sternarchorhynchus was discovered in samples from the easterly flowing rivers of the Guianas; that being S. galibi , a species apparently endemic to the Marowijne River (= Fleuve Maroni) system along the border between Suriname and French Guiana. Sternarchorhynchus galibi and S. oxyrhynchus differ in diverse features, most notably in mouth form.

Material examined

Holotype: – GUYANA. Essequibo River basin, Lower Potaro River at Amatuk (approximately 5°18′N, 59°18W), collected by C. H. Eigenmann, 10.x.1908; CAS 72246, (201, male).

298 C. D. DE SANTANA and R. P. VARI

Paratypes: – GUYANA. Essequibo River basin, Lower Potaro River at Amatuk (approximately 5°18′N, 59°18W), collected by C. H. Eigenmann with the holotype, CAS 227622 ex CAS 72246 , 1 (163, female; originally two specimens, but one female was lost during the clearing and staining process but photographed and radiographed), Essequibo River basin, lower Potaro River at Warraputa cataract; CAS 72248 , 1 (185).

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 38. Sternarchorhynchus freemani sp. nov., holotype, male, 201 mm total length, CAS 72246; Guyana, Amatuk, Lower Potaro River.