Sternopygus sarae, Torgersen & Galindo-Cuervo & Reis & Albert, 2023

Torgersen, Kevin T., Galindo-Cuervo, Aleidy M., Reis, Roberto E. & Albert, James S., 2023, A new species of barred Sternopygus (Gymnotiformes: Sternopygidae) from the Orinoco River, Neotropical Ichthyology (e 220088) 21 (1), pp. 1-30 : 8-23

Sternopygus sarae , new species urn:lsid:zoobank.org:act:3A5CDAFE-FFA7-4F03-89DA-C5E2E79A12B3

( Figs. 3–13 ; Tab. 4)

Sternopygus astrabes . —Mago-Leccia, 1994:79–80, 183 (designation of two paratypes in lot AMNH 58643 whose morphometric and meristic values fall outside the current diagnosis for S. astrabes ).

Sternopygus sp. ‘cau’. —Hulen et al., 2005:409–412, 416–426 (original mention). —Santos-Silva et al., 2008:1252, tab. 1 (mention).

Holotype. ANSP 209718, 407 mm TL (340 mm LEA), male, Venezuela, Bolívar, Río Orinoco basin, confluence of Orinoco and Caura (Las Piedras) rivers, 07°38’36”N 64°50’00”W, 20 Nov 1985, W. Saul, R. Royero, & L. Aguana.

Paratypes. All from Venezuela, Bolívar state, Orinoco basin. AMNH 58643 , 2 , 261–262 mm TL, creek tributary of Caura River near confluence of Orinoco River , approx . 07°36’N 64°55’W, 22 Nov 1985, B. Chernoff. ANSP 163043 , 7 , 161–302 mm TL, creek (possibly Caño Curimo) feeding Caura River near confluence of Caura and Orinoco rivers , 07°37’48”N 64°50’42”W, 22 Nov 1985, B. Chernoff, W. Saul, & R. Royero. ANSP 160357 , 27 , 171–392 mm TL, collected with holotype . MCP 15339 , 8 , 279–309 mm TL, collected with holotype .

Non-type. AUM 53682 , 1, 267 mm TL, Venezuela, Bolívar, Orinoco basin, Orinoco River at Caicara City , 07°38’44.5”N 66°10’46.3”W, 23 Apr 2010, N. K. Lujan, J. Birindelli & V. Meza.

Diagnosis. Sternopygus sarae can be distinguished from all other congeners by the presence of three broad, dark vertical pigment bars with irregular margins that extend across the mid-dorsum to the ventral margin of the pterygiophores on the lateral body surfaces (vs. no pigment bars in the S. aequilabiatus (Humboldt, 1805) group, S. arenatus (Eydoux & Souleyet, 1850) , S. branco Crampton, Hulen & Albert, 2004 , S. macrurus , and S. xingu Albert & Fink, 1996 ; pigment saddles that do not extend to the base of the pterygiophores in S. astrabes , juvenile S. obtusirostris , and juvenile S. sabaji , and no bars or saddles in adult S. obtusirostris and S. sabaji ; Fig. 1 ). See Discussion for comments on the appearance and regularity of the pigment bars. Sternopygus sarae further differs from all other congeners by a unique combination of characters of head and body proportions, and osteological traits. Sternopygus sarae differs from species of the S. aequilabiatus group, and from S. arenatus , S. macrurus , S. sabaji , and S. xingu in a relatively shorter head length (HL% 9.9–12.2 vs. 12.5–19.6), from members of the S. aequilabiatus group, S. branco , S. sabaji , and S. xingu in a relatively greater interorbital distance (IO% 27.5–37.6 vs. 14.4–26.5), and from S. arenatus and S. branco in a relatively greater mouth width (MW% 16.0–19.1 vs. 11.0–13.9). Sternopygus sarae has a proportionally shorter head and more slender body than sympatic congeners ( S. astrabes and S. macrurus ). Comparisons of relative head length (HL%) between S. sarae , S. astrabes , and S. macrurus are presented in Fig. 4 . HL% values overlap slightly between S. sarae and its sympatric congener, S. astrabes . Sternopygus sarae also has a more slender body shape in lateral profile (BD%) than its sympatric congeners ( S. astrabes and S. macrurus ) with overlap only in extreme values ( Fig. 4 ). While not diagnostic, the relatively shorter HL% and BD% of S. sarae are still useful in separating most adult specimens from S. astrabes and S. macrurus .

Sternopygus sarae is most similar to S. obtusirostris from the Amazon from which it differs by a lighter body coloration (lateral surfaces pale brown vs. dark brown), a lighter coloration of pectoral and anal fin membranes (hyalin vs. dark brown), a less tapered body shape in lateral profile (TR% 19.1-21.4 vs. 24.6-25.0; Tab. 5), and a less robustly ossified neurocranium with absence of paired ventral ridges of the posterior portion of the parasphenoid anterior limb (vs. present; Fig. 5 ). Sternopygus sarae further differs from the partially sympatric S. astrabes by having a relatively smaller eye diameter (ED% 6.7– 10.6 vs 13.8–19.5), a greater taper ratio (TR% 19.1–21.4 vs. 15.2–18.5), more precaudal vertebrae (PCV 24–26 vs. 18–19), and the lack of paired ventral ridges of the posterior portion of the parasphenoid anterior limb (vs. present). Sternopygus sarae can be further distinguished from S. sabaji by 11 pores in the preopercular-mandibular laterosensory line (vs. 10), presence of conical teeth on the ventral surface of the endopterygoid (vs. no teeth), more precaudal vertebrae (PCV 24–26 vs. 21–22), more anal-fin rays (AFR 278–325 vs. 204–237), and more scales above the lateral line (SAL 16–17 vs. 12–14). Sternopygus sarae also differs from S. xingu by fewer precaudal vertebrae (PCV 24–26 vs. 28–29). Morphometric and meristic comparisons among Sternopygus species are provided in Tab. 3. The new species readily differs from another barred sternopygid, Japigny kirschbaum Meunier, Jégu & Keith, 2011 from the Atlantic coast basins of the Guiana Shield, by the possession of all unbranched anal-fin rays and a free orbital margin.

Description. Head and body shape in Fig. 3 ; cephalic mechanosensory line and pore configuration in Fig. 6 ; morphometric and meristic data in Tab. 4; and body size distribution of type series in Fig. 7 . Maximum known body size 407 mm TL (340 mm LEA). Body elongate and slender compared to sympatric congeners, S. astrabes and S. macrurus ( Fig. 8 ), somewhat compressed laterally in body cavity region, more laterally compressed in post-coelomic body region; body widest immediately behind head; whole body covered in ovoid (axially elongate) cycloid scales, except head and fins. Lateral line complete and non-interrupted, extending onto tail posterior to last anal-fin ray. Longitudinal stripe thin and extending along posterior half of body, sometimes very faint or absent. Eyes relatively small, and not covered by layer of skin. Body relatively shallow with short head length relative to most congeners. Anterior naris slightly tubular, posterior naris non-tubular. 14–16 total pectoral-fin rays. 278–325 anal-fin rays, all unbranched. 24–26 precaudal vertebrae. Pigment pattern composed of three alternating dark bars. Bars (B1–B3) composed of less densely arranged chromatophores with larger diameters. Interbars (I1, I2) composed of more densely arranged chromatophores with smaller diameters ( Fig. 9 ).

Neurocranium. Neurocranium in dorsal, lateral, and ventral views in Fig. 10 . Preorbital region of neurocranium rounded and slightly decurved in lateral view, dorsal margin convex along its entire extent from tip of mesethmoid to posterior margin of parietal, parasphenoid ventral margin slightly concave in lateral view anterior to basipterygoid process (sensu Arratia, 2013: fig. 48), or parasphenoid lateral wing (sensu Adriaens, Verraes, 1998: fig. 19b). Neurocranium relatively well-ossified compared to other gymnotiforms, all bones of braincase ossified to their peripheral margins with little or no intervening cartilaginous plates. Supraorbital canal mostly fused to frontal. Lateral ethmoid cartilage well ossified, its ventral margin extending lateral to vomer. Sphenotic small, lateral process not extending beyond lateral neurocranium margin. Foramen between parasphenoid, pterosphenoid, and orbitosphenoid relatively small, bones of ethmoid region (e.g., lateral ethmoid, ventral ethmoid) relatively well-ossified as compared to congeners. Anterior and posterior cranial fontanelles large, separated by narrow interorbital bridge, posterior fontanelle extending posterior to about vertical with anterior margin of exoccipital. Antorbital and postorbital processes of frontal robust, anterior process tapering to distal tip. Parasphenoid anterior portion wider than posterior portion; parasphenoid ventral margin flat, without longitudinal ridges on lateral margins, but with pronounced transversely oriented basipterygoid ridges.

Oral jaws. Mouth terminal to slightly inferior, anterior margin of mesethmoid extending slightly anterior to anterior margin of dentary; Premaxilla ovoid in frontal view with five unevenly arranged tooth rows on each side, with nine large straight conical teeth on posterior margin, and approximately 35 large, straight conical teeth in anterior four rows. Maxilla broad, with pronounced lateral ridge, and angled at two thirds distance of posterior blade. Dentary of intermediate length, oral margin about as long distance from mandibular symphysis to dentary-retroarticular articulation. Dentary dentition “brush-shaped” (sensu Mago-Leccia, 1978), with approximately 85 mostly recurved teeth arranged in 4–5 irregular rows near mental symphysis tapering laterally to single tooth at posterolateral margin of tooth field. Dentary anteroventral margin without small ventral process. Anterior portion of preopercular-mandibular laterosensory canal completely ossified on medial margin with three constrictions along lateral margin of dentary descending process.

Suspensorium ( Fig. 11 ). All elements of suspensorium well-ossified. Hyomandibula broad with two large foramina on dorsomedial surface (through which pass nerves V, VII, and lateralis) and four separate foramina on lateral surface (through which pass posterior, supraorbital, infraorbital, and preoperculo-mandibular rami of same nerves); for comparison see Albert et al., 2005: fig. 14. Symplectic incompletely ossified at dorsoposterior margin. Quadrate well-ossified and abutting endopterygoid but with cartilaginous margin with metapterygoid. Metapterygoid lower portion poorly ossified. Endopterygoid broad, with about 16 pointed teeth on anterior margin of medial surface, medial margin not contacting other contralateral endopterygoid at midline of palate, endopterygoid ascending process tapering dorsally, connected by thin tendon to ventral surface of frontal. Ascending process of endopterygoid slightly curved posteromedially. Palatine unossified.

Opercular series ( Fig. 11 ). Opercle well ossified with rounded dorsal, anterior, and posterior margins, dorsal margin with broad median shelf, anterior and posterior margins convex, anterior articulating process large and horn-shaped, lateral opercular surface mostly smooth with large lacunae in ventral and dorsal fields. Preopercle poorly ossified, posterior, ventral, and anterior margins ragged, anterior margin unossified.

Branchial basket. Urohyal well-ossified, posterior blade extending to third branchial arch. Basihyal fan-shaped, basibranchial of third arch cone-shaped, fourth and fifth arches unossified. Hypohyals without medial process and not contacting each other at ventral midline. Five basibranchials, anterior two slender, posterior three broad. Pharyngeal jaws large and robust, pharyngobranchials of fifth branchial arch with 18–20 large conical teeth arranged in 3–4 irregular rows, opposed to large hypobranchials with 9–10 large conical teeth arranged in three irregular rows. Ceratobranchial of fifth arch with large triangular lateral margin. 5–6 squat gill rakers, about as wide as long, in irregular rows on anterodorsal and anteroventral margins of ceratobranchials of all five gill arches.

Pectoral girdle ( Fig. 12 ). Cleithrum well-ossified, anterior portion rounded and broadly contacting contralateral cleithrum at its anterior margin, ascending portion with sharp ridge on anterior margin of lateral surface. Anterior coracoid process thin and elongate, extending halfway to anterior tip of cleithrum. Supracleithrum fused to post temporal. Mesocoracoid unossified. Scapula fused to cleithrum. Five ossified proximal pectoral fin radials, lateral three fused at their bases.

Weberian apparatus ( Fig. 13 ). Composed of anterior four vertebrae and their articulating elements. Vertebral centra increasing in size posteriorly, both in diameter and axial extent. V1 being approximately half width and thickness of V4. Parapophysis of V2 (2 nd parapophysis) expanded laterally with large facet or hollow on its anterior margin and pointed distal margin (vs. truncate with vertical lateral margin in S. astrabes ). Suspensorium anterior margins well separated, posterior margins meet at point on ventral midline (vs. meet with broad symphysis in S. astrabes ). Intercalarium narrow, less than axial thickness of V1 (vs. broader in S. astrabes and S. macrurus from Amazon basin). Tripus sickle-shaped, its anterior margin broadly contacting intercalarium across its entire extent. Its posterior margin tapering and long and thin (vs. short and robust in S. macrurus ). Lateral process of 4 th parapophysis articulating with rib relatively small, anteriorly oriented, about as wide as first vertebral centrum (vs. large and posterolaterally oriented in S. macrurus ). Scaphium large, width approximately 1.5 that of V1 (vs. small approximately 1.0 that of V 1 in S. macrurus , and 1.2 in S. astrabes ). Supraneural gracile and curved, its dorsal limb oriented vertically (vs. robust vertical limb oriented anteriorly in S. astrabes and S. macrurus ). Neural spine of fourth arch tapering distally to acute point (vs. truncate in S. macrurus ).

Color in alcohol. Base color yellow to orange or light brown, without countershading. Two to four broad vertical bars with irregular margins along body extending from dorsomedial margin to anal-fin border. Vertical bars composed of large chromatophores spread at lower density than smaller chromatophores that compose base color. Cream colored longitudinal stripe thin and extending along posterior half of body, sometimes very faint or absent. Anal fin hyaline with dark anal-fin rays. Humeral blotch small, thin, and without well-defined margins ( Figs. 3 , 8 ).

Sexual dimorphism. Direct examination of the gonads of six specimens revealed no apparent sexual dimorphism in the proportions of body depth, head length, or any other obvious characters. However, we note that the largest specimens examined (271 mm, 307 mm, 340 mm LEA) were found to be males and the smaller specimens examined were either female or too immature to reliably sex.

Geographical distribution. Sternopygus sarae is known from the confluence of Orinoco and Caura rivers ( Fig. 14 ) and from the Orinoco River at Caicara City, Venezuela.

Ecological notes. Sternopygus sarae is known from river margins and small channels on the floodplain of a sediment-rich, white water river. Most of the specimens used in this description were collected from near the confluence of the Orinoco and Caura rivers, and a single specimen from Caicara City on the Orinoco River about 150 km upstream from this confluence.

Etymology. We name this species in honor of Dr. Sara Holmberg Albert, for her perennial support to the last author. A noun in the singular genitive case. Popular name: Sara’s Longtail Knifefish.

Conservation status. This species is currently known from limited collections in the

Orinoco River near the mouth of the Caura River, with an Extension of Occurrence

(EOO) calculated by the minimum convex polygon of approximately 1,500 km 2. As very little is currently known of its actual distribution range or populational trends, and considering existing threats caused by deforestation, extensive agriculture, and gold mining in the region, we suggest the species is preliminarily assessed as Data Deficient

(DD) according to the International Union for Conservation of Nature (IUCN)

categories and criteria (IUCN Standards and Petitions Subcommittee, 2022).

Figures

FIGURE 1 | Four species of barred Sternopygus. A. Sternopygus astrabes, ANSP 162663 (189 mm TL); B. Sternopygus n. sp., ANSP 160357 (284 mm TL, paratype); C. Juvenile Sternopygus sabaji, ANSP 189018 (146 mm TL); D. Juvenile Sternopygus obtusirostris, INPA 15787 (180 mm TL), photo taken at night from Crampton et al. (2004b). Dark outlines added to bars/saddles in all photos for emphasis. Scale bars = 1 cm.

FIGURE 3 | Sternopygus sarae, holotype, ANSP 209718, male, 407 mm TL. A. Full body; B. Closeup of head. Scale bar = 1 cm.

FIGURE 4 | Ridgeline plots of relative head length (HL%) and relative body depth (BD%) as a percentage of LEA for three sympatric Sternopygus species in the Guiana Shield region. Dark red regions indicate values outside 95% confidence interval from the mean; light red indicates overlapping values outside the 95% confidence interval.

FIGURE 5 | Micro-CT renderings of the ventral surface of the neurocrania of three species of Sternopygus with the endopterygoid highlighted in red. A. Sternopygus astrabes, INHS 61503; B. Sternopygus obtusirostris, INPA 6430; C. Sternopygus sarae, ANSP 209718. Arrows indicate ridges along ventral surface of endopterygoid in S. astrabes and S. obtusirostris, which are not present in S. sarae. Note: the presence of the ridges is not a function of body size. Scale bars = 5 mm.

FIGURE 6 | Diagram of cephalic sensory canal pore configuration of the head in left lateral view of Sternopygus sarae, ANSP 160357, paratype, 274 mm TL. Lateral sensory pores indicated by circles. Centerline of canals estimated by dashed lines. Anterior and posterior nares shaded gray. Supraorbital (so), infraorbital (io), posterior (pl), preopercular-mandibular (pm), and supratemporal (st) laterosensory canals. so0 indicates the otic canal. Scale bar = 1 cm.

FIGURE 7 | Histogram of specimen sizes, reported in LEA, of all 46 specimens comprising the type series of Sternopygus sarae.

FIGURE 8 | Full body photographs of three valid and sympatric species of Sternopygus from the Orinoco River drainage A. Sternopygus macrurus, ANSP 209719 (192 mm TL); B. Sternopygus sarae, ANSP 160357 (216 mm TL); C. Sternopygus astrabes, INHS 61503 (228 mm TL). Scale bars = 1 cm.

FIGURE 9 | A. Line drawing of five paratype specimens of Sternopygus sarae (ANSP 160357). Top two drawings depict typical coloration pattern, lower three drawings depict uncommon variations of pigmentation seen in preserved specimens. Scale bar = 1 cm. Bars (B1–B3) are composed of less densely arranged chromatophores that have larger diameters. Interbars (I1, I2) are composed of more densely arranged chromatophores that have smaller diameters. Homologous chromatophore fields with barred and interbarred regions are labeled for all four of the barred Sternopygus species. B. Close-up of specimen showing differences of size and density of chromatophores making up the bar and interbar regions. Note: bar appearance differs when viewed at whole body vs close-up such that pigment patches with large chromatophores and large inter-chromatophore spaces appear darker when specimen is viewed as a whole (e.g., at arms distance, panel A as compared to lighter when viewed close up panel B).

FIGURE 10 | Sternopygus sarae neurocranium 3D rendering from micro-CT scan of holotype, ANSP 209718. A. Dorsal view; B. Lateral view; C. Ventral view. The enlarged canal bones fused to the frontal and parietal bones of the neurocranium were included in those segments. Scale bar = 1 cm.

FIGURE 11 | Sternopygus sarae suspensorium and opercular series 3D rendering from micro-CT scan of holotype, ANSP 209718. A. Lateral view of left side; B. Medial view of same side (reversed). Scale bar = 5 mm.

FIGURE 12 | Sternopygus sarae pectoral girdle 3D rendering from micro-CT scan of holotype, ANSP 209718. A. Lateral view of left side; B. Medial view of same side. Scale bar = 5 mm.

FIGURE 13 | Sternopygus sarae Weberian apparatus 3D rendering from micro-CT scan of holotype, ANSP 209718. A. Dorsal view; B. Lateral view; C. Ventral view. Scale bar = 5 mm.

FIGURE 14 | Locality collection map of all known lots of Sternopygus sarae (red) and selected S. astrabes (blue) lots from the Orinoco River drainage. Holotypes indicated by a star. Symbols may represent more than one collection.