A new species of barred Sternopygus (Gymnotiformes: Sternopygidae) from the Orinoco River

Kevin T. Torgersen¹ , Aleidy M. Galindo-Cuervo², Roberto E. Reis² and James S. Albert¹

PDF: EN    XML: EN | Cite this article

Abstract​

---

Introduction​

---

With more than 1,000 described fish species, the Orinoco basin is one of the world’s hotspots of freshwater fish biodiversity (Lasso et al., 2004, 2011, 2016; Albert et al., 2011, 2020). Gymnotiform electric fishes (also called knifefishes) are an important component of the taxonomic and functional diversity of the Orinoco fauna (Lundberg et al., 1987; Albert, Crampton, 2005). Taxonomic knowledge of gymnotiform diversity in the Orinoco River has increased dramatically since the 1980s (e.g., Mago-Leccia, Zaret, 1978; Mago-Leccia et al., 1985, 1994; Lundberg, Stager, 1985; Lundberg, Mago-Leccia, 1986; de Santana, Crampton, 2011; Crampton et al., 2016). The results of these and other studies have more than tripled the number of described gymnotiform species known from the Orinoco basin from 20 to 65 over a period of 35 years (Machado-Allison, 1987; Maldonado-Ocampo, Albert, 2003; Van der Sleen, Albert, 2017; Peixoto, Waltz, 2017). These recent advances in our knowledge of gymnotiform species richness and species limits have improved our understanding of ecological and evolutionary processes (Marrero, Winemiller, 1993; Barbarino Duque, Winemiller, 2003; Winemiller, 2004; Lovejoy et al., 2010).

“Longtail electric fishes” of the genus Sternopygus Müller & Troschel, 1846 are widely distributed across the lowland river basins (<250 m elevation) of the humid Neotropics, from northern Argentina to Panama (Hulen et al., 2005; Waltz, Albert, 2017). Currently, 10 Sternopygus species are recognized as valid (Tab. 1; Hulen et al., 2005; Torgersen, Albert, 2022). However, differences in morphology (Albert, Fink, 1996), karyotypes (Santos Silva et al., 2008), and gene sequences (Maldonado-Ocampo, 2011) indicate that museum collections contain additional undescribed species. Only two Sternopygus species are known from the Orinoco basin: S. macrurus (Bloch & Schneider, 1801) (type locality unknown but in “Brazil”), and S. astrabes Mago-Leccia, 1994, which was described from a clearwater tributary of the upper Orinoco River. Sternopygus macrurus exhibits the broadest geographic distribution of all nominal gymnotiform species, with specimens ascribed to this species recorded from Pacific slope basins of Colombia to the Pampas of Argentina (Eigenmann, Ward, 1905; Eigenmann, Allen, 1942; Albert, Fink, 1996). Sternopygus macrurus is also thought to be among the most ecologically tolerant of all gymnotiform species, inhabiting water bodies of varying water chemistry (clearwater, blackwater, whitewater) and flow (riffles and runs) in lowland forests, seasonal floodplains, and even estuarine environments (Crampton, 1996, 1998a,b; Fernandes, 1999; Marceniuk et al., 2017). Due to its widespread distribution, unknown type locality, and conserved morphology, S. macrurus has long been a “wastebasket” taxon into which many specimens in museum collections have been ascribed.

TABLE 1 | Summary of all valid species of Sternopygus with information regarding primary type specimens and locality drainage for each species. Country of collection of primary types given in parenthesis.

Fishes ascribed to Sternopygus can be diagnosed from all other sternopygids by the following characters: (1) relatively larger gape (Mago-Leccia, 1978); (2) large branchial opening (Mago-Leccia, 1978); (3) long, evenly curved maxilla; (4) anterior process of maxilla extends as a narrow hook-like process (Lundberg, Mago-Leccia, 1986); (5) dorsal portion of ventral ethmoid elongate (Albert, Fink, 1996); (6) post-temporal fossa present between pterotic and epioccipital bones (Lundberg, Mago-Leccia, 1986); (7) gill rakers composed of three bony elements, the middle one with 3–10 small teeth (Mago-Leccia, 1978); (8) gill rakers not attached to branchial arches (Albert, Fink, 1996); (9) gap between parapophyses of second vertebra; (10) unossified post cleithrum (Albert, Fink, 1996); (11) long body cavity, with 18–30 precaudal vertebrae (Albert, Fink, 1996); (12) long anal fin with 170–340 rays, (13) unbranched anal-fin rays (Fink, Fink, 1981); (14) developmental origin of adult electric organ from both hypaxial and epaxial muscles (Unguez, Zakon, 1998; Albert, 2001); (15) absence of jamming avoidance response (Heiligenberg, 1991; Albert, 2001); (16) presence of a ‘medial cephalic fold’ (Triques, 2000), defined as a ridge of ectodermal tissue extending from the ventral limit of the opercular opening anteromedially to the branchial isthmus. Most Sternopygus species attain medium to large body sizes (40–50 cm Total Length (TL)), except the more diminutive S. astrabes which grows to about 20 cm TL. Most Sternopygus species are nocturnal predators of small animals (e.g., insect larvae, crustaceans) and occur in multiple habitats, including small streams, river margins, and deep river channels(Crampton et al., 2004a; Crampton, 2007, 2011; Brejão et al., 2013).

Most Sternopygus species share a similar color pattern with a base color composed of small, densely arranged gray chromatophores. Some species have a dark humeral blotch with variable contrast to the background coloration, and a distinctive yellow or white longitudinal stripe extending between the hypaxial and pterygiophore muscles on the posterior third of the body. These aspects of coloration are variable within and among nominal species and are sometimes absent, with some specimens ranging in color from deep black to pinkish white. At least three valid Sternopygus species possess a distinctive color pattern composed of 1–4 broad, dark vertical bars or saddles across the dorsal midline at some stage in their ontogeny: S. astrabes, S. obtusirostris Steindachner, 1881, S. sabaji Torgersen & Albert, 2022 (Fig. 1; Mago-Leccia, 1994; Crampton et al., 2004b; Torgersen, Albert, 2022). The monophyly, species limits, variation, and species richness of species with broad vertical pigment bars or saddles remains poorly understood and these topics are not addressed here.

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.

Here we describe a new species of barred Sternopygus from the lower Orinoco basin of Venezuela, bringingthe total number of species in the genus to 11, the number of species known in the Orinoco basin to three, the number of species in the Guiana Shield region to four, and the number of Sternopygus species possessing dark vertical bars to four.

Material and methods

---

A total of 46 specimens of the new species described herein were identified from museum lots collected in the lower Orinoco drainage of Venezuela between 1985 and 2010, with most specimens collected specifically from the confluence of the Orinoco and Caura rivers by L. Aguana, B. Chernoff, R. Royero, and W. Saul. Only specimens collected near the confluence of the Orinoco and Caura rivers were included in the type series. We were unable to deposit type specimens in Venezuela because of the ongoing political and economic instability. No animal experimentation or collection permits or approvals were necessary for the completion of this work.

Morphometric measurements followed Hulen et al. (2005). We used digital calipers and an ocular micrometer attached to an Olympus SZX12 dissecting microscope, measuring point-to-point linear distances from standard landmarks to the nearest 0.01 mm on the left side of the body when possible.

We measured: (1) length to the end of the anal fin (LEA) measured as the length from the tip of the snout (anterior margin of upper jaw at mid-axis of body) to the end of last anal-fin ray; (2) anal-fin length (AFL), measured from the origin of the anal fin at the isthmus to the end of the fin; (3) caudal appendage (CA), measured as the distance from the last anal-fin ray to the distal end of the caudal filament. Note: the CA in sternopygid fishes is often damaged, entirely missing, or in a variable state of regeneration. Therefore, the values reported here are not considered to have diagnostic value; (4) body depth (BD), measured as a vertical distance from the origin of the anal fin to the dorsal body border, (5) body width (BW), measured as body width at the origin of the anal-fin; (6) head length (HL), measured from the posterior margin of the bony opercle to the tip of the snout; (7) postorbital head length (PO), measured from the posterior margin of the bony opercle to posterior rim of free orbital margin of eye; (8) preorbital head length (PR), measured from the anterior rim of the orbital free margin to tip of snout; (9) eye diameter (ED), measured as the horizontal distance between the anterior and posterior rims of the free orbital margin; (10) interorbital length (IO), measured between the dorsomedial margins of the free orbital margin; (11) inter-narial distance (NN), measured from the posterior margin of the anterior nares to the anterior margin of the posterior nares; (12) mouth width (MW), measured as the horizontal distance of the gape at the rictus; (13) branchial opening (BO) measured as the distance from the posterodorsal to anteroventral extent of the skin fold of the branchial opening along the anterior margin; (14) head depth (HD), measured as the vertical distance at the nape to ventral body border with the lateral line held horizontal; (15) head width (HW) measured as the width at nape; (16) preanal distance (PA), measured from the origin of the anal fin to the posterior margin of anus; (17) pectoral-fin length (P1), measured from the dorsal border of fin base where it contacts the cleithrum to the tip of the longest ray. Morphometric data were standardized for size by reporting values as a percent of HL, except in HL %, BD %, BW %, and CA %, which are reported as a percent of LEA.

We assessed a body-shape tapering ratio (TR) as the ratio of BD at 75% LEA divided by BD at 25% LEA (Fig. 2). To reduce the effects of allometry, morphometric measurements used in the diagnosis were limited to morphologically mature specimens (more than 50% maximum known TL). Specimens that are damaged or with incompletely regenerated tails were excluded from analysis. Diagnostic trait values are reported as non-overlapping range values or range values within the 95% confidence interval (i.e., overlap less than 5.0%). Additional traits that are useful in identifying specimens of the new species are reported in the Diagnosis. The sex of six specimens was assessed by direct examination of gonads following Waddell, Crampton (2018) and Waddell et al. (2019).

FIGURE 2| Landmark scheme used in geometric morphometric analyses. Landmarks indicated by small red circles, pseudolandmarks by small blue circles. Landmarks described in Tab. 2. Body depth (BD) measured at 25% and 75% LEA to calculate Taper Ratio (TR). Photograph of S. macrurus, ANSP 209719.

TABLE 2 | Description of 13 landmarks used in 2D GMM analysis. Note: 7–12 are pseudo-landmarks.

Meristic counts also follow Hulen et al. (2005) and include: (1) anal-fin rays (AFR); (2) pectoral-fin rays (P1R) including all branched and unbranched rays; (3) precaudal vertebrae (PCV) including the four vertebrae that compose the Weberian apparatus; (4) scales above the lateral line (SAL) counted along a vertical line at the end of the body cavity; (5) scales below the lateral line (SBL) from the same point as SAL to the base of the anal-fin pterygiophores; (6) scales over the pterygiophores (SOP) counted from the same point as SAL at the base of the anal-fin pterygiophores to the anal-fin ventral border.

Micro-computed tomography (micro-CT) scans were made of 10 specimens from the type series of the new species using a Bruker SkyScan1273 with an x-ray source voltage of 65 kV. Only the head region, defined as the part of the body extending from the tip of the snout to a point between vertebrae 4–8 along the longitudinal axis of the specimens were scanned due to exceedingly large file sizes resulting from full-body scans and the relatively low return of information from scans past the body cavity. Osteological observations were made from 3D renderings of the micro-CT scans in the freeware Slicer (Fedorov et al., 2012) after being prepared in Fiji/ImageJ (Schindelin et al., 2012) following Buser et al. (2020). Precaudal vertebrae were counted from x-rays obtained from the CT scanner. Figures featuring images of 3D renderings were accomplished by taking screen captures of the renderings generated in Slicer before preparing them in additional photo editing software. The package ‘ggridges’ was used to create Ridgeline plots in R to facilitate the comparison of trait value distributions (Wilke, 2018).

Two-dimensional geometric morphometrics (2D GMM) were used to capture shape variation in the new species described herein and for comparison to congeners. Photographs of 91 specimens were taken using a Nikon Coolpix S9700 digital camera with all specimens in the same position in left lateral view with the dorsum forming a nearly straight line from the nape to the end of the anal fin. Photos were then converted to thin plate spline (.tps) files using tpsUtil (Rohlf, 2008), and seven homologous landmarks and six pseudo-landmarks (Fig. 2; Tab. 2) were placed on each photograph using FIJI (ImageJ) (Schindelin et al., 2012). Landmark coordinates were exported as .txt files and then imported into MorphoJ (Klingenberg, 2011) where a Procrustes superimposition was performed to remove the effects of size and scaling among specimens. A principal components analysis (PCA) was then performed in MorphoJ to identify the primary axes of variance. Data for comparison to the new species were collected from an additional 226 specimens from 116 lots of Sternopygus belonging to nine other species. These are listed in Tab. 3. Museum codes and abbreviations follow Sabaj (2020).

TABLE 3 | Summary of morphometric and meristic data for all valid species of the genus Sternopygus. Measurements reported in millimeters for LEA, AFL, and HL. Abbreviations given in Material and Methods.

Results​

---

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).

TABLE 4 | Absolute values of morphometric and meristic data for specimens of the type series of Sternopygus sarae. Abbreviations defined in Material and Methods.