Tenebrosternarchus, Bernt & Fronk & Evans & Albert, 2020

Bernt, Maxwell J., Fronk, Aaron H., Evans, Kory M. & Albert, James S., 2020, A redescription of deep-channel ghost knifefish, Sternarchogiton preto (Gymnotiformes: Apteronotidae), with assignment to a new genus, Neotropical Ichthyology (e 190126) 18 (1) : -

Tenebrosternarchus , new genus urn:lsid:zoobank.org:act:9205B11E-8A0F-461D-9B43-E309AFC4B204

( Figs. 1–11 ; Tab. 1)

Type species. Sternarchogiton preto Santana, Crampton, 2007 , by monotypy.

Diagnosis. Tenebrosternarchus is diagnosed from all other apteronotid genera by the following unique combination of four characters: three cranial fontanels present (vs. two in other apteronotids), bones 1 and 2 of the infraorbital laterosensory canal present as independently ossified tubes (vs. fused into a single bony element), ascending process of endopterygoid absent (vs. present), and brown to purplish-black pigment present over the dorsum, sides and fins (vs. absent or restricted to dorsum and distal fin margins). For field identification, this genus may be reliably distinguished from all other members of the Navajini by the combination of uniform dark coloration, the presence of five or more teeth on the premaxilla at all life stages, and gape not exceeding a vertical with eye.

Etymology. Tenebro from the Latin tenebrae meaning darkness in reference to the black pigmentation of this genus, and the Greek sternarchus (sternon and archos), a common generic suffix applied to apteronotids referring to the anterior position of the anus common to all Gymnotiformes .

Tenebrosternarchus preto ( Santana, Crampton, 2007) , new comb.

Sternarchogiton preto de Santana, Crampton, 2007 : fig. 5 table 1 (original description). —Crampton, 2007: 287, 291, 317, table 11.1, 11.2. —Crampton, 2011: 178, table 10.2. —Crampton, Cella-Ribeiro, 2013: 282–283 (photograph). —Silva et al., 2014: 638–645, fig. 2. — Tagliacollo et al., 2016: 30, fig. 6 (misidentification). — Smith et al., 2016: 306– 309, fig. 2. —Ferraris, Vari, de Santana, 2017: 12. — Bernt et al., 2018: 466, 471, 474– 477, table 2. — Bernt et al., 2019: 299–302, figs. 3, 4, 6. —Evans et al., 2019: 424–425, figs. 2, 3.

Sternarchogiton porcinum Cox-Fernandes, 1995: 32–33 , figs. 2–38, 2–39. —Crampton, 1996: fig. 6.1. —Crampton, 1998a: 817, 821, 830, 832, tables 3, 4, 5, 6. —Crampton, 1998b: 315, table 2.

Diagnosis. As for genus.

External morphology. Body shape and pigmentation illustrated in Fig. 1 . Summary of morphometric data and meristics shown in Tab. 1. Largest recorded size 330 mm TL. Body elongate and laterally compressed. Body depth greater than head depth. Forehead convex, sloped at approximately 45° in most specimens. Gape extending to or beyond vertical with posterior nares, but not reaching vertical with eye. Mouth subterminal. Eye diameter small, less than 10% head length, covered by thin layer of skin. Scales on body large and rhomboid forming 3–5 rows above lateral line at midbody. Scales absent on entire middorsum and over nape above lateral line to 5th lateral-line pore. Fleshy midsagittal electroreceptive organ originating on posterior third of dorsum. Nasal capsule closer to eye than to snout tip. Anterior nares tubular.

Neurocranium. Neurocranium illustrated in Figs. 2 and 3 . Paired frontals convex in lateral profile. Three cranial fontanelles present. Most apteronotid species have only two fontanelles with frontals merging at anterior margin of rear fontanel. In Tenebrosternarchus , however, posterior regions of frontals are fused on sagittal midline at margin with parietals, forming a broad isthmus dividing rear fontanel into two smaller openings, one between parietals at their anterior margin and the other at about the posterior third of the frontals. In some specimens, isthmus between frontals appears incompletely merged, with small openings appearing along suture. Mesethmoid decurved in lateral view and forked posteriorly, underlying nasals and bordering anterior fontanelle. Ventral ethmoid robust with wing-like processes extending posterolaterally to contact lateral ethmoid cartilage. Lateral ethmoid thin, slightly broader at ventral articular surface and angled obliquely to neurocranial axis. Orbitosphenoid contacting frontals dorsally, parasphenoid ventrally, and pterosphenoid posteriorly. Together, sphenoid bones form a prominent lateral fenestra. Margin of articulation between orbitosphenoid and pterosphenoid protruding ventrally into anterior third of this fenestra. Forming largest element of skull floor, parasphenoid contacts prootics posteriorly with long and narrow posterior process overlapping with most of basioccipital. Anteriorly, parasphenoid bifurcates into broad processes tapering to sharp points contacting cartilage of ventral ethmoid. Overlying these processes, vomer narrows posteriorly and broadens anteriorly at contact with ventral ethmoid.

Braincase composed of basioccipital, supraoccipital; and paired parietals, epiocciptals, exoccipitals, prootics, pterotics, and sphenotics. Pterotics support horizontal semicircular canals (visible ventrally, Fig. 3B ) and form lateral margin of widest region of skull. Prootic and exoccipital with prominent foraminae. Basioccipital approximately rectangular in ventral view, with deep medial groove and circular patch of cartilage at its posterior base. Sphenotics form prominent lateral processes at anterodorsal margin with frontals, nearly reaching width of pterotics. Supraoccipital crest reaching or slightly exceeding dorsal margin of parietals.

Suspensorium and oral jaws. Suspensorium and oral jaws illustrated in Figs. 2

and 4 . Opercle with slightly concave dorsal margin. This bone very weakly ossified with highly reticular structure (see Fig. 2 ), becoming more laminar at distal margins. Interopercle laminar, but weakly ossified, remaining mostly transparent ventrally after alizarin staining. Subopercle crescent-shaped and similarly thin and transparent distally after staining. Preopercle with vertically-oriented laterosensory canal tube fused to lateral surface and a narrow anteroventral process articulating with quadrate. Hyomandibula oriented about 120° to long axis of skull, with posterolateral ridge contacting dorsal half of preopercular anterior margin. Dorsal articulating head of hyomandibula about twice width of distal end and rounded at point of articulation with sphenotic. Prominent foramen at medial hyomandibular base associated with cranial nerves (V, VII, and lateral line nerves). Symplectic triangular, oblique to hyomandibula and separated by thick, obliquely-angled band of cartilage. Metapterygoid triangular with dorsal and ventral sides about equal in length. Endopterygoid edentulous, lacking ascending process, with blunt posterodorsal process. Endopterygoid broadly overlaps with metapterygoid and quadrate (visible medially). Autopalatine cartilage extending from anterior tip of endopterygoid and abruptly angled laterally, then ventrally to contact articular surface of maxilla.

Mandible comprised of dentary, anguloarticular, retroarticular, Meckel’s cartilage and coronomeckelian. Dentary slightly longer than deep with triangular coronoid process and narrow ventral process overlapping with anguloarticular laterally. Anterior ¾ of dentary bearing 13–16 conical teeth arranged in a single row and angled medially (except in some sexually mature males). Anguloarticular forked anteriorly. Dorsal process short, overlapping with dentary laterally. Ventral process long and triangular, broadly overlapping with dentary medially. Retroarticular short and rectangular, without anterior process, contacting only anguloarticular. Meckel’s cartilage visible on medial surface of mandible at center of dentary and anguloarticular. Coronomeckelian bone narrow and triangular, contacting dorsal surface of Meckel’s cartilage.

Maxilla and premaxilla illustrated in Fig. 5 . Maxilla with rounded anteroventral shelf, lacking free anterodorsal hook. Articular process rounded and angled posteriorly. Descending blade of maxilla thin and sharply pointed. Blade extending slightly below horizontal axis of maxillary shelf. Premaxilla broadest anteriorly and narrowing posteriorly. Five to nine large conical teeth present on premaxilla arranged in two uneven rows (but see discussion of sexual dimorphism).

Hyoid arch and branchial basket. Ventral hyoid arch of Tenebrosternarchus (illustrated in Fig. 6 ) notably similar to that of Apteronotus bonapartii (see Hilton, Cox-Fernandes, 2017, figs. 6a–b) and Melanosternarchus Bernt, Crampton, Orfinger, Albert, 2018 (see Bernt et al., 2018, fig. 8). Complex cartilaginous margin between ceratohyals. Posterior ceratohyal rounded at posterior margin with entire anterior surface articulating with anterior ceratohyal. Anterior ceratohyal narrow at posterior margin, broadening anteriorly at margin with dorsal and ventral hypohyals. Dorsal hypohyal articulating with superior surface of ventral hypohyal together forming a pyramidal shape. Ventral hypohyal triangular with point directed anteromedially, with all sides of approximately equal length. Urohyal with medial ridge, lacking posterior blade. Ventral surface with only a very short medial eminence. Four branchiostegal rays. Branchiostegal 1 and 2 slender and filamentous. Third branchiostegal crescent-shaped. Fourth branchiostegal with pointed anterior process, weakly ossified along middle-ventral margin.

Ventral branchial basket illustrated in Fig. 7A . We follow Hilton et al. (2007) in interpreting anteriormost element of ventral branchial skeleton as a fused basihyal and first basibranchial. This structure broadened anteriorly with cartilaginous tip and narrow posteriorly with thin dorsal ridge. Second basibranchial hourglass-shaped, convex at anterior and posterior articular surfaces. Third basibranchial ossified, broader anteriorly than posteriorly. Basibranchials 4 and 5 unossified, present as cartilaginous rods. First hypobranchials roughly conical and cartilaginous posteriorly. Second hypobranchials contacting medially beneath second basibranchial. Third hypobranchials contacting medially posterior to third basibranchial. Ceratobranchials 1 and 2 roughly rectangular, with cartilaginous cap at dorsal tip. Ceratobranchial 3 narrow at point of articulation with hypobranchials, with medial hook and short, ventral process. Ceratobranchial 4 very slender medially with medial process at about midlength of bone. Six gill rakers associated with each of first four ceratobranchial bones. Fifth ceratobranchials contacting medially around a small circular element of cartilage. Posteromedial surface of ceratobranchial 5 bearing 8–10 conical teeth.

Dorsal elements of branchial basket illustrated in Fig. 7B . Epibranchials 1 and 2 approximately rectangular, associated with few (1–2) or no gill rakers. Epibranchial 3 with short anteromedial process contacting infrapharyngobranchial 3. Epibranchial 4 Y-shaped, forked posteriorly, contacting epibranchial 5. Epibranchial 5 entirely cartilaginous, tapering posteriorly to a sharp point. Infrapharyngobranchials 1 and 2 broad posteriorly with thick cartilaginous articular surface contacting epibranchial laterally and next infrapharyngobranchial medially. Third infrapharyngobranchial rod shaped and entirely cartilaginous. Upper pharyngobranchial tooth plate positioned over articulation between infrapharyngobranchial 3 and epibranchial 4. Upper tooth plate substantially smaller than lower plate and weakly-ossified, bearing 6 teeth.

Cranial laterosensory system. Cephalic laterosensory canals illustrated in Fig.

8. Three nasal canal-bones present as small ossified tubes. Most posterior nasal canal

(superior to posterior naris) more than twice length of other two. Anterior half of this canal visible as a groove, or incompletely formed tube. Elongate supraorbital canal fused with frontal bone. Antorbital bone present as small tubular ossification immediately below posterior nares and anterior to eye. Infraorbital bones 1, 2, and 3 present as independently ossified tubes below anterior naris. Fourth infraorbital below antorbital. Infraorbitals 5 and 6 more than twice length of infraorbitals 1–4, positioned posterior to eye. Parietal canal not ossified in specimens observed. Horizontally-oriented otic canal forming longest element of cephalic laterosensory system immediately anterior to a short postotic canal. Supratemporal canal straight, vertically-oriented forming a right angle with postotic. Posterior-most element of cranial laterosensory system fused to posttemporal, ventroposterior to postotic and supratemporal canals. Five mandibular canals present as short, sharply-curved ossicles. Anterior four mandibular canals ventrolateral to dentary, posterior mandibular canal lateral to base of preopercle.

Weberian apparatus and vertebral column. Posterior to the skull, first four vertebrae and associated elements comprise the Weberian apparatus. Fifth vertebra articulating with first rib. First rib robust and with greatly expanded parapophysis,

contacting enlarged fourth parapophysis forming ventrally-angled concavity that contacts dorsolateral surface of anterior region of swim bladder. First neural spine distally broadened. Fifteen to 17 precaudal vertebrae including those associated with Weberian apparatus, extending to but not including vertebra articulating with anterior-displaced hemal spine. Anterior-displaced hemal spine long and saber-shaped, forming posterior margin of body cavity, its dorsal articular surface contacted by parapophysis of one or two vertebrae (see Fig. 9). Two to three posterior-displaced hemal spines articulating with base of hemal spines of first two caudal vertebrae, sometimes including vertebra articulating with anterior-displaced hemal spine.

Pectoral girdle. Pectoral girdle illustrated in Fig. 10 . Two weakly-ossified,

rectangular postcleithrae present. Dorsal extension of cleithrum with groove, closely associated with supracleithrum. Supracleithrum with notch-like articular surface for ventrally-broadened posttemporal. Anterodorsally angled laterosensory canal fused to posttemporal near point of contact with supracleithrum. Scapula broadest dorsally along contact with cleithrum, angling anteriorly to contact coracoid. Coracoid with dorsally expanded process contacting cleithrum near point of dorsal flexion and thin anterior process contacting ventral margin of cleithrum. Four irregularly-shaped proximal radials. Propterygium with broad cartilaginous base. Fourteen to 17 pectoral-fin rays. First ray about half length of remaining rays.

Fins, rays, and pterygiophores. Anal fin with 152–204 rays (mode = 193, n = 20).

Anal-fin pterygiophores longer than hemal spines along most of body length. Anal-fin pterygiophores broadened ventrally into symmetrical vanes above their articulation with distal radials. Caudal fin size and shape variable with 12–23 (mode = 14) rays. Caudal fin frequently absent, damaged, or regenerated (as in most apteronotids), presumably due to electroreceptive predators. Number of rays higher (and fin size larger) in many specimens with regenerated caudal fins. Ray counts for all fins include both branched and unbranched rays.

Color pattern. In life, Tenebrosternarchus ranges from slate gray to deep black in color. Pigment is darkest over top of head, middorsum, and nape. These regions devoid of scales are finely dotted with pale-colored tuberous electroreceptor organs. Dorsal organ paler in color than skin of other parts of dorsum, usually dark gray. Sides of body above anal-fin pterygiophores variable in coloration, scales darker at their bases. Region overlying pterygiophores have a deep purplish-red to lighter pink coloration. Anal fin dark brown to black, except in a narrow hyaline band along proximal margin. Pectoral and caudal fins uniformly dark brown to black. Opercular region with rosy coloration due to underlying gill lamellae. Fleshy region of upper lower jaw and margin of upper lip a pale cream color. Like Santana, Crampton (2007) we also notice that color varies with habitat. In particular we found that specimens from blackwater rivers (e.g., the rio Negro) were darker due to a visibly higher density of black chromatophores over body, while specimens taken from turbid whitewaters (especially the río Apure) were lightest in coloration with more gray and pink hues (see Figs 1 and 11 ). Coloration in ethanol similar to that in life, but with reddish to pink colors fading to brown and beige, and pale cream colors fading to dull yellow.

Sexual dimorphism. Three specimens of Tenebrosternarchus were found to exhibit a more laterally protuberant snout with a less steeply sloping forehead and a longer gape than the majority of samples (see Fig. 11 ). Upon inspection of the gonads, it was found that all of these hypertrophied specimens were male. Notably these male specimens were all near or exceeding 300 mm in length. Several males under 250 mm TL did not exhibit distinct cranial morphology and could be sexed only by the presence of testes. The two largest male specimens exhibited increased dentition with over 20 teeth on each premaxilla and two to three rows of dentary teeth. Both also showed scars and lacerations around the head and nape. This condition is commonly observed in male Sternarchogiton nattereri with enlarged external teeth and are presumed to be the result of male-male combat (Cox-Fernandes et al., 2009). The largest specimen of T. preto examined (330 mm TL) was a female that did not exhibit any distinct cranial morphology or increased dentition. The largest male specimen (MUSM 59463) was distinct enough from other T. preto that it was thought possible that it was a different species entirely. Genetic analysis of tissue taken from this specimen, however, showed it to be nested within Amazonian specimens of T. preto (specimen IQ17039).

Ecology. Little is presently known about the ecology of Tenebrosternarchus . Typically among ghost knifefishes, species inhabiting deep river channels (sensu Crampton, 2007)

have greatly reduced pigmentation, often appearing pink or white (e.g., Orthosternarchus

Ellis, 1912, Sternarchella Eigenmann, 1905 ), while those inhabiting smaller streams or marginal habitats of larger rivers have much darker coloration (e.g., Apteronotus

Lacepède, 1800, Platyurosternarchus Mago-Leccia, 1994 ). Interestingly, T. preto , with its dark coloration, appears to be an obligate channel-dweller, and does not appear to frequent shallower waters more than other members of the Navajini. This genus does seem to have a habitat preference for low-conductivity blackwater rivers as a majority of specimens we examined were collected from the rio Negro and the río Nanay (both blackwater rivers). This preference for blackwater does not appear to be as strict as that of Melanosternarchus (see Bernt et al., 2018), as specimens of T. preto are also collected from whitewater channels (e.g., the Amazon, Apure, and Madeira rivers). Specimens are also known from near the mouths of clearwater rivers (Tapajós and Xingu), but they are not known to be more abundant in these rivers than in whitewaters. Nearly all specimens from whitewater rivers have regenerated caudal fins (a common condition among apteronotids), while undamaged tails are more common from blackwater with about 70% of specimens showing signs of regeneration.

Tenebrosternarchus is typically found among aggregations of other channel knifefishes, especially Rhabdolichops Eigenmann, Allen, 1942 , Sternarchella , Orthosternarchus Ellis, 1912 , and Sternarchorhamphus Eigenmann, 1905 . It tends to be relatively uncommon in trawl samples, but was found to be locally abundant on the río Nanay in the vicinity of Iquitos, Peru during low water (June–August). Notably, specimens collected at this time and place were found to be substantially larger in average size (250–300 mm) than those from other collections. Examination of gut contents of several specimens from this habitat revealed that T. preto feeds most heavily on chironomid larvae, though coleopteran and ceratopogonid larvae were also present. These larvae were found to be mixed with sand and detritus, suggesting that T. preto is a primarily a benthic forager. Crampton (2007) also reports that this species feeds on freshwater sponges of the genus Drulia .

Distribution. The collection localities of samples analyzed in this study are summarized in Fig. 12 . Our examined material of Tenebrosternarchus ranges from the western Amazon at Iquitos, Peru to the mouth of the rio Xingu in Pará, Brazil, the rio Negro, from Manaus to Barcelos, and the Orinoco Basin in Venezuela, from the río Apure at San Fernando de Apure to near the mouth of the río Orinoco in Delta

Amacuro. Crampton, Cella-Ribeiro (2013) also report this species from the Madeira drainage above the extensive system of cataracts beginning at Porto Velho.

Phylogenetic relationships. The molecular phylogeny of Bernt et al. (2019) placed

Tenebrosternarchus sister to all other species in the Navajini, which form a clade comprised of the Apteronotus bonapartii group, Compsaraia , Melanosternarchus , Pariosternarchus

Albert, Crampton, 2006, Porotergus , Sternarchella , and Sternarchogiton . This phylogeny included eight specimens of T. preto . Five of these were from the western Amazon at

Iquitos, one was from the lower Amazon at Santarem, and two were from the Orinoco drainage (río Apure). The latter two samples are sister to the remaining six, however, the branch length separating them is comparable to intraspecific branchlengths throughout the family. Additionally, morphometric and meristic data do not suggest the presence of multiple allopatric species within the sample we analyzed. The relationship of

Tenebrosternarchus to other genera is well supported with a bootstrap value of 98% and a posterior probability of 1.

Material examined: Brazil: Amazonas: AMNH 221206, 5, rio Negro between Tarumã and Tarumã Mirim, 7 Nov 1997, C. Cox Fernandes, J. Lundberg. ANSP 189211, 1, rio Negro, 10 km downstream from Carvoeiro, 1°22’8’’S 61°54’22’’W, 9 Dec 1993, J. Lundberg et al. ANSP 189221, 1, rio Juruá downriver of Pauapixuna, upriver of Vitoria, 2°41’8’’S 65° 48’29’’W, 9 Nov 1993, J. Lundberg et al. ANSP 189222, 1, rio Juruá downstream of Humaitá, upstream of Pauapixuna, 2°45’19’’S 65°49’15’’W, 9 Nov 1993, J. Lundberg et al. ANSP 189223, 1, rio Negro, 34.3 km downriver of Novo Caioe, 4.4 km upriver of São Francisco de Assis, 1°58’37’’ S 61°15’21’’W, 5 Dec 1993, J. Lundberg et al. ANSP 189224, 1, rio Negro, 62 km downriver of Vila Guajara, 47.2 km upriver of Moura, 1°14’51’’S 61° 58’52’’W, 9 Dec 1993, J. Lundberg et al. ANSP 189225, 2, rio Juruá 17.3 km downriver of Pauapixuna, 15.7 km upriver of Tamanicoa, 2°37’36.9’’S 65°47’01.8’’W, 7 Nov 1993, O. Oyakawa, J. Lundberg, et al. ANSP 199741, 1, rio Tefé, in floating macrophytes (Projeto Mamirauá), 1994, W. Crampton. ANSP 195681, 14, rio Negro, 4.3 km downstream of Carvoeiro, 37.0 km upstream of Moura, 1°20’26’’S 61°55’19’’W, 9 Dec 1993, J. Lundberg et al. ANSP 195682, 2, rio Solimões below mouth of rio Purus, 3°35’51.4’’S 61°07’40.8’’W, 31 July 1996, M. Toledo-Piza, J. Lundberg et al. ANSP 195683, 1, rio Amazonas downriver of mouth of rio Madeira, upriver of Itacoatiara, 3°20’45.8’’S 58°37’50.3’’W, 8 August 1996, M. Toledo-Piza, J. Lundberg et al. ANSP 195684, 1, rio Amazonas, 159 km downstream of Manaus, 30 km upstream of Itacoatiara, 3°19’60’’S 58°35’44’’W, 10 Aug 1996, C. Cox Fernandes, J. Lundberg et al. ANSP 195686, 1, rio Negro downstream of Vila Guajara and upstream of Carvoeiro, 1°13’31.5’’S 62°13’57.5’’W, 10 Dec, 1993, J. Lundberg et al. ANSP 195688, 3, rio Negro above rio Jufari, 34.3 km downriver of Vila Guajara, 1°12’52’’S 62°14’41’’W, 10 Dec 1993, J. Friel, J. Lundberg et al. ANSP 195689, 10, rio Negro, 1.9 km above confluence with rio Branco, 9.3 km downstream of Carvoeiro, 34.8 km upstream of Moura, 1°22’21’’S 61°54’32’’W, 9 Dec 1993, J. Lundberg et al. ANSP 195690, 9, rio Negro, 26.3 km downriver of Vila Guajara, 35.4 km upriver of Carvoeiro, 1°13’33’’S 62°13’54’’W, 10 Dec 1993, M. Garcia, J. Lundberg et al. ANSP 195691, 1, rio Solimões, 27.8 km downriver of Foz do Jutaí, 6.9 km upriver of Ponta Grossa, 2°33’52’’S 66°35’37’’W, 12 Nov 1993, J. Lundberg et al. ANSP 195692, 1, rio Negro, 10.6 km downriver of Leprosario, 14.8 km upriver of Manaus, 3°6’0’’S 60°9’33’’W, 10 Oct 1994, J. Lundberg et al. ANSP 195693, 1, rio Negro, 12.6 km downriver of Moura, 20.7 km upriver of São Francisco, 1°31’45’’S 61°31’52’’W, 7 Dec 1993, M. Garcia, J. Lundberg et al. ANSP 195694, 1, rio Purus upstream from confluence with rio Solimões, downstream of town of Beruri, 3°49’53’’S 61°24’7’’W, 26 Jul 1996, A. Zanata, J. Lundberg et al. ANSP 195695, 1, rio Purus, 4 km upriver of São Tomé, 8 km downriver of Beruri, 3°50’35’’S 61°23’45’’W, 26 Jul 1996, F. Langeani, J. Lundberg et al. ANSP 195696, 1, rio Madeira, 25.9 km downriver of Nova Olinda do Norte, 0.7 km upriver of Rosarinho, 3°39’50’’S 59°4’15’’W, 16 Oct 1996, F. Langeani, J. Lundberg et al. ANSP 195697, 1, rio Solimões, below mouth of rio Purus, 3°36’1.2’’S 61°19’20.3’’W, 28 Jul 1996, A. Zanata, J. Lundberg et al. ANSP 195698, 2, rio Purus above confluence with rio Solimões, 10 nautical miles downriver of Surará, 8 nautical miles upriver of Beruri, 3°58’02.4’’S 61°28’01.6’’W, 27 Jul 1996, J. Lundberg et al. ANSP 195699, 2, rio Amazonas, upstream of Itacoatiara, 3°15’33’’S 58°58’42’’W, 5 Aug 1996, A. Zanata, J. Lundberg et al. ANSP 195700, 1, rio Madeira, upriver of Urucurituba, 3°37’25’’S 58°59’33’’W, 7 Aug 1996, A. Zanata, J. Lundberg et al. ANSP 195702, 1, rio Solimões, 37.2 km downriver of Anori, 3 km upriver of Porto S. Francisco, 3°36’23’’S 61°20’11’’W. ANSP 207840, 5, 2 CS, rio Demeni, 18 km upstream of confluence with rio Negro, 0°38’28.8’’S 62°53’47.5’’W, 29 Nov 1993, J. Lundberg et al. ANSP 207903 (5), Furo of rio Demeni near mouth of Parana do Camuqual, 0°24’57.6”S 62°53’37.8”W, 12 Aug 2018, M. Bernt, G. Costa Silva, B. Waltz. ANSP 195704, 4, rio Negro, 5.0 km upriver of mouth of rio Branco, 7.9 km downriver of Carvoeiro, 1°23’19.9’’S 61°54’29.2’’W, 8 Dec 1993, J. Friel, J. Lundberg et al. ANSP 207873, 2, rio Araçá immediately upstream from confluence with rio Demini, 0°24’54.4’’S 62°56’9.1’’W, 11 Aug 2018, M. Bernt, G. Costa Silva, B. Waltz. ANSP 207797, 1, rio Negro at Lagoa do Pato, 1°12’16.5’’S 62°23’9.3’’W, 6 Aug 218, M. Bernt, G. Costa Silva, B. Waltz. Pará: ANSP 195687, 9, rio Amazonas, 9.7 km downstream of Vila Canaã, 10.2 km upstream from Porto S. José, 1°33’22’’S 50°44’5’’W, 16 Nov 1994, A. Zanata, J. Lundberg et al. ANSP 195685, 3, rio Acarai upstream from confluence with rio Xingu, Porto de Moz, 2°4’34’’S 52°20’42’’W, 10 Nov 1994, A. Zanata, J. Lundberg et al. ANSP 195701, 2, rio Trombetas, 3.6 km downriver of Oriximiná, 2.9 km upriver of Fazenda Paraiso, 1°48’7’’S 55°50’52’’W, 24 Oct 1994, J. Lundberg et al. ANSP 195703, 2, rio Amazonas above mouth of rio Trombetas, 13.3 km upriver of Óbidos, 1°55’10.5’’S 55°37’26.9’’W, 23 Oct 1994, M. Westneat, J. Lundberg et al. ANSP 200380, 1, rio Tapajós at mouth, ca. 18 km northnorthwest of Santarém, 2°16’52.7’’S 54°47’59.9’’W, 18 Jun 2015, J. Albert, V. Tagliacollo et al. Peru: Loreto: ANSP 200246, 3, río Nanay near confluence with río Amazonas, 3°41’32.9’’S 73°14’32.6’’W, 18 Aug 2015, K. Evans et al. ANSP 200470, 2, río Nanay upstream of Pampachica, 3°44’50.6’’S 73°18’45.1’’W, 2 Jan 2016, M. Bernt, E. Chota et al. ANSP 200551, 3, río Amazonas near mouth of río Nanay, 3°41’32.9’’S 73°14’32.6’’W, 2 Sep 2016. MUSM 54617, 13, 2 CS, río Nanay upstream from Manacamiri, sandy beach across from small quebrada, 3°44’29.55”S 73°17’3.88”W, 4 Jul 2014, M. Bernt, D. Saldaña et al. MUSM 54656, 1, río Amazonas near Iquitos 3°49’11.5”S 73°09’31.2”W, 7 Jul 2014, M. Bernt, D. Saldaña et al. MUSM 59447, 8, 2 CS, río Nanay, beach upstream of Pucayacu a Orillas, 3°45.712’S 73°18.901’W, 2 Jan 2017, M. Bernt, R. Cahuaza et al. MUSM 59463, 2, río Nanay at Pampachica, 3°45’09.0180”S 73°16’54.6924”W, 5 Jan 2017, M. Bernt, R. Cahuaza et al. Venezuela: Apure: ANSP 198346, 1, río Apure, channels on both sides of small island, ca. 12 km downstream of Aeropuerto Las Flecheras, ca. 15.5 km east-southeast of Puente María Nieves, San Fernando de Apure, 7°49’04.2’’N 67°21’30.6’’W, 10 Apr 2015, M. Sabaj, M. Bernt, O.Castillo et al. ANSP 198373, 1, río Apure, ca. 5 km upstream of mouth of río Portuguesa, 7°57’51.1’’N 67°34’36.4’’W, 9 Apr 2015, M. Sabaj, M. Bernt, O.Castillo et al. ANSP 198391, 1, río Apure, just downstream of Puente María Nieves, San Fernando de Apure, 7°53’55.3’’N 67°27’32’’W, 8 April 2015, M. Sabaj, M. Bernt, O.Castillo et al. Bolivar: ANSP 149504, 1, río Orinoco, south side of river at Ciudad Bolivar, 241 nautical mi from sea buoy, 8°9’N 63°32’W, 8 Nov 1979, E. Marsh et al. ANSP 199016, 1, río Orinoco, electric power line crossing at narrows below Ciudad Bolivar, 234 nautical miles from sea buoy, station no. 36917, 300 meters from shore, 8°11’06’’N 63°25’30’’W, 9 Nov 1979, J. Lundberg et al. Delta Amacuro: AMNH 217738, 1, río Orinoco, from buoy 129.7 to buoy 130.4 at Isla Tres Caños, downstream from El Consejo and Barrancas, 14 Nov

1979, J. Baskin, G. Miller et al. ANSP 149529 , 3 , río Orinoco , downstream from mouth of río Arature , ca. km 51. ANSP 199012 , 2 , río Orinoco just downstream of Isla Portuguesa ca.116.5 naut. mi. from sea buoy, 8°36’12’’N 61°46’24’’W, 24 Feb 1978, J. Lundberg, J. Baskin et al. ANSP 199018 , 1 , río Orinoco , just downstream of Isla Portuguesa , ca. 116.5 nautical miles from sea buoy, 8°36’12’’N 61°46’54’’W, 14 Nov 1979, J. Lundberg et al. ANSP 199019 , 1 , río Orinoco , old shipping channel S of Portuguesa, ca. km 117, 20 Feb 1978, J. Lundberg et al. ANSP 199248 , 1 , Small caño (caño “Electrica”) and swamp off río Orinoco , near buoy 85.7, on N side of Isla Carosimo, 8°25’36’’N 61°20’54’’W, 21 Nov 1979, D. Stewart et al. ANSP 199006 , 1 , río Orinoco: near mouth of río Arature, ca. km 53, 24 Feb 1978, J. Lundberg, J. Baskin et al .

Figures

FIGURE 1 | Lateral view of live Tenebrosternarchus preto. A. MUSM 54656 (243 mm TL) from the río Amazonas at Iquitos, Peru; B. ANSP 207797 (232 mm TL) from the rio Negro downstream from Barcelos, Brazil; and C. Detail of the head of ANSP 207797 (232 mm TL).

FIGURE 2 | CT scan reconstructions of Tenebrosternarchus preto, MUSM 59447 (male) 264 mm TL in dorsal A., ventral B., and lateral C. views.

FIGURE 3 | Illustrations of the neurocranium of Tenebrosternarchus preto MUSM 54617 (198 mm TL) in dorsal A., ventral B., and lateral C. views. Cartilage shown in black. Abbreviations: bao, basioccipital; epo, epioccipital; fro, frontal; let, lateral ethmoid; met, mesethmoid; obs, orbitosphenoid; par, parietal; pas, parasphenoid; pro, prootic; pto, pterotic; pts, pterosphenoid; soc, supraoccipital; spo, sphenotic; vet, ventral ethmoid; vom, vomer.

FIGURE 4 | Suspensorium, opercular series, and lower jaw of Tenebrosternarchus preto, MUSM 54617 (198 mm TL) in A. lateral and B. medial view (image reflected). Thin, unstained hard tissue associated with opercular bones shown in light gray. Abbreviations: ang, anguloarticular; ap, autopalatine cartilage; cm, coronomeckelian; d, dentary; enp, endopterygoid; h, hyomandibula; iop, interopercle; m, Meckel’s cartilage; mpt, metapterygoid; op, opercle; pop, preopercle; q, quadrate; rar, retroarticular; sym, symplectic.

FIGURE 5 | Maxilla of Tenebrosternarchus preto. A. MUSM 54617 (198 mm TL) in lateral view; and B. the paired premaxillae of the same specimen in ventral view.

FIGURE 6 | Elements of the ventral hyoid arch of Tenebrosternarchus preto in A. lateral and B. ventral view MUSM 54617 (198 mm TL). Anterior facing left. Abbreviations: br, branchiostegal; cha, anterior ceratohyal; chp, posterior ceratohyal; hhd, dorsal hypohyal; hhv, ventral hypohyal; uh, urohyal.

FIGURE 7 | Gill arches of Tenebrosternarchus preto, MUSM 54617 (254 mm TL) Ventral arches A. shown in dorsal view and dorsal arches B. in ventral view. Abbreviations: bb, basibranchial; bh, basihyal, cb, ceratobranchial; eb, epibranchial; hb, hypobranchial; iph, infrapharyngobranchial; uptp, upper pharyngeal toothplate.

FIGURE 8 | Cephalic lateral-line bones of Tenebrosternarchus preto, MUSM 54617 (198 mm TL). Abbreviations: ant, antorbital; io, infraorbital; na, nasal; otc, otic canal; mco, mandibular canal ossicles; pa, parietal canal; poc, postotic canal; prc, preopercular canal; soc, supraorbital canal; stc, supratemporal canal; trc, trunk canal.

FIGURE 10 | Pectoral girdle of Tenebrosternarchus preto, MUSM 54617 (198 mm TL) in lateral A., and medial B. views. Abbreviations: cl, cleithrum; co, coracoid; pt, posttemporal; ptg, propterygium; ra, radial; sca, scapula; scl, supracleithrum.

FIGURE 11 | Heads of male Tenebrosternarchus preto A. ANSP 198373 (302 mm TL) from the río Apure at San Fernando de Apure, Venezuela B. MUSM 59463 (291 mm) and C. MUSM 59447 (320 mm TL) both from the río Nanay at Iquitos, Peru.

FIGURE 12 | Collection localities of Tenebrosternarchus preto, star indicating type locality. Some points indicate multiple collections or lots from proximate locations.