Sternarchella orthos Mago-Leccia, 1994

Evans, Kory M., Crampton, William G. R. & Albert, James S., 2017, Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species, Neotropical Ichthyology 15 (2), No. e 160168, pp. 1-30 : 19-21

Sternarchella orthos Mago-Leccia, 1994

Fig. 20, Tab. 7

Sexual dimorphism. Mature female specimens of S. orinoco have wider heads than male specimens of comparable sizes ( Mago-Leccia, 1994).

Distribution and habitats. ( Fig. 19). This species is known only from the Orinoco River basin, where it is inhabits deep river channels. It has been most commonly collected in the Amacuro estuary and Llanos floodplains.

Remarks. While superficially resembling S. sima , this species possesses several differences including tail length, gill raker condition (contacting gill arch vs. non-contacting in S. sima ), a straight ventral profile, and skull width. Females have a wider head and a deeper body (Mago- Leccia, 1994:99).

Material examined. Holotype. MBUCV-V-10514, 1, 201 mm. TL, Venezuela, Río Orinoco, between km 153 and 152 in front of Isla Iguana, Território Delta Amacuro, collected by J. G. Lundberg, 18 Feb 1978. Non-types. Venezuela: USNM 228727 , 8 , (3 C&S) 189-221 mm LEA, Venezuela, Delta Amacuro, Río Orinoco, Old Shipping Channel of Isla Portuguesa.

Sternarchella orthos Mago-Leccia, 1994:188 , fig. 93: tab. 5 [type Locality: Venezuela, Río Apure, near, Apure].

Sternarchella cf. terminalis . - Lundberg, 2013:163, fig. 5C [type locality: Brazil, rio Amazonas , above rio Tapajos].

Sternarchella terminalis . - Ivanyisky, Albert, 2014:569, fig. 2 [type locality: Peru, Madre de Dios, Río Los Amigos]. - Crampton, Albert, 2006:386 [ Brazil, rio Amazonas near Tefé]. - Crampton, 2007:316, fig. 11.6 3E [ Brazil, rio Amazonas near Tefé]. - Crampton, 2011:165-189 [ Brazil, rio Amazonas near Tefé].

Diagnosis. Sternarchella orthos can be diagnosed from all congeners by the presence of a terminal mouth (vs. sub-terminal in S. orinoco and S. sima , and superior in S. calhamazon , S. duccis and S. patriciae ) (shared with S. raptor , S. rex , and S. schotti ) the possession of a long body cavity with 14-16 pre-caudal vertebrae (vs. less than 14 in S. calhamazon ) (shared with S. duccis , S. orinoco , S. patriciae , S. raptor , S. rex , S. schotti , and S. sima ), the presence of a fan or rod-shaped second basibranchial (vs. hour-glass shaped in S. calhamazon , S. duccis , S. patriciae , S. raptor , and S. rex and triangular in S. sima and S. orinoco ) (shared with S. schotti ) and the presence of a crown of thorny projections present at border between parietals and supraoccipital, continuing to epioccipital (vs. no crown present in S. calhamazon , S. duccis , S. orinoco , S. raptor , S. rex , S. schotti , and S. sima ) (shared with S. patriciae ).

Description. Medium to large size species reaching 256 mm LEA. Pectoral fin size small, less than 80% HL. PA% moderate, 46-49% HL. Head narrow, distance between lateral margins 36-57%. Preorbital (snout) length moderate, 28-32% HL. Postorbital distance small, 63-70% HL. Eye diameter small, 4-7% HL. Interorbital distance small, 12-18% HL. Mouth narrow, distance between ricti 15-41% HL. Body depth less than HL. Scales absent on posterolateral portion of body. Scales large in size with 5-8 present above lateral line at midbody. Scales dorsal to lateral line rhomboid at mid-body. Rictus extends to a vertical with mental symphysis, gape very small, less than twice eye diameter. Oral aperture terminal, upper and lower jaws equal in length. Body cavity long; 14-16 pre-caudal vertebrae present. Proximal surface of first displaced hemal spine narrower then descending blade. One to two displaced hemal spines. Swim bladder not extending posterior to body cavity. Anal-fin pterygiophore length equal to or shorter than hemal spines. Anal-fin proximal length shorter than hemal spine. One row of bones in caudal peduncle visible externally. Caudal peduncle shallow 8-22% HL. Dark spot on caudal peduncle absent. No apparent connective tissue between anal-fin base and caudal peduncle. Caudal peduncle length less than HL. Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla. Premaxilla triangular in ventral view. Three rows of teeth present on premaxilla. Anterior hook of maxilla absent, anterior process of maxilla broad and triangular with a continuous ventral margin with descending blade. Anterior process of maxilla extending as a shelf of bone less than one-third length of descending blade. Ventral margin of maxillary blade curves evenly towards its distal tip. Descending blade maxilla thin, evenly curved. Two rows of teeth present on dentary. Dentary longer than deep, oral margin of dentary longer than length of angular articular. Dorsal margin of dentary slightly concave in lateral view. Posterior margin of dentary curves gradually to descending limb. Ventral ethmoid large and robust with a large fan shaped lateral process. Dorso-anterior portion of mesethmoid straight. Anterior tip of mesethmoid scyphate on dorsal surface. Anterior fontanel longer than posterior fontanel. Lateral ethmoid very robust and large, may contact ventral portion on frontals, hour-glass shaped with most narrow portion at mid-length. Orbitosphenoid broad, well ossified in median nasal septum with ventral margin longer than dorsal margin. Dorso-medial portion of orbitosphenoids in contact (visible through anterior fontanel in dorsal view). Absence of ventral process of pterosphenoid, anterior ventral margin of pterosphenoid similar to posterior ventral margin of orbitosphenoids. Small, lateral margins of parasphenoid not extending to a horizontal with trigeminal foramen. Parasphenoid ventral margin straight or slightly curved. Narrow, just lateral to supraoccipital, parietal ridges are very large and pronounced. Crown of thorny projections present at border of parietals and supraoccipital, continuing to epioccipital. Dorsal margin of supraoccipital crest exceed dorsal margin of parietals. Supraoccipital crest extends to a dorsal distal tip. Internal carotid foramen reduced. Ventral surface of basioccipital smooth. Anterior extension of infraorbital canal shorter than width of canal pore, anterior canal pore of infraorbital near first infraorbital. Supraorbital canal fused to frontal. Mandibular canal size small. Mandibular canal ossicles form long slender tubes. Supratemporal laterosensory canal curved at a sharp angle on surface of parietal, extending posterior onto epaxial surface of body, terminal canal pore oriented posteriorly, epidermis overlying supratemporal canal depigmented. Endopterygoid process small, not contacting frontal. Endopterygoid process extends vertically at or near a 90º with dorsal surface of endopterygoid. Hyomandibula short, its width half its length. Dorsal margin of opercle concave. Opercle broad, width over half depth. Base of gill rakers contacting gill arch. Gill rakers long with ossified distal tips. Dorsal surface of basihyal convex forming a robust ridge posteriorly. Second basibranchial fan-shaped, extending to long narrow end. Third basibranchial unossified. Fourteen or more teeth present on pharyngobranchial. Eight or more teeth present on sixth hypobranchial. Medial surface of fourth hypobranchial with a process or bridge extending to meet contralateral process on midline. Urohyal blade unossified. Anterior limb of cleithrum length greater than cleithrum ascending limb length. Post-temporal fused with supracleithrum in mature specimens. First hypohyal bell- or cylinder shaped.

skull, resembling the head and skull shape of Sternarchella duccis ( Lundberg et al., 1996) . This morphology does not appear to be a secondary sexual phenotype of mature males or females ( Lundberg et al., 2013; fig. 8). It is unclear if a concave head and skull is linked to trophic differences. In the Western Amazon, S. orthos exhibits a wider head, a straighter skull, deeper body, and larger body size than observed in populations from the Eastern Amazon. In the Orinoco basin, Sternarchella orthos exhibits a longer tail, a narrower head and a shallower body than populations from the Eastern and Western Amazon. No discrete osteological, or genetic differences are yet known between populations of S. orthos in the Amazon and Orinoco basins.

Craniofacial variation in Sternarchella orthos . Sternarchella orthos exhibits a wide range of craniofacial variation, from a slender head with a superior mouth to a deeper head with a terminal mouth ( Figs. 21-22). Lundberg et al. (2013: fig. 8) concluded that these differences do not indicate species-level differences, as the variation is continuous and does not correlate to other morphological characters. Here we report similar findings in head shape using geometric morphometrics. We detected no discrete differences among three populations of S. orthos from the Western Amazon, Eastern Amazon and Orinoco basins ( Fig. 22).

Coloration in alcohol. Yellowish brown color, with striations demarcating the ribs at the body cavity. In life, this species is pale white in color with a pink hue ( Fig. 12d).

Sexual dimorphism. No sexual dimorphism found in 9 male and 11 female specimens.

Distribution and habitat. ( Fig. 17). This species is widely distributed throughout the Amazon and Orinoco River basins.

Remarks. This species is highly variable throughout its range with narrower and longer individuals found in the Orinoco River basin, and deeper-bodied populations found in the Amazon basin ( Fig. 21-22). This species is also highly variable in mouth position ranging from terminal in the Western Amazon to superior in the Eastern Amazon, as also observed by Lundberg et al. (2013).

In comparison to congeners, S. orthos exhibits a wide range of variation in head shape and body depth throughout its geographic range. Sternarchella orthos in the Eastern Amazon often possess a concave head and Material examined. Porotergus terminalis: CAS 54912, 1, Peru, Loreto, Iquitos, 1 Jan 1922, S. Morris. Sternarchella orthos: USNM 228839, 6, 229-256 mm LEA, Delta Amacuro, Río Orinoco, Old Shipping Channel S. of Isla Portuguesa, Venezuela. USNM 228725, (2 C&S) Delta Amacuro, Río Orinoco, Old Shipping Channel S. of Isla Portuguesa, Venezuela. USNM 228840, 13, 176-259 mm LEA, Delta Amacuro, Río Orinoco, deep river channel near Los Castillos, 161 Nautical miles upstream from sea buoy, Venezuela. USNM 228871, 8, 235-301 mm LEA, Delta Amacuro, río Orinoco, deep river channel near Los Castillos, 162 Nautical miles upstream from sea buoy. “ Sternarchella terminalis ”: Brazil: ANSP 199199, 5, 159-173 mm LEA, rio Amazonas below rio Xingu. FMNH 115236, 3, 122- 175 mm LEA, Pará, Brazil, rio Amazonas, between tributaries Furo de Urucuricaia and Paraná dos Arraiolos, between towns Almeirim and Gurupa. IDSM 495, 2, 110-110 mm TL, Mamirauá Lake System, Paraná Maiana station A, 2.5 km 045° from Comunidade Boca do Mamirauá, 03º06.74’S 64°47.53’W, 9 Sep 1999. INPA 18265, 1, 252 mm TL, Mamirauá Lake System, Paraná Maiana station A, 03º06.74’S 64°47.53’W, 28 Jan 1999. MCP 33370, 2, 79-93 mm TL, Mamirauá Lake System, Lago Promessa, 03º04.38’S 64°46.97’W, 19 May 1998. MCP 49432, 1, 155 mm TL, Mamirauá Lake System, Paraná Maiana station A, 03º06’44”S 64°47’32”W, 13 Jan 1999. MCP 49433, 2, 231-267 mm TL, Mamirauá Lake System, Paraná Maiana station C, 03º04’06”S 64°47’31”W, 5 Feb 1999. MCP 49435, 4, 125-246 mm TL, Río Japurá, West bank at Boca do Lago Mamirauá, 03º07’36”S 64°46’15”W, 3 Dec 1999. MCP 49436, 4, 160-250 mm TL, Río Japurá- Solimões Confluence, Praia Caborini, 03º09’08”S 64º47’04”W, 19 Feb 2001. ZUEC 12340, 1, 262 mm, Mamirauá Lake System, Paraná Maiana station B, 03º04.83’S 64°47.30’W, 25 Jan 1999. ZUEC 12341, 1, 219 mm TL, Río Tefé, Toco Preto, 03°47.31’S 64°59.91’W, 22 Oct 1999. Peru: MUSM 45236, 2, Madre de Dios state, Río Madre de Dios 2 km upstream bridge at Puerto Maldonado. UF 238215, 1, 271 mm LEA, Peru, Isla Milagro beach, East bank Río Amazonas, upstream of Iquitos, 7.64 km and 21.86° from Plaza de Armas.