Sternarchella sima Starks, 1913

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 : 26-28

Sternarchella sima Starks, 1913

Fig. 25, Tab. 10

Sternarchella sima Starks, 1913:22 , plate 4 [type locality: Brazil, Pará, Belém market].

Diagnosis. Sternarchella sima is readily distinguished from congeners by the presence of a short snout (preorbital length 18-28% HL) (vs. 29-36% HL in S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , S. rex , and S. schotti ) (shared with S. calhamazon ), the possession of a large interorbital distance (19-25% HL) (vs. 11-19 %HL in S. calhamazon , S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , and S. schotti ) (shared with S. rex ), the possession of a deep body (body depth equal to or greater than HL (vs. shallow body depth, less than HL in S. calhamazon , S. duccis , S. orinoco , S. orthos , S. patriciae , S. raptor , S. schotti ) (shared with S. rex ), and the presence of gill rakers in which the base does not contact the gill arch (vs. contacting in all other Sternarchella species ).

Description. A medium-sized species, to 189 mm LEA. Pectoral fin size large, 80% HL or greater. PA% small, 24-36% HL. Head width narrow, distance between lateral margins 40-49% HL. Preorbital (snout) length small, 18- 28% HL. Postorbital distance large, 64-75% HL. Eye diameter small, 6-9% HL. Interorbital distance large, 19- 25% HL. Mouth narrow, distance between ricti 12-16% HL. Body depth equal or greater than HL. Scales absent on posterolateral portion of body. Scales large in size with 5-8 present above lateral line at mid-body. 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 sub-terminal, upper jaw extends anteriorly to lower jaw. Body cavity long, 15 precaudal vertebrae. Proximal surface of first displaced hemal spine narrower then descending blade. 1-2 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 short, length shorter than hemal spine. One row of bones in caudal peduncle visible externally. Caudal peduncle shallow less than 30% HL Dark spot on caudal peduncle absent. No apparent connective tissue between anal-fin base and caudal peduncle. Caudal peduncle length short, less than HL. Premaxilla large, lateral margin of premaxilla longer than lateral margin of maxilla. Premaxilla square in ventral view. Four rows of teeth present on premaxilla. Anterior hook of maxilla absent, anterior process 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 curved evenly towards its distal tip. Descending blade maxilla thin, evenly curved. Three to four 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 curved gradually to descending limb. Endopterygoid large, contacting frontal. Angle of endopterygoid process with dorsal margin of endopterygoid oblique (greater than 90° with dorsal surface of endopterygoid). Endopterygoid process slender with a narrow tip. Hyomandibula short, its width half its length. Dorsal margin of opercle concave. Opercle broad, width over half depth. Anterior limb of cleithrum length greater than cleithrum ascending limb length. Post-temporal fused with supracleithrum in mature specimens. Ventral ethmoid size large and robust with a large fan-shaped lateral process. Dorso-anterior portion of mesethmoid strongly curved from anterior tip to frontal boundary. Anterior tip of mesethmoid convex and rounded. Anterior fontanel longer than posterior fontanel. Lateral ethmoid large hour-glass shaped, narrowest 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. Distance between parietal ridges short, just lateral to supraoccipital, parietal ridges very large and pronounced. No crown of thorny projections present at border of parietals and supraoccipital. Dorsal margin of supraoccipital crest extends beyond 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. Base of gill rakers not contacting gill arch. Gill rakers long, with ossified distal tips. Dorsal surface of basihyal convex, forming a robust ridge posteriorly. Second basibranchial triangular with short stem, length of descending rod less than dorsal margin. 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. First hypohyal triangular, base longer than any other margin of bone.

Coloration in alcohol. Yellowish brown color, with a darker brown stripe confined to the dorsum running the length of the body, with striations demarcating the ribs at the body cavity.

Sexual dimorphism. Insufficient specimens are available to ascertain differences between males and females. However, Mago-Leccia (1994) reported that mature female specimens of S. sima have wider heads in the lateral dimension than males of comparable sizes.

Distribution and habitat. Known from the eastern portion of the lowland Amazon basin, with most specimens known from near its mouth ( Fig. 19).

Remarks. Sternarchella sima superficially resembles, S. orinoco in possessing a sub-terminal mouth, three rows of teeth on the pre-maxilla and dentary, and ventral flexion of the mesethmoid region. However, S. sima is a smaller species, reaching 189 mm LEA (vs. 221 mm in S. orinoco ), with a slightly shorter face, pre-orbital length 18-28% HL (vs. 26- 32% HL in S. orinoco ). Two specimens catalogued as S. sima from the western Amazon are not recognized in this species herein. The paratype of S. sima designated by Starks (1913) described the specimen as having 193 anal-fin rays, here we describe that specimen as possessing 191 anal-fin rays.

Material examined. Paratype. Sternarchella sima: AMNH 3864, 1, Brazil, Pará, Belém Market, E. C. Starks. Non-types. Sternarchella sima: ANSP 192108, 1, 146 mm LEA, Brazil, rio Pará above rio Tocantins 83.3 km upriver from town of Abaetetuba, 9.1 km downriver from Boa Vista. USNM 373114, 2, 132-139 mm LEA, Brazil, río Amazonas, 15 km above Breves. ANSP 192107 (3 C&S), Brazil, Pará, rio Jari downstream of Monte Dourado, upstream of Bom Jardim. INPA 48443, 1, Brazil, Pará,Amazon above rio Tapajós.