Sternarchella orthos

description

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

Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species

diagnosis

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).

Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species

discussion

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 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 Weste

Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species

distribution

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

Taxonomic revision of the deep channel electric fish genus Sternarchella (Teleostei: Gymnotiformes: Apteronotidae), with descriptions of two new species