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Orthosternarchus tamandua

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Orthosternarchus tamandua is a species of weakly electric knifefish in the family Apteronotidae, native to the deep river channels of the Amazon River system. It is the only species in its genus. This species is characterized by its long tubular snout, long dorsal appendage, and tiny, bilaterally asymmetrical eyes.[1]

O. tamandua was originally described as Sternarchus tamandua by George Albert Boulenger in 1880; its species name is from the Tupi word tamanduá, meaning “anteater”, in reference to its long snout. In 1905, Eigenmann and Ward placed this species in the genus Sternarchorhamphus, though noting that it may represent a distinct taxon. In 1913, Ellis placed this species into its own genus Orthosternarchus, from the Greek orthos (“straight”), sternon (“chest”), and archos (“rectum”), referring to the straight snout and the forward placement of the urogenital opening.[1]

Ellis saw O. tamandua as a close relative to Sternarchorhamphus, which has been corroborated by recent morphological and molecular analyses. The relationship between these two genera and the rest of the Apteronotidae is less clear, though they are generally regarded as basal within the family. Some authors consider them to be sister taxa to Platyurosternarchus and Sternarchorhynchus, which also have elongate heads. However, in those taxa the elongation is accomplished by lengthening of the jaws, whereas in Orthosternarchus and Sternarchorhamphus it is accomplished by the lengthening of the head while the jaws remain relatively short. This suggests that elongate head shape was independently evolved between the two groups.[1]

O. tamandua is a relatively rare species found in the Amazon River basin, and is most abundant in the Rio Negro and the Rio Purus. It inhabits both whitewater and blackwater rivers, usually occurring at depths of 6-10 m and occasionally shallower or deeper, but never beyond 20 m down. It is absent from floodplain channels, river edges, and small lakes. A few specimens are known from the confluence of the Rio Solimões and the Rio Negro, and from Lake Prato, Anavilhanas.[1]

One of the gymnotiforms most specialized to living in deep river channels, O. tamandua resembles fishes adapted to caves in several respects. Their elongated, laterally compressed bodies are nearly unpigmented, appearing bright pink due to the blood underneath. The eyes are tiny and virtually non-functional, and are placed asymmetrically on the head.[2] The asymmetry of the eyes is not correlated with size, sex, or environment, but may be related to their degenerate state. The snout is distinctive, being a long, evenly tapering, nearly straight tube measuring four times as long as high. The mouth is relatively small. The dorsal throng (a whip-like appendage used for electroreception) is unusually long and thick, which led it to originally be described as a “very strongly developed adipose fin” by Boulenger.[3] The throng originates close to the rear margin of the skull, which is much more anterior than in other apteronotids.[1]