Appearance
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. 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 is unusually long and thick, which led it to originally be described as a "very strongly developed adipose fin" by Boulenger. The throng originates close to the rear margin of the skull, which is much more anterior than in other apteronotids.''O. tamandua'' has a long anal fin, a small caudal fin, and tapering pectoral fins; the dorsal and pelvic fins are absent. The caudal fin shows a great deal of variation due to regeneration after tail loss; in some cases the regenerated fin becomes merged with the anal fin. The fin rays number 9 in the caudal fin, 14-15 in the pectoral fins, and 207-256 in the anal fin. Almost the entire body, except for the dorsal midline, is densely covered with flimsily attached scales, being small and circular towards the front and larger and more rectangular towards the back. There are around 12 rows of scales above the lateral line and 40-42 rows below. The first 5-10 scales along the lateral line are modified into overlapping elongate tubes. The maximum known size is 44 cm long and 125 g.
Distribution
''O. tamandua'' is a relatively rarely recorded species found in the Amazon River basin, and is most abundant in the Rio Negro and the Rio Purus.Behavior
As in other apteronotids, ''O. tamandua'' generates a continuous weak electric field for electrolocation and communication. At 452-605 Hz, the electric organ discharge of ''O. tamandua'' has a much lower frequency than most other apteronotids and approaches the frequencies of the sternopygid knifefishes. The EOD is also unusual in that the waveform is monophasic, with a head-negative spike followed by a positive baseline . The only other apteronotid known to have a monophasic EOD is the related ''Sternarchorhamphus muelleri''. The monophasic waveform of these two genera is similar to the EOD of a larval ''Apteronotus'', suggesting that it may be a paedomorphic or evolutionarily ancestral trait.Habitat
It inhabits both whitewater and blackwater rivers, usually occurring at depths of 6–10 m and occasionally shallower or deeper, but has not yet been recorded beyond 20 m down. It is absent from floodplain channels, river edges, and small lakes.References:
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