Arapaima
Morphology

Arapaima can reach lengths more than 2 m (6 ft 7 in), in some exceptional cases even exceeding 2.6 m (8 ft 6 in) and over 100 kg (220 lb). The maximum recorded weight for the species is 200 kg (440 lb), while the longest recorded length verified was 3.07 m (10 ft 1 in). Anecdotal reports suggest that specimens as long as 4.57 m (15 ft 0 in) exist, but verification is deemed impossible, and thus considered questionable. As a result of overfishing, arapaima more than 2 m (6 ft 7 in) are seldom found in the wild.

The arapaima is torpedo -shaped, with large, blackish-green scales and red markings. It is streamlined and sleek, with its dorsal and anal fins set near its tail.

Arapaima scales have a mineralised, hard, outer layer with a corrugated surface under which lie several layers of collagen fibres in a Bouligand -type arrangement. In a structure similar to plywood, the fibres in each successive layer are oriented at large angles to those in the previous layer, increasing toughness. The hard, corrugated surface of the outer layer, and the tough internal collagen layers work synergistically to contribute to their ability to flex and deform while providing strength and protection—a solution that allows the fish to remain mobile while heavily armored. The arapaima has a fundamental dependence on surface air to breathe. In addition to gills, it has a modified and enlarged swim bladder, composed of lung -like tissue, which enables it to extract oxygen from the air.
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