Arapaima Fishing, Amazon River, Brazil

Arapaima gigas scales have a highly mineralised, very hard outer layer with a corrugated surface under which lie several layers consisting of collagen fibres. In a structure similar to 'plywood', the fibres in each successive layer are oriented at right angles to that in the previous layer for maximum toughness. The hard corrugated surface of the outer layer, the soft but tough internal orthogonal collagen layers and the hydration of the scales all contribute to their ability to flex and deform while remaining strong—a solution that allows the fish to remain mobile while heavily armoured and enables it to live in piranha-infested lakes, where no other animals could survive.
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. This is an adaptation to the often hypoxic conditions of the Amazon floodplains, but requires the arapaima to surface for air every 5 to 15 minutes. This tendency to stay at the surface makes it more vulnerable to attacks from spear fishermen.