cardinal tetra has bright red ventral parts and an iridescent blue line that runs horizontally along its body. The characteristic iridescence of this and related fishes, such as the neon tetra, is a structural color, caused by refraction of light within guanine crystals that develop within special cells called iridocytes in the subcutaneous layer. The exact shade of blue seen depends on the viewing angle of the viewer relative to the fish – if the viewpoint changes so as to look at the fish more from below, the colour will change hue, becoming more deeply sapphire blue and even indigo. Change of viewpoint to one above the fishes, however, and the color becomes more greenish. Cardinal tetras appear to grow larger in captivity than they do in the wild. They have a large stomach and small gut.
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