Big Blue Octopus

The common daytime octopus of the reef: a mantle to 22 cm (9 in) and arms to 80 cm (31 in), usually found not as an animal but as a pile of empty shells outside a hole in the coral. That midden is the reliable way to locate one, because the octopus itself will be whatever texture and color the rock behind it is.

The camouflage runs on hardware no other animal has. Millions of pigment sacs in the skin are wired directly to the brain and open or close in milliseconds, while separate muscles raise the skin into spikes and ridges to change its texture. One study counted an animal changing appearance around a thousand times in seven hours. It does all this while being, as far as anyone can tell, colorblind.

One display is hunting rather than hiding. When a crab freezes rather than fleeing, the octopus runs a dark band across its own body toward the prey, over and over, like a shadow passing. The crab breaks and runs, and running is fatal. This trick, called the passing cloud, is one of the few known cases of an animal using a moving image on its own skin as a tool.

Unlike most of its relatives it hunts by day, especially at dawn and dusk, walking over the reef with its arms spread in a web and dropping the whole membrane over a coral head to trap whatever is inside. It lives about a year, breeds once, and dies.