You’re sitting on the seabed, just off the coast of the Indonesian island of Lembeh. You’re not deep—20 feet or so—and there’s plenty of light. As you’d expect in such a tropical place, the water is warm. All around, you see ripples of a fine gray-black sand, covered, in places, with a kind of greenish scum. As you explore, you notice a conch shell. Stoutly made, it has six heavy spikes coming off it. Perhaps the maker is within. Or perhaps the maker is long dead, and the shell now belongs to a hermit crab. Curious, you flip it over. A row of suckers. A pair of eyes.
An octopus. In particular, Amphioctopus marginatus, also known as the coconut octopus. Its common name comes from its habit of hiding in…