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 discarded coconut shells (sometimes it even picks them up and carries them about, for use as an emergency shelter). But in fact, any big shell will do—such as a conch.
With a few of its suckers, this octopus is holding two halves of a clamshell. As you watch, it drops them and hoists itself up a little. It gives the impression of evaluating the situation. You make like a statue. After a moment, the octopus climbs out of the shell. Its body is the size of your thumb, its arms perhaps three times that. As it moves onto the sand, it turns a matching shade of dark gray. Is it leaving? No. It snakes several of its arms over the sand, and the rest over the shell. With a single heave, it flips the shell back over and flows inside.