On the soft-sediment bottoms this species inhabits, there is little natural cover. Coconut octopuses excavate discarded coconut half-shells and large bivalve valves, clean them of sediment, and assemble them into a portable refuge, sometimes closing two halves around themselves like a shell.
The transport is the striking part. Rather than swimming with the load, the octopus gathers the shells beneath its body and moves on stiffened arms in a gait divers described as stilt-walking. During transport the animal is slower and more exposed than it would be unencumbered, so the shell carries a net cost until it is deployed.
That cost is the basis of the claim, published by Julian Finn and colleagues in 2009, that this qualifies as tool use: the object is carried for anticipated future benefit rather than immediate use. The argument is not unanimous, and some researchers prefer to call it object manipulation or the use of a portable den, since the boundary between tool use and shelter transport is a definitional dispute rather than a factual one.