Wood favors a conservative definition of tool use. By that standard—an animal using a solid object to solve an immediate problem, rather than just to provide defense against potential predators—there aren’t any real clear examples of octopuses using tools. But, he said, defining tool use isn’t a black or white issue. There’s no single, official right answer.
Mather works from a different, but still valid, definition. She doesn’t count the coconut shell as a tool, because the octopus isn’t modifying the shell in any way, and isn’t using it to alter other things in its environment.
But she does think octopuses use tools. In 1991, she documented octopuses collecting rocks and stacking them, outside the opening of a shelter, to form a protective fence. In fact, she said, the octopuses tended to do this before they went to sleep. That doesn’t count as tool use to Wood, but with several scientifically sound definitions, there’s room for interpretation.
Cephalopods lovers should also check out Fuck Yeah Octopus.