Blue-ringed octopuses are bottom animals. They are not the octopus of the blue-water documentary, jetting across open sea. They live where the bottom is complicated and the water is thin. Tide pools, reef flats, coral rubble, sandy and silty pockets, and seagrass are the habitats named again and again. Depth for the greater species is mostly the top twenty meters. The southern species is happy deeper, down toward fifty, and still on the bottom.
A den can be anything with a door. Rock crevices are the classic. Empty shells are common. So are the objects people leave: bottles, cans, a bit of pipe. An octopus that can pour itself through a gap the width of its beak does not need a purpose-built cave. It needs a space predators cannot easily follow into, and a view of the crabs walking past.
They will also improve a door. Like other octopuses, they sometimes pull small rocks and shells across the entrance. It is a modest piece of construction, and it is worth noticing because it tells you the animal is not only hiding in the landscape. It is editing it. A pile of pebbles against a hole, in the right water, is a reason to look and a reason not to dismantle the pile to see.
The same habits put them in the path of waders, reef walkers, and anyone collecting shells. The overlap is the whole problem. Their habitat is our shoreline. The journal’s map is not a remote trench. It is the pool at the edge of a beach in the right part of the world.