Blanket octopus

An open-ocean octopus that never touches the bottom. The female grows to about two metres and carries two enormously extended dorsal arms joined by a sheet of webbing — the blanket — which she unfurls and spreads when threatened, and can shed in strips that trail behind her like a decoy. The male grows to about two and a half centimetres. The ratio is the most extreme sexual size dimorphism known in any animal that is not parasitic, and the two sexes look so unlike each other that they were described independently.

The male has one arm, the hectocotylus, loaded with sperm. He detaches it into the female’s mantle cavity and dies shortly afterwards; she can carry several from different males and fertilise her eggs at leisure. This is why a photograph of a male is a much rarer thing than a photograph of a female — the male’s adult life is short and almost entirely spent as plankton.

Both sexes, while small, are immune to the venom of the Portuguese man o’ war and tear off lengths of its tentacle to hold in the suckers of the first pair of arms. The stolen tentacle is used defensively and offensively. It is the reason the species carries a hazard note here: the nematocysts are still live, and they do not care that they are being held by an octopus rather than a siphonophore.

Blackwater diving is what put this animal within reach of a camera. Before drifting night dives over deep water became normal practice, the blanket octopus was known mostly from strandings and from stomach contents.