The Portuguese man o' war, Physalia physalis, carries some of the more potent nematocysts in the open ocean. Blanket octopuses appear to be immune to them, and small individuals have repeatedly been found holding severed lengths of Physalia tentacle in the suckers of their dorsal arms.
The interpretation is that the octopus wields them, in the manner of a weapon it does not have to grow. Direct observation of use is limited, and the association is documented mainly from collected specimens and a small number of field encounters, so the behaviour is better described as well attested than as fully characterised.
The size difference is stranger still. Females reach around two metres, while males are roughly the size of a walnut, a mass ratio often cited as among the greatest known in animals. Males mate by detaching the hectocotylus arm entirely, which remains in the female's mantle cavity; the arm was originally described as a separate parasitic organism. The webbing of the female, sometimes deployed as a broad sheet, is thought to serve as a threat display rather than a net for prey.