Tremoctopus is a genus of pelagic cephalopods, containing four species that occupy surface to mid-waters in subtropical and tropical oceans. They are commonly known as blanket octopodes, in reference to the long, transparent webs that connect the dorsal and dorsolateral arms of the adult females. The other arms are much shorter and lack webbing.
Tremoctopus violaceus exhibits the most extreme degree of sexual size-dimorphism known in any non-microscopic animal. Females may reach 2 m (6.6 ft) in length, whereas the males are 2.4 cm. The weight ratio is at least 10,000:1, and can probably reach as much as 40,000:1. The males have a large arm in a spherical pouch modified for mating, known as a hectocotylus. During mating, this arm is detached, and kept by the female in her mantle cavity until used for fertilisation. The male almost certainly dies shortly after mating. There is competition between the males, multiple male arms have been found in the mantle cavity of females. The females carry more than 100,000 eggs attached to a sausage-shaped calcareous secretion held at the base of the dorsal arms and carried by the female until hatching.
Blanket octopodes are immune to the venomous Portuguese man o’ war, whose tentacles the male and immature females rip off and use for offensive and defensive purposes. Like many other octopodes, the blanket octopus uses ink to intimidate potential predators. Also, when threatened, the female unfurls her large net-like membranes that spread out and billow in the water, greatly increasing her apparent size.