Invertebrates
Blanket Octopus
Tremoctopus violaceus · also called blanket octopus, violet blanket octopus
The blanket octopus lives its whole life adrift in the open ocean, and almost everything about it is strange. The female can spread to two metres across on great sheets of iridescent webbing. The male is the size of a walnut and dies not long after mating, having handed over one of his arms. And when young, both sexes rip the stinging tentacles off Portuguese man o’ war and carry them around like whips.
Overview
Blanket octopuses of the genus Tremoctopus drift through the warm surface waters of oceans worldwide, never touching the seafloor. Females reach up to 2 m across their outstretched blanket; adult males are only about 2.4 cm. This is one of the most extreme size differences between the sexes of any large animal.
The blanket
Long, thin sheets of webbing join the female’s four longest arms. Normally she keeps them furled, but when threatened she unfurls them into a broad, billowing cape, instantly appearing several times her real size. If a predator seizes the blanket, she can tear away the damaged section and escape, the way a lizard sheds its tail.
A borrowed weapon
Young blanket octopuses and adult males are immune to Portuguese man o’ war venom. They break off lengths of the man o’ war’s stinging tentacles and hold them in their suckers, brandishing them at predators and prey and swapping them for fresh ones when they wear out.
The tiny male
The male matures at a fraction of the female’s size. To mate, he charges a specialised detachable arm, the hectocotylus, with sperm, passes the entire arm to the female, and dies soon after. The female stores the arm until she is ready to fertilise her eggs, which she carries and guards herself.
Rarely seen
For much of the twentieth century the blanket octopus was known mostly from dead specimens and predators’ stomachs. Because it lives dispersed across open water, a live sighting of a female cruising with her blanket spread is still rare enough to make the news, and no one has a population estimate for it.
Blanket Octopus: common questions
Why is the female blanket octopus so much bigger than the male?
The blanket octopus shows one of the largest size differences between the sexes of any animal that is not microscopic, with a female potentially tens of thousands of times heavier than a male. The likely reason is the open-ocean lifestyle. Mates are scarce and hard to find in the vast pelagic zone, so a male gains little from growing large and much from maturing fast, finding a female quickly, and mating once. A female, by contrast, benefits from being big enough to carry and protect a huge number of eggs. The result is a female the size of a person's arm span and a male the size of a walnut.
How does the blanket octopus use Portuguese man o' war tentacles?
Juvenile blanket octopuses and adult males are immune to the venom of the Portuguese man o' war, a colonial relative of jellyfish whose stings are agonising to most animals. The octopus deliberately breaks off pieces of the man o' war's stinging tentacles and holds them in the suckers of its arms. It can then wave these venomous whips at a predator or at prey, effectively wielding a borrowed weapon. When the tentacles lose their potency the octopus discards them and collects fresh ones.
What is the blanket, and what is it for?
The blanket is a pair of thin, wide sheets of webbing that connect the female's four longest arms, a bit like the webbing between a normal octopus's arms but enormously extended. Most of the time she keeps it rolled up out of the way. When a predator appears she unrolls it and spreads her arms, transforming instantly from a slender shape into a large, rippling, iridescent sheet that can be several times her real size. If something bites the blanket she can tear off the damaged part and swim away, much as a lizard drops its tail.
Where and how would you ever see a blanket octopus?
Only in the open ocean, usually in warm water and near the surface, and usually by luck. Blanket octopuses spend their whole lives adrift far from land, so they are almost never encountered by shore divers. The sightings that make headlines are typically made by scuba divers on offshore trips, by snorkellers over very deep water, or by people on boats, and they most often involve a swimming female with her blanket at least partly unfurled. Small juveniles and the tiny males are even harder to spot and are more often found in plankton samples than seen alive.