A Gulf in Size
In the open tropical oceans, the blanket octopus (Tremoctopus) is the most extreme example of sexual size dimorphism in any large animal. Females of the common blanket octopus, Tremoctopus violaceus, can reach up to 2 meters (6.6 feet) in length. The males, by contrast, grow to a mere 2.4 centimeters (about 1 inch). This size difference is staggering; a female can be up to 40,000 times heavier than her mate. To put this in perspective, if the female were the size of an average human, the male would be the size of a walnut.
The female is named for the spectacular, iridescent webs of skin that connect her dorsal and dorsolateral arms. When threatened, she unfurls this "blanket" to dramatically increase her apparent size, deterring potential predators. If a predator attacks, she can detach sections of the blanket as a decoy, much like a lizard shedding its tail. The male has no such blanket. He is a tiny, pelagic drifter, spending his entire life in the open water column, far from the seafloor.
A Disposable Mate
The reproductive life of the blanket octopus is as strange as its size difference. The male’s third right arm is a specialized reproductive tool called a hectocotylus, which is filled with sperm. When a male finds one of the colossal females, he fills this arm with sperm, detaches it from his body, and gives it to her. The arm then crawls into the female's mantle cavity, where she stores it until she is ready to fertilize her eggs. A female may collect and store hectocotyli from multiple males.
This act of mating is fatal for the male. Having fulfilled his only purpose, he dies shortly after relinquishing his reproductive arm. The female continues to live, carrying the fertilized eggs in a sausage-shaped calcareous case held at the base of her arms until they hatch. She can lay between 80,000 and 100,000 eggs.
Stolen Weapons
Both male and juvenile female blanket octopuses have a remarkable defense strategy. They are immune to the venom of the Portuguese man o' war (Physalia physalis), a highly venomous siphonophore. They actively seek out these creatures, rip off their stinging tentacles, and wield them as weapons. Holding the stolen tentacles in their suckers, the octopuses use them for both defense against predators and to stun their own prey.
This behavior is most common in the small males and females (less than 7 cm long). As the females grow to their massive adult size, this defense mechanism becomes less practical, and they rely more on their intimidating blanket. The male's reliance on this stolen weaponry may be one evolutionary reason for his diminutive size. The first live male blanket octopus was not observed by scientists until 2002, during a night dive off the Great Barrier Reef.