Blanket octopus pairs are some of the undersea world’s oddest couples. What’s so startling is the size difference: Males are about the size of a walnut—less than an inch long—but some females can reach a whopping six feet long. They can also weigh up to 40,000 times more than males.

That's one of the largest size differences between males and females—called sexual size dimorphism—in the animal kingdom. Why the dramatic disparity? It’s not fully known, but it’s thought that males put their energy into looking for females, not growing.

Reproduction

Mating happens at arm’s length for the four species of these cephalopods. The tiny male detaches its hectocotylus—a modified arm that holds its sperm—and gives it to the female, who keeps it in the mantle cavity until needed for fertilization. When it’s time, the octopus lays upwards of 100,000 eggs, then retrieves the hectocotylus and spreads the sperm out over the egg bundle.

Armed and dangerous

Another unusual aspect of the blanket octopus: It’s immune to the stinging cells of the highly dangerous (to humans at least) jellyfish, the Portuguese man-of-war, which it uses to its advantage by yanking the siphonophore’s tentacles off and brandishing them as weapons against predators.

Blanket octopuses get their name from sheets of webbing that stretch between some of their arms. When threatened, they stretch their arms out, creating a blanket-like silhouette meant to frighten would-be attackers away. They are always in the open ocean—in the Atlantic and Pacific oceans, as well as in Info-Pacific waters—and so never rest on the seafloor. They need all the defense they can get.