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.

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