The female blanket octopus glides through the ocean like a winged phantom. When she’s threatened, she extends some of her arms. That spreads the webbing between the arms, like a flowing cape. The shiny cape makes the octopus look bigger—perhaps scaring away predators.

The octopus is impressive even without the cape. An adult female can be six and a half feet long—the size of a basketball player. Her mate, on the other hand, is about as big as a walnut—perhaps an inch across. And a female may weigh up to 40,000 times as much as a male. That’s the biggest difference in the size of adult males and females in the animal kingdom.

Blanket octopuses are found around the world. They’re in the open ocean and around coral reefs. They’re immune to the sting of a Portuguese man-o’-war, so males and young females sometimes tear off the tentacles and use them to defend themselves against predators.

These octopuses are rarely seen. In fact, the first live male wasn’t discovered until 2001. In part, that’s because of its size—it’s tough to spot something that small in the open ocean. In addition, the male is almost colorless.

A male grows a long arm that it fills with sperm. When he finds a mate, he rips off the arm and hands it to her—then dies. She then stores it in a pouch until she’s ready to fertilize her eggs. She may accept the arms of several suitors. After the eggs hatch, she may die as well—the final act for this phantom of the oceans.