An octopus has three hearts, nine brains, and blue blood. Two hearts pump blood to the gills, while a third circulates it to the rest of the body.

You can tell a male octopus from a female by looking at the tip of its third arm on the right (starting between the eyes and going clockwise). Males have a special tip on this arm that has no suckers on the last few inches. This specialized tip is visible even in immature octopus. To mate, the male inserts this special arm into the female’s body cavity. A captured male will try to protect this arm.