Discover how researchers identified an octopus' ancient sex chromosomes and why their genetic history is such a mystery.

Blanket octopus pairs are some of the undersea ... 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 ...

TAYLOR MAGGIACOMO, NGM STAFF After attaching to the female ... This species of octopus is also known as the paper nautilus (the shell it builds to hold its eggs is made of paper-thin calcium ...

For many species of octopus, you can tell if they are male or female if they have a groove under their third right arm. If there is, it’s a male and they use this arm in mating. What they do is ...