https://blog.nature.org/2021/05/24/meet-the-argonaut-the-worlds-weirdest-octopus/

> Male argonauts use a modified arm, known as a hectocotylus, to transfer their sperm to the female. This special tentacle, found in a pouch under their left eye, has small grooves to hold the sperm securely in place. But the males don’t just reach out and pass off a parcel of sperm. Oh no. They detach their own arm and give it to the female. 
The female inserts the hectocotylus into her egg case, holding onto it until she’s ready to fertilize her eggs. The male dies, or so we think.

> Scientists weren’t aware that male argonauts existed until the 19th century. Knowing little about the species reproduction, they thought all argonauts were the larger, shell-building females. Famed naturalist Georges Curvier observed the dismembered arms and thought they were a species of parasitic worms, which he named Hectocotylus octopodis. Worms they were not, but they still bear the name hectocotylus.

> We know a bit more about argonaut reproduction today, but scientists still have never observed live, male argonauts in the wild.