Sea hares are hermaphrodites, with fully functional male and female reproductive organs. The penis is on the right side of the head while the vagina is situated in the mantle cavity, beneath the shell, deep down.

They can mate in pairs with one acting as a male, the other as a female, but they commonly occur in crowded numbers during the mating season, and this often leads to chains of three or more sea hares mating together. The one at the front acts as a female and the one at the back as a male. The animals in between are acting as both males and females, in other words, the animal receiving sperm passes its own sperm to a third sea hare.