Almost all of these creatures, which are also known as nudibranchs, are thought to be “simultaneous hermaphrodites”. This means they have both male and female sexual organs and can use them both at the same time.
Bernard Picton, curator of marine invertebrates at the National Museums Northern Ireland, explained: “The genital apparatus is on the right hand side of the body. So two nudibranchs come together and one faces one way and one faces the other way, with the right hand side of their bodies touching.
“The penis from one fits into the female opening of the other one, and the penis from that one fits into the female opening of the first one, if you see what I mean.