Octopus populations have been booming in the last 60 years to the point where some marine biologists even call them "the weeds of the sea". Their main purpose in life is to breed and make babies, but the way they do that is real weird.
Male octopuses have a specialized tentacle called a hectocotylus (which is basically a penis). In order to copulate, they must successfully insert that arm into a female octopus's mantle cavity, or as Aristotle called it "the nostril of the female." That may sound simple, but the male must be careful considering female octopuses are often larger and can have cannibal tendencies.
Male octopuses have come up with various methods to get around this dilemma, ranging from extending their hectocotylus into the female's mantle cavity from a distance to actually detaching it and swimming away.