Argonaut or paper nautilus is a weird species of octopus. First, they have a highly divergent sexual dimorphism. That’s science-speak for the difference in body sizes between males and females. A female argonaut grows up to 10 cm (~ 4 in.) with shells as large as 45 cm (~ 18 in.) The male, however, is only 2 cm (3/4 in) long!

But that’s not why argonaut is on this list. The male argonaut produces a ball of spermatozoa in a special tentacle called a hectocotylus [wiki]. When meeting a female it fancies, the male then detaches its penis to swim by itself to the female!

Hectocotylus (Image Credit: Julian Finn, Macalogist)

This detachable swimming penis was actually first noted by an Italian naturalist back in the 1800s, who mistook it for a parasitic worm!