Four photos (contributed by Dr. Steve O'Shea) of a fully mature male Ocythoe tuberculata. This is a pelagic octopus, closely related to Argonauta. The fourth photo is a close up of the hectocotylus, which in this monotypic family/genus/species is enclosed within a large sac-like pouch. It explodes off the male and “swims” to the female's mantle cavity, nestling itself amongst her gills, releasing the sperm for internal fertilisation (in the female's ovariducts you can find developing embryos, which means that fertilisation must be internal).