Argonauts

These are basically a slightly strange species of Octopus. Firstly, they have a highly divergent sexual dimorphism, which is otherwise known as extremely different body sizes of males and females. But that's not really why the argonaut makes it on this list. 

When its time for copulation, males and females don't even have to make direct contact. Rather, the male produces a ball of sperm in a special tentacle called the hectocotylus and when it meets a female it fixates on, he then detaches its penis and allows it to swim to the female and impregnate it. Incidentally, the detachable penis was first noted by an Italian naturalist back in the 1800s who mistook it for a worm!