Barnacles have the biggest penises – up to 8 times their body size

With penises up to 8 times their body size, Barnacles have the biggest penises of all animals.
Compelled to mate, yet firmly attached to the rock, barnacles have evolved the longest penis of any animal for their size – up to 8 times their body length – so they can find and fertilize distant neighbours.

Source: Science Daily, Wikipedia