We often think of animals hunting and fighting for food, but many flatworms appear to hunt and fight for mates. Each worm is hermaphroditic, containing both ovaries with eggs and testes with sperm. Some even have two penises and one or more genital pores for receiving a unique, two-tailed sperm delivered during copulation.
Using new camera technology, Marine Biologist Leslie Newman of Australia's Southern Cross University participated in filming the marine flatworm Pseudobiceros hancockanus engaging in some odd reproductive behavior -- referred to as penis fencing.
During penis fencing, each flatworm tries to pierce the skin of the other using one of its penises. The first to succeed becomes the de facto male, delivering its sperm into the other, the de facto female. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.
The Shape of Life . The First Hunter | PBS
(Article found at The Shape of Life)