The Intense Underwater Duel of Penis Fencing Flatworms

This footage captures a natural biological behavior known as 'penis fencing' performed by hermaphroditic flatworms. These marine creatures engage in a competitive mating ritual where they use their dual reproductive organs to attempt to inseminate one another. Because both individuals act as both male and female, they 'duel' to determine who will take on the role of the sperm donor and who will become the recipient, as the latter incurs the higher metabolic cost of pregnancy. The white, sword-like structures visible in the video are the specialized organs used for this process, which often resembles a high-stakes fencing match on the ocean floor.