When flatworms mate, both use their sharp, bifurcated penises to fence each other in a battle that can last up to an hour. The winner stabs the loser, injecting sperm through the body wall. The 'loser' then carries and bears the offspring—a costly role these hermaphrodites actively try to avoid.
Flatworm Sex: A Sword Fight Nobody Wants to Lose
Somewhere in the warm, shallow waters of a coral reef, two flatworms are locked in mortal combat. Not over territory. Not over food. They're fighting to avoid becoming a mother.
Welcome to penis fencing—and yes, that's the actual scientific term.
The Rules of Engagement
Marine flatworms of the genus Pseudobiceros are hermaphrodites, meaning each individual has both male and female reproductive organs. In theory, this should make mating simple. In practice, it's turned reproduction into a gladiatorial sport.
Both flatworms extend their bifurcated penises—essentially twin-pronged daggers—and attempt to stab each other while simultaneously dodging incoming attacks. These bouts aren't quick jabs; they can last anywhere from a few minutes to over an hour of relentless parrying and thrusting.
Why Fight at All?
The answer comes down to economics. Being the "female" in this transaction is expensive:
- Energy costs: Producing eggs and bearing offspring requires significant metabolic resources
- Time investment: The pregnant flatworm can't immediately mate again as the male
- Risk: Carrying developing embryos makes the flatworm more vulnerable to predators
Being the "male"? You stab, you leave, you're done. No wonder everyone's fighting for that role.
Traumatic Insemination
The winner doesn't deposit sperm in any designated reproductive tract. Instead, it pierces the loser's body wall and injects sperm directly into the flesh. This sperm then migrates through the body to fertilize the eggs.
Scientists call this traumatic insemination, and while it sounds brutal, it's surprisingly common in the animal kingdom. Bedbugs do it. Some sea slugs do it. But flatworms have elevated it to an art form with their elaborate fencing matches.
The Wound That Keeps on Giving
Being stabbed isn't just metaphorically costly—it's literally damaging. The puncture wounds take time and energy to heal. Researchers have observed flatworms with multiple scars from previous mating attempts, suggesting some individuals lose repeatedly.
The losing flatworm will absorb the sperm through its body tissues, eventually producing eggs that it must then carry to term. Meanwhile, the winner glides away, unencumbered and ready to fence another day.
Evolution's Strangest Arms Race
This bizarre mating system has driven the evolution of increasingly sophisticated penis weaponry. Some species have developed longer, more flexible stylets. Others have evolved faster strike speeds or better defensive maneuvers.
It's an evolutionary arms race—except the arms are penises, and the prize is not getting pregnant.
For these beautiful, ribbon-like creatures gliding across coral reefs, every romantic encounter is a duel. And in the world of flatworm love, chivalry isn't just dead—it was stabbed to death with a bifurcated penis.