|No, that's not fancy lettuce, it's flatworms penis-fencing|
Flatworms are hermaphrodites where every member of the species sports a pair of dagger-penises. Flatworms end up fighting with their dagger-penises in attempt to make the other flatworm bear the burden of motherhood (a tactic we too wish we could use). This epic penis-fencing can last up to an hour and is quite violent. The flatworms use their whole bodies to twist and flip around one another to try and jab each other with their double-pronged shlongs.
The first flatworm to deliver a successful jab is the winner and continues to shoot semen into the skin of its opponent. The losing flatworm must use up valuable resources to provide motherhood, which is why they fight for the role. The loser can come away with dozens of gaping cuts on their bodies and sperm can be absorbed anywhere on their skin.
In 2000 Michiels & Bakovski wrote a research paper with the greatest title ever: "Sperm trading in a hermaphroditic flatworm: reluctant fathers and sexy mother". It is handy to know that when these worms stab each other the sperm is actually digested into the body of the other flatworm and can be used to fertilize eggs after that. Secondly, sperm trading isn't always one-sided. Sometimes neither worm gets fertilized (about 35% of the time), other times it's reciprocal (38%), and least likely is only one worm getting spunk (27%).
|From Michiels & Bakovski 2000|
The researchers also found that the amount of sperm transferred depended on how much spunk was already in that worm's trunk prior to mating, and was NOT dependent on the amount of spunk being delivered by the current sparring partner. They found that flatworms with more allosperm from previous matings had more self-sperm and consequently transferred more. What does that really mean? It means that sperm digestion actually boosts sperm production. YUM YUM CUM GUZZLERS.
Michiels, N. K., & Bakovski, B. (2000). Sperm trading in a hermaphroditic flatworm: reluctant fathers and sexy mothers. Animal behaviour, 59(2), 319-325.