This unusual nature story/discovery comes from the recent article at Discover titled:
These flatworms plunge their penises into their own heads to inject themselves with sperm (when they must).
As the article tells us:
“The flatworm Macrostomum hystrix isn’t exciting to look at. Its diet of microalgae doesn’t raise any eyebrows, and you probably wouldn’t even notice one if you came across it in its native habitat. But in the bedroom, these flatworms take kink to a whole new level: when they can’t find a partner, they will stab themselves in the head with their needle-like penises and inject sperm to self-fertilize.
In principle, it is what scientists call hypodermic insemination: the practice of forcefully depositing sperm outside a female genital tract—and yes, it’s as usually as rough as it sounds. Bed bugs are infamous for it, as this type of insemination (also called traumatic insemination) causes major damage to the female and reduces her fitness, though a number of species have been demonstrated to reproduce in this way, including flatworms.
Since most flatworm species are hermaphrodites (possessing both male and female sex parts), the decision of who gets to inseminate whom is settled in a rather special way: penis fencing. Two individuals will, quite literally, use their male copulatory parts (called stylets) in bizarrely beautiful combat until one is able to plunge his sperm-delivering organ into the other’s body. The then-labeled “male” in this exchange earnestly injects his sperm, then disengages, leaving the now-female with the burden of bearing the pair’s young.
But what if a penis-fencing male can’t find a sparring partner? According to new research just published in Proceedings of the Royal Society B, they just inject themselves instead.
Scientists from the University of Basel in Switzerland and Bielefeld University in Germany had recently demonstrated that M. hystrix was capable of self-fertilization — not an uncommon ability amongst species that possess functioning male and female reproductive parts simultaneously. But they weren’t sure how the flatworms moved their sperm, which is neatly packaged in their male bits at the tail end of their bodies, to somewhere where the sperm could migrate towards the female bits nearer to the head. Luckily, the see-through nature of these flatworms allowed the scientists to observe sperm moving about inside their bodies, so they placed mature flatworms into wells of a 24-well plate either alone (“isolated”) or with two others (“triplet”), and checked on them over time.
The triplet flatworms, unsurprisingly, had sperm injected into their bodies — most often in the tail to mid-body region — after eleven to thirteen days of snuggling up together. The isolated worms also had sperm in their bodies, but oddly, the sperm were found somewhere else: in the animals’ heads.
“Such a pattern strongly suggests that outcrossing in triplet worms normally occurs through hypodermic insemination into the tail region of the
worm,” the authors explain, “whereas self-fertilization in isolated worms is likely achieved through the extraordinary mechanism of hypodermic
self-insemination.” In short — when there’s no one else to stab a stylet into, these flexible worms whip their tails around, penis at the ready, and inject their own heads with sperm.”