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Who’s Your Daddy? This Worm Injects Itself With Sperm to Get Pregnant

Image: Flatworm

The flatworm known as Macrostomum hystrix has a needle-like male organ as well as a female reproductive organ in its tail region. And it's found a way to put both to use, all by itself. S.A. Ramm, A. Schlatter, M. Poirier, L. Scharer / Proc. B

Researchers say the flatworm known as Macrostomum hystrix has one of the craziest reproduction systems ever conceived: In a pinch, it can plunge a natural hypodermic needle filled with sperm into its own head for self-insemination.

Does that make you go "eww"? Or "wow"?

"To us, the idea of self-injecting sperm does indeed sound pretty gruesome, amusing even," Steven Ramm, an evolutionary biologist at Bielefeld University in Germany, told NBC News in an email. "But it may well be that under conditions where they can't find a mate, it really is the best option available to these worms."

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Ramm and his colleagues lay out their findings about M. hystrix in the Proceedings of the Royal Society B.

Some species of hermaphrodites have been known to be capable of self-fertilization, and some species use a needle-like organ known as a stylet to inject sperm into their mates. But this is the first case where there's evidence that the needle is being used to self-fertilize, Ramm said.

Image: Stylet
This extreme closeup highlights the flatworm's male copulatory apparatus, including a needle-like stylet in the worm's tail region that is used for sperm injection. Ramm et al. / Proc. R. Soc. B

The biggest missing piece in the study is that researchers haven't actually seen the flatworm do the deed.

"What we do not have, and have not directly observed, is a video of the act of self-insemination itself, which is rather inferred from the resulting distribution of received sperm observed in isolated worms," Ramm acknowledged.

Every M. hystrix worm has a male stylet as well as a female part known as a parenchyma. Usually, one worm sticks its stylet into another worm's parenchyma, and motherhood ensues. But Ramm and his colleagues put some worms in isolation and compared them with cohabitating worms to compare where their sperm ended up. They found that the isolated worms had more sperm in their head regions but less in their tail regions (where the needle would usually go).

And yes, the isolated worms had hatchlings.

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The researchers surmise that after injection, the sperm migrates through the worm's body to reach the site of fertilization. This weird scenario probably comes into play naturally when the flatworm population falls to low levels for whatever reason, Ramm said.

"I think what's really striking here is the inventiveness of Mother Nature," he told NBC News. "The evolution of hypodermic insemination and a needle-like copulatory organ to try and inject sperm into the body of another individual in the context of sexual conflict is interesting enough in itself, but the fact that these worms apparently turn that weapon on themselves, and can thereby reproduce even without a mating partner, is truly fascinating."

In addition to Ramm, the authors of "Hypodermic Self-Insemination as a Reproductive Assurance Strategy" include Aline Schlatter, Maude Poirier and Lukas Schärer.