Flatworm can self-fertilize by stabbing itself in the head

Macrostomum hystrix flatworm

If there’s no other flatworm around for mating, a tiny translucent Macrostomum hystrix flatworm will inject sperm into its own head or any other convenient body surface. 

Lukas Schärer/FLICKR (CC BY-SA 2.0)


Lonely hermaphroditic flatworms with needle-tipped male organs apparently inject themselves with sperm in whatever body region is easy to stab.

If raised alone in the lab, a tiny Macrostomum hystrix flatworm ends up with sperm distributed oddly around its body, researchers report July 1 in Proceedings of the Royal Society B. More sperm gets injected into the head and upper body and less into their rears than in counterparts with social opportunities, says coauthor Steven Ramm of Bielefeld University in Germany. He knew isolated flat worms self-fertilized somehow. The new work suggests they just curl their rears forward for a convenient jab.

Hypodermic-style mating is the norm. When not alone, the flatworms vie to deliver sperm rather than receive it in encounters Ramm characterizes as “hit and run.” But if there’s no partner around, a solo flatworm will stab itself to become a single mom-dad.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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