Flatworm Reproduces by Using its Penis to Inject Sperm Into its Head

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Aamna Mohdin

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852 Flatworm Reproduces by Using its Penis to Inject Sperm Into its Head
Lukas Schärer

Scientists have discovered a hermaphrodite flatworm that literally fucks itself in the head. The flatworm resorts to self-insemination when it’s unable to find a mate. How's that for kinky?

The flatworm, Macrostomum hystrix, is able to use this reproductive strategy as they have both male and female sex organs. Though this produces ‘inbred’ offspring, it’s still better than not being able to reproduce at all, researchers suggest. 


Normally, these flatworms reproduce by exchanging sperm with others using their stylet – a sharp, needle-like protrusion – to penetrate their partner’s outer body membrane in a process that scientists describe as ‘traumatic insemination.’

Researchers from the University of Basel in Switzerland and Bielefeld University in Germany studied this bizarre reproductive strategy, called hypodermic insemination, in transparent flatworms. The study, published in the journal Proceedings of the Royal Society B, showed just how the flatworm developed this mechanism.

For the study, the flatworms were split into two groups: one batch as isolated individuals and the other in small groups. Researchers then measured how much sperm each worm contained. Isolated flatworms were found to have more sperm in their head. Though researchers didn’t directly observe this self-insemination, the interesting placement of the sperm suggests a strange insemination route. The sperm migrated from the head to the center of its body, which is the creature's site of fertilization. In comparison, the flatworms kept in groups contained more sperm in their tails.

While some hermaphrodites are known to be able to self-fertilize, this may be the first case of them using their needle-like penis to inject sperm into their own head. Researchers suggest this ‘convoluted’ mechanism is needed as there is no internal linkage between the flatworms’ male and female sex organs.


  • tag
  • flatworm,

  • asexual reproduction,

  • evolutionary biology