Hermaphrodite flatworm that fucks itself in the head

Today marks the International Polychaete Day and you can use #InternationalPolychaeteDay  on twitter to follow interesting discussions on Polychaetes but while the focus is on Polychaetes, I have a rather interesting flatworm that I will discuss in this post.

Flat worms are of the phylum Platyhelminthes, the main difference between the two groups is that polychaetes are segmented and plathyhelminthes are not. This particular post is about the mating styles of Macrostomum hystrix a hermaphroditic flatworm that quite literally “fucks itself in the head” – yup, you read that right!

Flat worms are capable of self fertilization because they are Hermaphrodites – animals that produce both sperm and eggs and although they do prefer mating with other worms they do have the ability to self fertilize as a back up plan. A very good description of this can be seen in this YouTube video which describes the mating ritual as “penis fencing”.

In a paper just published in the Proceedings of the Royal Society B, a group of scientists studying the mating behavior and adaptations to hypodermic insemination in M. hystrix researched this ability by splitting 100 individuals into two groups, one batch had isolated individuals and the other had animals in small groups (three to a group) and kept them for a month before measuring the sperm living in their bodies. What they found was that isolated flatworms had more sperm in their heads compared to those in groups who had sperm more towards their tail ends leading to the discussion that isolated worms must have self fertilized themselves. While the act of self insemination itself was NEVER observed, the placement of the sperm does suggest a strange insemination route – animals living in isolation must have swung their sharpened penis around to inject themselves in the head.

While lots of animals are able to self fertilize, this is the first example of one that uses a hypodermic appendage to do so.


Ramm Steven A., Schlatter A, Poirier M, Scharer (2015) Hypodermic self-insemination as a reproductive assurance strategy.  DOI 10.1098/rspb.2015.0660

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