Flatworms Self-Fertilize By Injecting Sperm Through Their Heads

Flatworms have some bizarre mating habits and now, researchers have taken a closer look. They've found that the flatworm, Macrostomum mystrix, injects its sperm into its own head in order to reproduce.

Not having a mate usually spells disaster for animals that reproduce sexually; not so for the flatworm. Like other simultaneous hermaphrodites, this worm has both male and female sex organs at the same time. This means that it can self-fertilize.

The researchers found that when flatworms self-fertilized, they had very few sperm in their tail region. Instead, they have sperm in their head region. This is in stark contrast to worms kept in a group; most of them contained most sperm in their tails, close to where fertilization actually occurs.

So what does this mean? Using its needle-like male copulatory organ, an isolated worm can self-inject sperm into its own anterior body, from where the sperm then moves through the body and toward the eggs.

"As far as we know, this is the first described example of hypodermic self-injection of sperm into the head," said Steven Ramm, first-author of the new study, in a news release. "To us, this sounds traumatic, but to these flatworms it may be their best bet if they cannot find a mate but still want to reproduce."

The findings reveal a bit more about the strange sexual behavior of these worms. By injecting sperm into their heads, the worms and reproduce even when there are no viable mates present for the time being.

The findings are published in the journal Proceedings of the Royal Society B.

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