The microscopic flatworm (Stenostomum brevipharyngium) is one of nature's weirder creatures. Chop off its head, and it'll grow one back. Cut it in half, and it'll become two separate healthy worms. And now scientists have discovered an even stranger trick. These unsegmented, soft-bodied invertebrates can grow two heads, a normal one and a backward-facing one, in the middle of their bodies where their tail should be. This developmental error lets the worms permanently reverse their head-to-tail body axis without any noticeable harmful effects.

Scientists have long known how to create double-headed flatworms by applying electrical currents. But this is the first time the two-headed variety has been observed occurring naturally and spontaneously. The discovery was made by accident by zoologists Katarzyna Tratkiewicz and Ludwik Gasiorowsk from the University of Warsaw in Poland during routine maintenance of their laboratory cultures. Intrigued, they decided to investigate what was going on.

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

Flipping tails to heads

First, they watched their cultures under powerful microscopes and added chemical markers to confirm that the tail was missing and had been replaced by a functional head. Then they cut up the two-headed worms into three pieces and tracked the healing and survival of each fragment.

The first and third sections mostly healed normally, but the middle section with the backward-facing head did something unexpected. At the opposite end of the head was an open cut, and when it healed, it didn't grow a second head. Instead, it regenerated a tail. This meant that the worm had successfully and permanently flipped its head-to-tail direction.

Causes

To rule out a permanent genetic cause of the error, the researchers monitored the offspring of the double-headed worms and found that they were all normal. This proves that their axis reversal was due to developmental flexibility rather than genetic mutations.

"Our findings suggest that whole body regeneration based on pluripotent stem cells provides a flexible developmental framework in which even a reversal of the A-P axis of the adult bilaterian body can be accommodated," wrote the researchers.

While a genetic mutation has been excluded, the cause of the unusual two-headed creature has yet to be determined. It could be an environmental trigger or even a random developmental error. It's also unknown whether this occurs in the real world. The scientists observed this in the lab, and so we'll have to wait for other research to see if it is a trick the flatworms use in the wild.