The regenerative power of flatworms — which can regrow into complete individuals after they've been cut into pieces — is well-known among scientists. But a group of flatworms that recently visited the International Space Station (ISS) had a few surprises to share when they returned to Earth.

Scientists sent the worms into space to observe how microgravity and fluctuations in the geomagnetic field might affect the worms' unusual ability to regenerate. This was done to better understand how living in space could affect cell activity.

Compared with a group of flatworms that never left Earth, the spacefaring worms showed some unexpected effects from their time off the planet: most notably, the rare sprouting of a second head in an amputated piece of a worm, the researchers documented in a new study.

Planarian flatworms ( Dugesia japonica) are very flat and tiny, measuring about 0.2 to 0.4 inches (0.5 to 1 centimeter) in length, study co-author Michael Levin, a professor of biology at Tufts University in Massachusetts, told Live Science in an email. (Levin is also the director of the Allen Discovery Center at Tufts and the Tufts Center for Regenerative and Developmental Biology.)
One flatworm could result in multitudes, under the right conditions. Individuals can perform fission — dividing to form two distinct individuals — and severed flatworms can grow new heads or tails , depending on where the body was cut.

To find out how factors such as gravity and Earth's magnetic field affect the worms' ability to regrow themselves, scientists sent sets of whole worms and amputated worms to the ISS for five weeks, the study authors wrote. Researchers sealed the worms inside tubes with varying ratios of air and water, and then observed the animals when they came back, the authors wrote.

Return of the regenerating space worms

After the worms returned, the researchers tracked changes in the animals' bodies and in their microbes, comparing the test worms with flatworms that had never left Earth. And the researchers continued observing the worms over 20 months, to see whether any changes were long-lasting.

The scientists found several significant differences between the flatworms that went to space and the Earth-bound worms. For instance, during the first hour of immersion in containers of fresh spring water, the spacefaring worms appeared to experience "water shock"; they curled up and were "somewhat paralyzed and immobile," the researchers wrote, suggesting that the worms underwent metabolic changes while in space. The space-y flatworms exhibited normal behavior after about 2 hours, but further analysis revealed that their microbial communities had changed, hinting at metabolic shifts caused by the unusual conditions the worms encountered on the ISS, the study authors wrote.

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