This Flatworm on the ISS Grew Two Heads in Space
published during a waning gibbous moon.

Microgravity has negative effects on human physiology, so if we’re going to put humans on Mars or build space colonies, we have to learn more about why and how gravity (or lack thereof) affects our bodies, behavior, and biomes, as well as figure out ways to mitigate those deleterious consequences. That’s where worms come in. Researchers from Tufts University sent planarian flatworms to the ISS for five weeks in order to study what happens to them in space, and they’ve published the results in Regeneration.

One of the benefits of these worms is their ability to regrow amputated segments. Scientists wondered how the lack of gravity and geomagnetic fields would affect this ability. Would the shapes of their bodies change? What about cell activity? They also wanted to see whether returning to Earth would reverse any changes or whether they’re permanent.


Normal flatworm. Credit: Eduard Solà

Back in 2015, the researchers sent flatworms in tubes with water and oxygen on a SpaceX resupply mission. Some of them were whole and others had been amputated into various parts. They also monitored two Earth-bound control groups, one of which lived in water with a consistent 68˚ temperature in a dark place and the second group was exposed to the same temperature changes as their ISS counterparts.

The worms stayed on the ISS for five weeks and scientists monitored them for almost two years after they came back. Perhaps the most unexpected result that one of the severed worm pieces grew into a two-headed worm. The researchers, who have nearly 20 years of experience with flatworms, had never seen this happen spontaneously. Even more surprising was what happened next: after the scientists cut the heads off the worm, its middle segment grew two more heads. The worm’s cells not only developed a new plan in space, but that plan remained after its return.


Double-headed flatworm. Credit: Junji Morokuma, Allen Discovery Center at Tufts University

Other space worms experienced something called “spontaneous fission,” which means their bodies split into two identical worms. Something about living in space caused increased growth to the point of proliferation. When the space worms were put into spring water, they curled into motionless balls for hours, while the terrestrial worms responded normally. Researchers believe the unusual reaction indicates a biological change. Bacterial differences also exist between the worms, which researchers think indicates that space habitation changes the worms’ metabolism and excretion.

Another difference between the worm groups involves their responses to light. 20 months after their return, the worms were put into container half under red light, which flatworms can’t see, and half under blue light, which they can. The space worms chose to hang out in the blue light roughly 30% of the time, while the terrestrial worms chose the darkness over 95% of the time. Researchers aren’t yet sure why the ISS flatworms seemed to prefer light far more than their Earth-bound counterparts.


Image representation of worms with one, two, three and four heads. Credit: Dr. Michael Levin, Tufts University

Despite the small sample size of the space flatworms, as well as researchers’ inability to precisely replicate the temperatures, take-off, and splashdown they experienced, the study represents an important step in gauging the effects of space. Future experiments will replicate these conditions more exactly and will include real-time data gathering.

Perhaps on the next go, ISS astronauts will perform amputations on the worms and complete other aspects of the experiment instead of researchers doing them before liftoff. Each time the process improves, scientists get a step closer to devising research protocols that will help both animals and humans maintain their health in space.