There are lots of strange things to discover in space, but flatworms sent to the International Space Station saved the really weird stuff for when they returned to Earth. That's when one worm, which had been partially amputated before being launched into space aboard a SpaceX rocket in 2015, grew a second head.

Planarian flatworms are often used in biological studies because of their impressive ability to regenerate parts of their bodies after amputation. A set including both whole and amputated worms was sent to the ISS for a five-week stay and then evaluated back on Earth for an additional 20 months.

Worms returning from five weeks in space / image by Junji Morokuma / source Junji Morokuma Worms returning from five weeks in space curled up and were immobile when transferred to petri dishes containing fresh spring water. By contrast, stay-at-home control worms moved rapidly and fully extended themselves.

One of the space-flown worms regenerated into a rare double-headed specimen. That's something the researchers running the experiment from Tufts University say they'd never seen in their combined 18 years of maintaining a colony of more than 15,000 flatworms.

But that's just the beginning of the strangeness. When the researchers amputated both of the heads from the newly twin-headed worm, the headless middle section grew back two heads. In other words, something that happened after the worm was launched into space caused its body to be "reprogrammed" to consider itself to be some sort of new two-headed species.

The worms that left Earth whole also did odd things, including spontaneously splitting into two or more unique individuals. The scientists note, however, this could be a result of the different temperatures experienced by the worms during their space journey.

"As humans transition toward becoming a space-faring species, it is important that we deduce the impact of space flight on regenerative health for the sake of medicine and the future of space laboratory research," said Junji Morokuma, lead author of the study in a statement.

Normal Flatworm Sent Into Space Grows 2 Heads

Flatworms have some rather impressive regenerative powers, but even they generally don’t just go from being single-to-double headed creatures. It appears a trip to space can change that.

A study detailing the worms' long, strange trip is described in detail in the journal Regeneration: Planarian regeneration in space: Persistent anatomical, behavioral, and bacteriological changes induced by space travel

Regeneration is regulated not only by chemical signals but also by physical processes, such as bioelectric gradients. How these may change in the absence of the normal gravitational and geomagnetic fields is largely unknown.

Planarian flatworms were moved to the International Space Station for 5 weeks, immediately after removing their heads and tails. A control group in spring water remained on Earth. No manipulation of the planaria occurred while they were in orbit, and space-exposed worms were returned to our laboratory for analysis.

One animal out of 15 regenerated into a double-headed phenotype—normally an extremely rare event. Remarkably, amputating this double-headed worm again, in plain water, resulted again in the double-headed phenotype. Moreover, even when tested 20 months after return to Earth, the space-exposed worms displayed significant quantitative differences in behavior and microbiome composition.

These observations may have implications for human and animal space travelers, but could also elucidate how microgravity and hypomagnetic environments could be used to trigger desired morphological, neurological, physiological, and bacteriomic changes for various regenerative and bioengineering applications.

Flatworm amputation and space-exposed and Earth-bound worm sample schematics / image by Junji Morokuma, Fallon Durant, Katherine B. Williams, and others / source OnlineLibrary.wiley.com (A) Approximately a third of the anterior part of the worm was cut off to create the head (H) fragment; then the posterior half was cut in half to create the pharynx (P) and tail (T) fragments, respectively. A total of 15 flatworms were cut and collected into three separate 50 mL conical tubes per fragment. (B) An identical number of worm samples, both whole and amputated fragments, were either sent into space or left on Earth for 32 days. (C) Immediately upon return to Earth, both space-exposed and Earth-only control worms from each sample tube were transferred to a Petri dish containing fresh Poland Spring water individually to identify any phenotypic changes.

Water shock / image by Junji Morokuma, Fallon Durant, Katherine B. Williams, and others / source OnlineLibrary.wiley.com (A), (B) Control worms left on Earth. (A) Representation of Earth-only control worms, with full extension and rapid movement. (B) Close-up image of representative Earth-only control worm. (C), (D) Worms from space. (C) Representation of space-exposed worms, in a state of water shock (ventrally curled and no movement). (D) Close-up image of representative stocked space-exposed worm.