Biologists that sliced off the heads of trained planarian flatworms have discovered that the regenerative creatures can retain memories and transfer these to their new, regrown brains.

The astonishing discovery was made by Michael Levin, a professor at Tufts University, and former colleague Tal Shomrat, and published in the latest edition of the Journal of Experimental Biology. The coauthors are hopeful that the find marks the beginning of our understanding of how memory can potentially be stored in tissue and salvaged, helpful for when regenerative medicine advances to such a stage that human brain tissue can be replaced or replenished with stem cells.


Planarians, non-parasitic flatworms, are fascinating creatures known for their regenerative capabilities. They have the capacity to regenerate any part of their body, made possible by the fact that their adult pluripotent stem cells -- of which they have abundance (about 20 percent of their total cells) -- have the potential to become any cell in the body. As such the flatworms, which continually grow and shrink, make an excellent model for regeneration studies. An 1898 study even proved that a planarian could rebuild itself from a fragment as tiny as one 279th the size of its original body.

According to the authors, the reason more investigations have not been made of late into its regenerative capabilities, are the difficulties associated with studying the cognition of such a basic invertebrate. However, Levin and Shomrat believe they have overcome that with their new experimental model using video-tracking technology.

Knowing planarians have an aversion to bright lights, they built a scenario where they would be encouraged to go into the light for food and get used to doing so regularly. Two groups of flatworms were placed in two different environments, one with a ridged service, one with a smooth surface (a Petri dish). The researchers then illuminated a portion of the environment and placed a piece of liver there to see if they would seek it out. The worms were filmed over ten days with tracking technology analysing and measuring which was more reluctant to go into the light. Those on the rough textured surface appeared to get over their bright light qualms far faster than the Petri dish-living worms.

To follow up this find, Levin and Shomrat cut off their subjects' heads. Around 14 days later, when their heads had had time to regrow, they were all placed in a Petri dish and tested for an aversion to light once more. All were reluctant to go into the light, but those that had originally been living on the rough surface and had become adept at venturing into the light, seemed slightly quicker. When given a refresher course, however, then a four-day break, they found that the worms that had originally started on the rough-textured surface were far, far quicker than the Petri-dish living flatworms. "We show that worms exhibit environmental familiarisation, and that this memory persists for at least 14 days -- long enough for the brain to regenerate," write the authors. "We further show that trained, decapitated planaria exhibit evidence of memory retrieval in a savings paradigm after regenerating a new head. "We propose planaria as a key emerging model species for mechanistic investigations of the encoding of specific memories in biological tissues. Moreover, this system is likely to have important implications for the biomedicine of stem cell-derived treatments of degenerative brain disorders in human adults."

The authors are not entirely sure how or where the flatworms are storing their memories, but it's clear the action is happening somewhere outside of the brain, whether it's the nervous system retaining the information or some kind of hypothesised but never proven cellular memory function.