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Scientists at Harvard University are attempting to replicate a controversial 1960s study that claimed flatworms could learn behaviors and transfer those memories to other worms through cannibalism. Led by neuroscientist Sam Gershman, the research team dispatched assistants to rivers and lakes to collect wild planarians in an effort to solve a decades-old scientific mystery.
The Origins of Worm Learning
In the 1960s, behavioral psychologist James McConnell argued that planarian worms could undergo classical conditioning and that trained memories could be passed on to untrained worms that ate them. McConnell famously beheaded his trained worms and found that both regenerated heads and tails retained their training. He then fed pureed trained worms to their cannibalistic peers, who appeared to immediately know the correct response without training.
McConnell suggested that memories were encoded in RNA structures, a theory that gained massive media attention. He became a prominent public science figure, publishing studies in his own countercultural journal, The Worm Runner’s Digest. However, other researchers struggled to replicate his results. Nobel laureate Melvin Calvin failed to reproduce the worm experiments in 1965, and interest faded as scientists shifted their focus to other animals.
Modern Attempts and Modern Mysteries
When Gershman and his team attempted to condition planarians using modern protocols, they could not get the worms to learn anything. As WIRED reported, despite trying various stimuli and sourcing 12 different strains of planarians—including trips to McConnell’s former fishing grounds in Michigan—none of the modern worms showed any learning.
To investigate further, the Harvard team tracked down Daniel and Reeva Kimble, former McConnell lab members in their nineties living in Eugene, Oregon. The Kimbles provided the researchers with the complete print archive of The Worm Runner’s Digest from their basement.
Researchers suggest several reasons for the discrepancy. Project researcher Snyder noted that McConnell and other historical “worm runners” may have misinterpreted normal worm movements as trained reactions, influenced by the era’s funding pressures and a charismatic leader. Alternatively, planarians themselves may have changed over six decades due to pollution or genetic drift, though Gershman finds that explanation unlikely.
The Revival of Memory Transfer Research
While planarian learning appears to be a dead end in modern labs, memory transfer research in other organisms has returned to scientific favor. In 2018, neuroscientist David Glanzman of the University of California, Los Angeles, performed a memory transplant on sea slugs via genetic material injections. Later, in 2021, Princeton University geneticist Coleen Murphy discovered that microscopic roundworms can learn to avoid harmful bacteria by consuming or interacting with trained peers, identifying genetic material that appears to carry memories.
At Harvard, Gershman is now shifting his focus from planarians to the microscopic roundworm, which has a well-documented ability to learn. With new experiments underway, the team hopes to study memory at a molecular level without falling into another historical trap.
