The long-running controversy over whether planarians can learn and transfer memories remains unresolved. Some researchers suspect that the animals used in earlier experiments may have changed over time because of pollution or genetic drift, while newer work on sea slugs and roundworms suggests that memory-related information can, under certain conditions, pass between individuals.
Why planarian learning remains controversial
One remote possibility is that planarians themselves have changed over the past 60 years, perhaps as a result of pollution or genetic drift. Gershman considers that explanation unlikely. “What are the chances that many researchers just happened to be conducting these studies at the particular time this phenomenon occurred?” he asked incredulously. “Did they just get very lucky over the millions of years of evolution of planarians? So has our luck run out?”
Whatever the explanation, as of 2026 there is still no convincing evidence that planarians can learn, despite their nervous systems and simple brains. From an evolutionary perspective, that may make sense. “The reason we learn some associations is so we can anticipate and avoid danger,” Snyder says.
Planarians, however, have a different relationship with danger. Their regenerative physiology is central to McConnell’s experiments and protects them from blunt trauma. If a planarian is bitten in half, it can grow back. What use would memory be to an organism with that ability? “That’s a whole other philosophical conundrum,” Snyder said.
Evidence from other organisms
Planarian learning may be a dead end, but memory-transfer experiments involving other organisms have returned to scientific research—and some appear to be producing results.
In 2018, neuroscientist David Glanzman of the University of California, Los Angeles, conducted a memory transplant experiment on the sea slug Aplysia, a popular model organism for memory research because it has a relatively simple nervous system and giant neurons. After training the slugs to respond to tail shocks, Glanzman transferred the sensitization from one slug to another by directly injecting genetic material. The findings suggest that some aspects of memory may be stored in RNA, as McConnell argued.
In 2021, Princeton University geneticist Colleen Murphy and her colleagues reported that Caenorhabditis elegans, or C. elegans—a microscopic roundworm with 302 neurons—could learn to avoid pathogens after exposure to them. The group identified jumping segments of genetic material called retrotransposons, including Cer1, that may carry memories.
A few years later, a group from the Indian Institute of Science trained C. elegans and found that the worms released extracellular vesicles—small lipid particles containing genetic information—that can convey their training to other worms.
These researchers have not attracted the same attention as McConnell, but their work suggests that he may have been right about memory transfer in worms. He may simply have chosen the wrong kind of worm and continued to defend the idea despite inconsistent evidence. In the end, he lost his reputation, but his enthusiasm helped spark the curiosity of another unconventional scientist. In this case, that curiosity has followed the evidence.
A new focus on C. elegans
At Harvard, Gershman is shifting his attention from the cryptic question of planarian learning to the more tractable biology of C. elegans. The roundworm may not regenerate when cut in half, but it is a long-standing model organism in neuroscience and has repeatedly been shown to learn.
With new experiments underway, Gershman is cautiously optimistic. “I just hope we don’t go down another rabbit hole,” he said. With a worm, on the other hand, there are definitely holes.
Original story. Reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation, whose mission is to enhance the public’s understanding of science by covering research developments and trends in mathematics, the physical sciences and the life sciences.
Source: www.wired.com
