Scientists Discovered ‘Zombies’ With ‘Immortal’ Body Parts. This Could Redefine What We Think Life Is.

5 min readSea cucumbers are weird and diverse animals, with roughly 1,500 known species roaming the ocean floors worldwide. Shaped like their name implies, these tube-like creatures use their feet to help them eat. When threatened, some types entangle predators with sticky threads that shoot from their bodies, while others can mutate to reveal toxic inner organs. If they lose a limb, they simply regrow it. And now, researchers have found a species whose severed tissues—such as a lost foot—appear to be immortal, raising novel questions about what it means to be alive.Many animals can lose appendages—think of a lizard losing its tail—but “typically we don’t see the tail come back to life,” says Sara Jobson, a PhD student in the ocean sciences department at Memorial University in Newfoundland and Labrador. “Ultimately, that’s the broad bizarre nature of what we’re seeing. We’ve compared it to ‘Thing’ from The Addams Family; you wouldn’t expect your hand or your foot to be able to survive on its own and continue existing.”While lizards are the only vertebrate animal known to have the ability to regrow limbs, thousands of invertebrates have this evolutionary superpower, including all echinoderms, a phylum of animals that includes sea cucumbers, starfish, sand dollars, and sea urchins. But no one has ever witnessed discarded tissues that stayed alive for a long period of time, says Jobson, who led a recent study at the Ocean Sciences Centre of Memorial University to investigate the curious anomaly discovered in Psolus fabricii, a cold-water sea cucumber native to the North Atlantic.During a routine research procedure in which a sea cucumber was removed from a tank in the lab, the animal left behind a couple of its small tube feet, which often happens in these situations. But instead of dying and decomposing, the feet stayed attached for weeks, showing no signs of decay. That’s when Jobson and a team of researchers decided to take a closer look, studying small pieces of the animal (called explants) for more than three years, during which the tissues persisted.“One of our first questions was, ‘What types of tissues can do this?’” Jobson says, noting that their work relied on decades of prior research on sea cucumber biology by other scientists. “We looked at a couple different tissue types, including the feet and the tentacles, to see if there were other tissues that showed the same thing, which they did.”The team also confirmed that the tissues were healing and regenerating, and that their immune systems were still functional. Next, they wanted to know what was fueling their growth and survival, since the severed tissues were no longer attached to a mouth or digestive system.“We saw that they were absorbing nutrients across the cell membrane,” Jobson explains. “That, in theory, seems to be how they’re able to fuel their long-term existence.”Furthermore, the tiny pieces of sea cucumber were interacting with their environment, meaning their neural network showed some healing and function.“If you poked them or moved the water around them, they would retract back into themselves,” Jobson says. “They were responding to changes in their environment, which potentially means they could learn and become better adapted.”The one major function the tissue cannot perform is reproduction. But because the tank used in the experiments contained water pumped straight from the nearby bay—microorganisms, bacteria, and all—Jobson suspects there are lots of “zombie” cucumber explants surviving in natural habitats.“Why is this an evolutionary advantage for them to maintain this high level of regeneration when they’re not reproducing?” Jobson asks. “They seem to almost kind of reform into a new biological unit. They’re not functioning the way a sea cucumber would, but they’re not dying either, so they’re in this gray zone.”José García Arrarás, PhD, a regenerative biologist at the University of Puerto Rico in San Juan, says it’s exciting that the cells remain alive for such a long time, but he doesn’t believe these tissues redefine the boundary between life and death.“The cells are definitely alive,” García Arrarás says. He says the boundary is between what appears to be a simple tissue that survives a long time without being able to reproduce and what it takes to become an organism that might not be immortal but can reproduce.For Jobson, who, along with the research team, has jokingly called the cucumber parts “lab zombies,” the explants are straddling the line of functioning as a third state of life.“It gets kind of philosophical sometimes because we’re not totally sure what this means in this type of existence, as they’re maintaining a lot of the fundamental processes that we would associate with life,” she says. “But they’re not passing on their genetics, which is kind of a key pillar of evolution.”Charlotte Seid, PhD, collection manager of the Benthic Invertebrate Collection at UC San Diego’s Scripps Institution of Oceanography, says the surprising work underscores the importance of fundamental research on diverse organisms.“I appreciated that this study took the first steps to inquire how widespread this survival ability may be across the tree of life, by testing other cold-water echinoderms including a contrasting lineage of sea cucumbers,” Seid says. “Next, I wonder whether closer relatives of Psolus fabricii may share this intriguing ability. Marine invertebrates are too often overlooked and underfunded because studying their basic biology and well-being does not always point to immediate payoffs for humans.”Since the severed tentacles in the study maintained the ability to respond to their environment in the same way they would respond when attached to the original organism, investigating that phenomenon could tell researchers more about how or where instinctual memories are stored in neural networks that don’t have brains. “Even in animals that do have brains, we’re not totally sure where memories are kept or where instinct is kept,” Jobson says.While the initial samples of explants were discarded, there are currently lots of different kinds of “zombies” living in the team’s lab for future research.“I’d like to look at whether or not the telomeres in the individual cells are maintaining their length between cell divisions because basically that tells us if cells are aging,” Jobson says. “It would confirm whether or not these tissues are actually immortal, in a definitive sense.”Katie Neith is a science writer based in Los Angeles. Her favorite celebrity sightings are the great horned owls that live in her hillside neighborhood. A recent graduate of the Johns Hopkins MA in Science Writing program, her work has appeared in Nautilus and Live Science, among other publications. Connect: www.katieneith.com