The Headless Horsemen of Slugs

From lizards dropping their tails to one-armed sea stars regenerating their limbs, various animals have displayed the ability to self-amputate and regrow organs as a means to survive. Recently, a group of scientists from Japan have discovered that two species of sea slugs—Elysia marginata and Elysia atroviridis—have a remarkable ability: detaching their heads off their bodies and regrowing their bodies from their heads. What’s even more astonishing is that the head and the body of the slug can survive separately for weeks, even months, before restoring their missing parts and living on with a new body.

Sayaka Mitoh, a doctoral student at Nara Women’s University of Japan, first observed this phenomenon in 2018 when she was astonished to find a decapitated sacoglossan sea slug head circling inside the tank and eating algae. Mitoh noted that the wound on the head appeared to be self-inflicted. While she thought the slug head would die due to its lack of internal organs, it began regrowing its body back instead. Three weeks later, the slug head grew 80 percent of its body and vital organs back and was observed to be perfectly replicated from its old body.

This led Mitoh and her fellow scientists to closely study groups of Elysia marginata and Elysia atroviridis sea slugs. They observed that many of the slugs decapitated themselves at least once during their lifetimes and that slugs have a breakage plane on their neck to ensure a clean cut to remove the head. While the detached heads could grow back bodies, the bodies could not grow back heads. However, just like the head, the separated body was responsive to stimuli for months until decomposition.

Limb and organ regeneration is a unique adaptation that certain organisms possess to varying degrees, for example lizards, axolotls, zebrafish, and planarians, a group that includes flatworms. Planarians, famous for their ability to regrow their head or even the rest of their body, rely on stem cells, which have the power to transform into many other types of cells. They have a certain type of stem cell called neoblasts throughout the body, which allow them to regrow masses of tissues that have been removed. On the other hand, zebrafish, which can repair major organs like the heart, brain, and pancreas through regeneration, take a more localized approach. For example, initiation molecules from wounds in the heart signal heart regeneration to the zebrafish. The presence of these molecules results in increasing gene activity in the heart cells, specifically genes that promote wound healing and increase growth factors that activate cell division to replace dying cells and grow new ones.

For the Elysia marginata and Elysia atroviridis sea slug species, Mitoh and her group believe that stem cells, as in planarians, play a significant role in the slug's regrowth capabilities, and hope to further study the sea slugs to understand their regenerative abilities on a molecular level. As for the regeneration’s function, they are still unsure, especially because decapitation is not necessary for survival. They theorize that the mechanism is used not only to flee from predators but also to remove parasites within their old bodies, after the group found that all of the slugs that detached their heads had internal parasites in their old bodies. After amputation, their heads grew new bodies free from parasitic infections. Another potential purpose for regeneration is reproduction. The researchers observed that only younger slugs were able to live freely as heads and grow new bodies. The older slugs, on the other hand, only lived up to 10 days and were unable to eat or regrow new bodies. This indicates that the abilities of regeneration and autonomy are an indicator of youth for the slugs, as they lose them over time.

While the Elysia marginata and Elysia atroviridis sea slugs can regenerate, the regrowth process expends large amounts of energy. Despite this need for energy, the heads are still able to live, wandering freely and eating as if they are still one whole organism. What allows the sea slugs to remain mobile is a process called kleptoplasty, where slugs steal chloroplasts from the algae they consume. They then use the stolen chloroplasts to perform photosynthesis, converting sunlight into energy which allows them to sustain the regeneration process. However, the chloroplasts only last for a few days, so the slugs must continue eating algae until they fully restore their bodies.

Mitoh and her team’s discovery of the regenerative abilities of the Elysia marginata and Elysia atroviridis sea slugs have only left scientists more curious about what other organisms possess such impressive survival mechanisms, and how. Her research on these slugs at a molecular level may allow us to not only understand the mysteries of how these slugs can regenerate, but also give insight into the growing field of stem cell research. By expanding our knowledge of stem cells and learning how they function, new possibilities in regenerative medicine and other medical fields may open in the future. Individuals suffering from medical conditions, such as spinal cord injuries and blood-related diseases like leukemia, could one day be cured using stem cells as a treatment, putIndividuals suffering from medical conditions, such as spinal cord injuries and blood-related diseases like leukemia, could one day be cured using stem cells as a treatment, putting their lives at ease, all thanks to the decapitated head of a sea slug. their lives at ease, all thanks to the decapitated head of a sea slug.