Imagine biologist Sayaka Mitoh’s surprise the day she found that a sea slug in her lab was suddenly missing its body. Or its head, really—depends on your perspective. Either way, the sea slug was in two pieces, both of which seemed to be alive, in the sense that they were both still moving. Somehow, they kept on living for days, and then weeks, even though the head was minus a heart and digestive system.
Among biologists, this kind of body-splitting maneuver is known as autotomy—lizards, for instance, shed their tails to escape predation. But what the sacoglossan sea slug does next puts it in a class of its own. “We were surprised to see the head moving just after autotomy,” says Mitoh. “We thought that it would die soon without a heart and other important organs, but we were surprised again to find that it regenerated the whole body.”
That’s right: It pulled a Deadpool. Just a few hours after its self-decapitation, the head began dragging itself around to feed. After a day, the neck wound had closed. After a week, it started to regenerate a heart. In less than a month, the whole body had grown back, and the disembodied slug was embodied once more. Several slugs have actually done this in Mitoh’s lab, so this is a feature, not a bug. One slug—apparently a show-off—even self-decapitated twice.
The previously owned bodies, though, don’t make it. As Mitoh puts it quite poetically in a new paper describing the phenomenon in the journal Current Biology, “The bodies gradually shrank and became pale, apparently from losing chloroplasts, and eventually decomposed. The beating of the heart was visible just before the body decomposed.”
Now, before we get to the question of why on Earth a sea slug would decapitate itself, let’s talk about the how, and those chloroplasts. Mitoh actually observed this behavior in several individuals from two different species of sacoglossan sea slug. This group of mollusks is famous—at least among biologists—for its “kleptoplasty,” or the way it steals its source of energy. In the algae that the animals eat, photosynthesis hums along in structures known as chloroplasts. Instead of digesting these, the sea slug actually incorporates them into its own tissues. These chloroplasts can remain photosynthetically active for months, allowing their adoptive sea slug to draw energy from the sun. The animal is very much solar powered.
So even after the sea slug’s head divorces itself from its body and digestive system, these chloroplasts may be what keeps the animal alive. “We expect that they can get energy by photosynthesis using chloroplasts incorporated into the digestive cells distributed all over the body, including the head, even when they do not have their body,” says Mitoh.
Furthermore, Mitoh suspects that stem-like cells around the neck are what allow a slug’s head to fully regenerate the body—though this will require more research. Yet unlike a lizard losing its tail, the slugs don’t break when Mitoh simulates an attack (by pinching them, if you’re curious), so this doesn’t seem to be a defensive strategy.
So what is it then? Generally speaking, it’s not ideal for an organism to lose its body. These sea slugs would need a very good reason to self-decapitate, and that reason may be parasites. Three of the individuals that jettisoned their bodies in Mitoh’s lab were infested with tiny parasitic crustaceans called copepods. Another 39 infected individuals lost only part of their bodies, and 13 of those ended up regenerating bodies. However, all of the 64 parasite-free sea slugs stayed intact.
It may be, then, that these animals can detect when they’ve got a particularly bad infestation of parasites and write the body off as a lost cause. This may seem … extreme, but from an evolutionary perspective, it makes sense. If a sea slug is overburdened by parasites, it can’t devote energy to reproduction. And living to reproduce is the reason why any animal exists. By self-decapitating, the sea slug head can survive just fine on solar power until it regrows a new digestive system. It’ll also regrow a fresh set of reproductive organs, so it can finally get back to fulfilling its purpose on this planet.
But hold up, says Terrence Gosliner, a senior curator at the California Academy of Sciences, who studies sea slugs. Scientists don’t tend to see fragmented individuals in the wild, “unless they have serious bite marks from predators,” says Gosliner. “And if it was a natural means of getting rid of parasites, you would expect to see that.” In Gosliner’s view, the body-splitting is more likely to be an adaptation that allows a sea slug to recover from predators making off with a significant amount of its body.
“To me, the most exciting part of the paper is showing there is this incredible power of regeneration that occurs in these animals,” says Gosliner. “Like most good scientific experiments, it raises almost as many questions as it answers.”