Introduce biologist Sayaka Mitoh’s surprise the day she discovered that a sea slug in her lab was suddenly missing its body. Or his head really depends on your perspective. Regardless, 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 survived for days and then weeks after that, even though the head was minus heart and digestion.
Among biologists, this type of body splitting maneuver is known as autotomy – for example, lizards drop their tails to escape predation. But what the sacoglossan sea snail does next sets it apart. “We were surprised to see the head move just after autotomy,” says Mitoh. “We thought it would die quickly without a heart and other major organs, but we were surprised again to find that it regenerated the whole body.”
That’s right: it drew a Deadpool. Just hours after his self-decapitation, the head began to drag around to feed. After a day the neck wound was closed. After a week, it started to regenerate a heart. In less than a month, the whole body had grown back and the disembodied snail was reincarnated. Several slugs actually did this in Mitoh’s lab, so this is a feature, not a bug. A snail – apparently a showpiece – even decapitated itself twice
However, the bodies previously owned do not make it. As Mitoh puts it quite poetically in a new article describing the phenomenon in the magazine Current biology, “The bodies gradually shrank and turned pale, apparently from the loss of chloroplasts, and eventually decomposed. The beating of the heart was visible just before the body fell apart. “
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 even observed this behavior in different individuals of 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 energy source. In the algae that the animals eat, photosynthesis co-hums in structures known as chloroplasts. Rather than digesting them, the sea snail actually absorbs them into its own tissues. These leafy green granules can remain photosynthetically active for months, allowing their adoptive sea snail to get energy from the sun. The animal works very much on solar energy.