Imagine the biologist Sayaka Mitoh’s surprise the day he found a sea slug in his lab suddenly missing his body. Or your boss really depends on your perspective. Either way, the sea slug was split into two pieces, both of which appeared to be alive, in the sense that both were still in motion. Somehow, they continued to live for days and then weeks, even though the head was less a heart and a digestive system.
Among biologists, this type of maneuver to divide the body is known as autotomy: lizards, for example, throw their tails to escape predation. But what the sacoglossal sea slug does next places it in a class of its own. “We were surprised to see the head move right after the autotomy,” Mitoh says. “We thought he would die soon without a heart and other important organs, but we were again surprised to see that he was regenerating his whole body.”
That’s right: he pulled out a Deadpool. Just hours after his self-decapitation, the head began to crawl to feed. A day later, the neck wound had closed. A week later, he began to regenerate a heart. In less than a month, the whole body had grown back and the bodyless lemon was incarnated once again. Several slugs have done so in Mitoh’s lab, so this is a feature, not a mistake. A slug – apparently an ostentation – even self-escaping twice.
However, the previously owned bodies fail to do so. As Mitoh says very poetically in a new article describing the phenomenon in the magazine Current biology, “The bodies gradually shrank and turned pale, apparently due to the loss of chloroplasts, and eventually decomposed. The heartbeat was visible just before the body broke down.
Now, before we get to the question of why a sea slug would be beheaded on Earth, let’s talk about how, and those chloroplasts. Mitoh observed this behavior in several individuals of two different species of sacoglossal sea slugs. This group of mollusks is famous — at least among biologists — for their “kleptoplasty” or for the way they steal their energy source. In algae eaten by animals, photosynthesis is humming in structures known as chloroplasts. Instead of digesting them, sea slugs incorporate them into their own tissues. These chloroplasts can remain photosynthetically active for months, allowing their sea slug to take in energy from the sun. The animal runs on a lot of solar energy.