Researchers are astonished to discover that when their captive slugs spontaneously discarded their bodies, they immediately started growing them back.
When a person loses a limb it's a life-changing tragedy. For a salamander, it is an inconvenience, for the limb will soon grow back. Distantly related animals from gastropods and arthropods to lizards and amphibians all possess the ability to regenerate lost body parts. Some can ditch their extremities when they become infected or injured, a process known as autotomy.
Researchers have long been interested in these regenerative powers. Some hope that unravelling the biological mechanisms underlying them might one day have medical applications in humans. Now, though, a pair of biologists have turned up the most drastic example of regeneration so far. A paper in Current Biology reports two species of sea slug that are capable of jettisoning their bodies below the neck, and then building new ones from scratch.
Mitoh Sayaka, a PhD student, and Yusa Yoichi, her supervisor at Nara Women’s University, in Japan, were studying a species called Elysia marginata that had, until now, been mostly overlooked by science. They were astonished when five of their captive slugs spontaneously discarded their bodies and then started growing them back. (One slug performed the trick twice.) In every case, the creatures dumped their hearts, kidneys, intestines and reproductive organs along with their bodies.
Those bodies kept moving for several days before their hearts stopped beating and their tissues began to decay. (A couple persisted for several months.) The heads, meanwhile, busied themselves crawling around and capturing algae in their mouths. Digestion being difficult without a stomach, the slug heads instead collected the photosynthesising organs (known as chloroplasts) from these algae, and incorporated them into their remaining tissues.
This sort of behaviour has been seen before, albeit only with intact slugs. It is, presumably, the photosynthesised nutrients created by those chloroplasts that allow the slugs to regenerate their missing bodies.
To probe what was going on more closely, Ms Mitoh and Dr Yusa captured more slugs, waited for them to breed and then began monitoring their descendants. Regeneration, it seems, is a power that declines with age. The researchers saw both old slugs and young ones autotomise.
Those between 226 and 336 days old found the process least troubling. In these animals, wound closure finished within a day, hearts reappeared in a week, and a new body grew within 20 days. In slugs between 480 and 520 days old regeneration did not happen at all, and the heads died about ten days after disconnecting. (Why older animals continue to shed their bodies despite its being fatal remains a mystery.)
To explore why the slugs might have evolved such drastic reconstructive abilities in the first place, the researchers tried pinching them to simulate an attack by a predator. This did not work. Follow-up experiments with a related species, Elysia atroviridis, suggested that the regeneration might instead be a defence mechanism against parasites.
Of the 146 slugs collected by the researchers, 82 were found to be infected with a copepod parasite (a type of crustacean). None of the parasite-free slugs shed their bodies, but three of the infected ones did. A further 13 discarded bits of their bodies, and in every case the parasites were ejected along with the body parts.
Ms Mitoh and Dr Yusa therefore propose that autotomy in sacoglossans helps the animals clear parasitic infections, as an alternative to activating a costly immune response that might fail. It is a drastic strategy - but one that seems to work.