Imagine being like the salamander and having the ability to loose a limb without fear because it will soon grow back. Or to have the ability to ditch extremities when they become infected or injured, a process known as autotomy. Researchers have for some time been interested in these regenerative powers.

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 were overlooked. Both were extremely surprised when five of their captive slugs spontaneously discarded their hearts, kidneys, intestines and reproductive organs along with their bodies, then grew them all back. (The bodies in some cases lasted several days, a couple persisted for several months.)

The heads then ate algae in their mouths and collected the photosynthesising organs (known as chloroplasts) from these algae, and incorporated them into their remaining tissues.

The slugs between 226 and 336 days old could close wounds 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 (heads died about 10 days later).

Follow-up experiments with a related species, Elysia atroviridis, suggested that the regeneration might instead be a defence mechanism against parasites.

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.

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