Here's an article about sea slugs that purposely decapitate themselves:Decapitated Sea Slugs
It's believed they occasionally "jettison" their bodies to get rid of parasitic infestations. The abandoned torso (if that word applies to slugs) swims independently, sometimes for months, before eventually decomposing. The severed head, however, grows a whole new body, often within three weeks. Meanwhile, it continues to feed on algae as if it doesn't notice it has no digestive system, not to mention other essential organs such as a heart.
Self-amputation, called "autotomy," shows up in other species, such as lizards who let their tails detach to escape predators, then grow new tails. Starfish can generate new arms to replace severed limbs, and in some case a segment of a dismembered starfish can develop into a separate animal. In my high-school biology class, we bisected flatworms to watch the pieces regenerate over several days. Sea cucumbers sometimes eject their internal organs under stress and regrow the lost organs. Mammals, in contrast, have limited capacity for regeneration, but (according to Wikipedia) two species of African spiny mice shed large areas of skin when attacked by predators: "They can completely regenerate the autotomically released or otherwise damaged skin tissue — regrowing hair follicles, skin, sweat glands, fur and cartilage with little or no scarring."
Why do plants regularly lose limbs and grow new ones anywhere on their trunks, while most animals are much more limited in this respect? Why the difference in regenerative capacity between lizards and mammals, although they're all vertebrates? The sea slug's self-decapitation makes tales of vampires and other monsters that can re-grow lost appendages seem more plausible. The slug's independently moving detached parts remind me of a vampire novel by Freda Warrington in which a decapitated vampire regenerates a complete body from his severed head. Meanwhile, the headless corpse grows a new head; however, the resulting individual rampages mindlessly like a zombie. In the science fiction genre, I once read a story whose protagonist hosts a visiting alien at a house party. This alien's species has detachable limbs, so losing an appendage is no big deal for them. Also, this particular ET has a totally literal comprehension of English. When the protagonist compliments a concert pianist with the remark, "I wish I had her hands," the alien tries to do the host a favor by amputating those hands and presenting them as a gift. . . .
It's hoped that understanding the sea slug's remarkable ability "could one day lead to advances in regenerative medicine and other fields." Many science-fiction future technologies include medical treatments that enable injured patients to grow new organs and limbs. Maybe that vision might eventually come true.
Margaret L. CarterCarter's Crypt