In some animals, as is well known, severed limbs or the tail grow back – but what researchers have now discovered in the Sacoglossa sea snails is the most bizarre kind of regenerative capacity: the molluscs can shed their lower body, including their hearts and other internal organs, in order to then emerge from their heads completely restore. Presumably, the animals use the bizarre body amputation to get rid of parasites, say the scientists.
The most famous example of animal regeneration can be found in the lizards: When threatened, some species specifically throw off their tails so that predators can pounce on this “bait” and the lizard can escape. The lost body part then grows back in some species. However, even more impressive regeneration abilities are known from some amphibian species. The main focus of science is the newt axolotl. It can regrow whole limbs and even damaged organs and is therefore the focus of medical research.
Even more drastic than previously known
Sayaka Mitoh and Yoichi Yusa from the Japanese women’s university in Nara present a particularly extreme example of animal regeneration. As they report, the discovery was a coincidence. Actually, the Sacoglossa sea snails from the genus Elysia were in their focus because of their ability to perform so-called kleptoplasty: The molluscs eat algae, whose photosynthetic organelles (chloroplasts) they incorporate into their body cells. Like “animal plants”, they are able to generate energy from light and can do without food for a long time.
As part of their investigations, the researchers observed one day how the head of one of these Sacoglossa snails was traveling in an experimental container without the rest of the body. “We thought that this piece would soon die without a heart and other important organs, but to our surprise we found that the head eventually regenerated the whole body,” says Mitoh. The researchers then devoted a closer look at the phenomenon.
Body amputation to fight parasites?
In the case of the species Elysia marginata and Elysia atroviridis, for example, they were able to demonstrate that the snails have a kind of seam in the “neck area” at which they can tie off their lower body area. The separated body area with the heart, digestive system and other organs then die off after a while. But the head lives on, can move, take in food and then finally produce a lower body with all its organs again. About a week after the self-decapitation, the snails were largely complete again in this way. Apparently they are even able to regenerate several times in a row – but only up to an age of 480 days, the scientists found.
So far it is unclear what the astonishing ability to regenerate is based on. But the researchers assume that certain forms of stem cells in the lower area of the severed head are capable of building up the entire lower body of the animals. They also suspect that the snails’ ability to supply energy through photosynthesis plays a role in the phenomenon. This allows the severed head to survive without digestive organs long enough to spawn a new body, say Mitoh and Yusa.
But what could the “costly” shedding of the lower body serve the snails? Since the constriction takes a relatively long time, a function as a bait for predators – as with the lizards – is hardly an option. As the researchers report, however, they have already found indications that it is, as it were, a medical amputation: the snails apparently always shed their bodies when they are infected with parasites. The scientists suspect that they can then regenerate and regain a healthy lower body.
However, Mitoh and Yusa only want to uncover the more precise background of the phenomenon through further investigations on the astonishing marine animals.
Source: Cell Press, technical article: Current Biology, doi: 10.1016 / j.cub.2021.01.014
Video: This video shows how the head of the sea snail Elysia marginata moves around the rest of its body after self-amputation. (CREDIT: Sayaka Mitoh)