Elysia marginata and Elysia atroviridis are sea slugs from the family of Plakobranchidae. While these marine slugs are typically small (>10 cm), they have some of the coolest superpowers found on the planet. Not only are these amazing animals able to convert solar energy into chemical energy, but they are also able to detach and regrow their entire body from just their head in only twenty days. 

Autotomy

Autotomy is the voluntary shedding of a body part, often used to escape predation, and is fairly common in some orders found in the animal kingdom, such as arthropods, asteroids, gastropods, amphibians and lizards. Autotomy is generally followed by the subsequent regeneration of the lost limb. A familiar example of this is when lizards lose their tails when escaping predators, such as cats, which the lizards are then able to regrow.

A new type of “extreme” autotomy has recently been discovered in two species of sacoglossan sea slugs by scientists in Japan. 

They announced their findings in a peer-reviewed correspondence article published in Current Biology Magazine. What is surprising about these sea slugs is their ability to voluntarily detach their heads and then to regrow their entire body, including the heart and other important organs, from just the detached head. One individual sea slug in the study even autotomized twice. 

Elysia marginata

While the head is busy regenerating it is able to move autonomously, without the aid of the heart, immediately after severing itself from the body. It begins to feed on algae within a couple hours. The wound from the autotomy exercise heals itself within a day and the regeneration of the heart begins within seven days. The entire process of regeneration is complete within around twenty days in E. marginata. 

Interestingly, the bodies of the sea slugs also survived after being decapitated and remained responsive to tactile stimulation until it gradually decomposed – likely from the loss of functioning chloroplasts (refer to Kleptoplasty and photosynthesis below) which need to be continually ingested.

Elysia atroviridis

Elysia atroviridis is another sea slug which was found to have this regeneration ability. It is the faster of the two sea slugs and was able to regenerate within a week. In this species, the body was once again reactive to stimuli even after being removed from the head. Other individuals in the study also gradually lost other body parts without autotomy during rearing, presumably through autolysis in which the body produces enzymes that digest its own cells.

Why Regenerate?

Although most animals autotomize to avoid predation, the authors of the paper suggest that it is unlikely that these sea slugs follow the general trend, as their main defenses appear to be cryptic camouflage and toxicity. E. marginata also took several hours to autotomize – which is likely to be ineffective as a predation defense mechanisms and experiments imitating predator attacks did not induce autotomy. The authors instead suggest that the two species have evolved this mechanism to remove internal parasites. 

Dissections of the decapitated bodies from the autotomized E. atroviridis individuals, as well as the autolyzed ones, were all shown to contain parasites whereas the newly regenerated bodies and the individuals which did not attempt autotomy were found to be parasite free. Elysia atroviridis is commonly parasitized by Arthurius copepods. These parasites occupy a large proportion of their host’s bodies and strongly inhibit their reproductive abilities. By removing the parasite, the sea slugs recover their reproductive abilities which enhances their contribution to the next generation.

This is not the only example of such “parasitic autotomy”. It has been shown that earthworms do something similar to protect themselves against a protozoan parasite.

Alternative Theories

The authors also suggest alternate theories, which are not mutually exclusive, for why the sea slugs autotomize. They suggest that it may also act as a mechanism which would enable the sea slugs to escape from tangled algae or to remove accumulated toxins from their bodies.

Kleptoplasty and Photosynthesis

Elysia marginata and E. atroviridis are both sacoglossans. Sacoglossans are a superorder of marine gastropods that are commonly known as “solar-powered sea slugs” for their unusual ability to ingest living chloroplasts (photosynthetic organelles) from their algal diet and to use them to photosynthesize. This unusual ability is known as kleptoplasty, which derives from the Greek word Kleptes meaning thief. These slugs are commonly found in the Indo-Pacific and typically live at depths of 0-10m giving them lots of exposure to sunlight.

The sea slugs have a highly branched digestive gland which is spread all over its body, including the head. This gland houses specialized cells which are able to absorb the chloroplasts while the rest of the algae is digested and maintain the chloroplasts while they continue to survive and provide the sea slug with energy from the sun. The regeneration process requires a lot of energy. It is likely that it is this amazing ability which allows the head to survive without the rest of the body and the other important organs, in addition to regrowing the entire body.

Younger Sea Slugs Are Better At Regeneration 

Although the researchers observed older sea slugs autotomize, only the young ones were capable of regenerating their bodies. The older sea slugs’ heads survived for about ten days before they died. The main difference between the older heads and the younger heads is that the older heads never started eating again once separated from their bodies – it is likely that they survived only for as long as the chloroplasts they ingested survived before they ran out of energy. 

Multipotent Stem Cells

Researchers are still unsure exactly how the sea slugs are able to regenerate their bodies, but it is likely that the sea slugs use some form of stem cells (special undifferentiated cells that can potentially be turned into any type of cell – such as those which are found on the human umbilical cord). 

Future Possibilities

The authors of the paper are, unsurprisingly, very excited to further their research to attempt to discover the mechanisms behind such an incredible superpower and to determine if there are any other closely related species which have the same ability. An interesting spin-off for this type of research is that if we were to discover the secret to limb regeneration it is possible that we would be able to use this knowledge in the medical field to help people regrow lost limbs. Who knows what the future holds?