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Scientists Discover Decapitated Sea Slug Heads Grow New Hearts, Bodies
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March 9, 2021
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1:26Now PlayingJapanese sea slugs like this one show how it can live on despite getting decapitated.
In the ultimate case of regeneration, the sea slugs' lonely heads don't just chug along, but regrow hearts and whole new bodies, according to a study published today (8 March 2021)in Current Biology.
Biology researcher Sayaka Mitoh loves studying Japanese sea slugs because they are small, cute and weird they can even briefly photosynthesise like a plant drawing food from the sun.
Mitoh says the process was completely unexpected: "We have been rearing Elysia marginata (scientific name of a sea slug breed) to study the benefits of photosynthesis and how it impacts the life-history trait of sea slugs. Then, one day one of the sea slugs was found decapitated by chance. We kept it as it was until the regeneration started - that's the process of this discovery. So, I think it is fair to say that this was purely by accident."
The doctoral student and Nara Women's University aquatic ecology professor Yoichi Yusa decided to create a study cutting the heads off 16 sea slugs.
Six of them started regeneration with three living on fully.
One slug even lost and regrew its body twice. Two different species of Japanese sea slugs did this regeneration trick, called autotomy.
"When I tied the neck of one of the sea slugs, one which did not cut its part of the body apart by itself, the autotomy happened as expected and regenerated. That convinced me of its ability to autonomise," explains Mitoh.
One of these sea slug species can grow to as much as six inches.
The researchers can't work out how such a relatively large animal could survive without a heart to pump blood and nutrients to the brain.
"Some animals can autotomise their legs, appendages or tails, but no-other animal shed their whole body. So we think this is the most extreme case," says Yusa.
When these sea slugs eat a certain type of algae they can photosynthesise their food from sunlight and oxygen, just like a plant, for about 10 days according to Yusa.
He believes what's happening after decapitation is that the head sort of acts like a plant, turning a shade of green it gets its energy from oxygen and sunlight.
The fact that it becomes tiny after decapitation helps this process.
"So, one of the functions of the heart is to circulate blood and convey oxygen and in the case of the autotomised sea slug, they may not need the heart for that purpose," says Yusa.
The researchers think this species may have developed the ability to regenerate to fight off parasites in the body.
"Yes, we think at least for one species, Elysia atroviridis, the function may be to remove the parasites. And for the other species which we found the autotomy the individuals we saw were parasite-free because we reared them in the laboratory. But in the wild they have parasites. We think that it is possible that parasites is (are) important for autotomy, but there may be other reasons for autotomy because in the laboratory we don't have parasites for at least one species," says Yusa.
Scientific commentators say what's especially intriguing is that these sea slugs are more complex than flatworms or other regenerating species and is doing more than other animals.
This trick of the sea slug may help us understand more about our own biology, but any of that research is a long way off according to Yusa.
He says: "We think it is difficult to directly utilise that information because humans and sea slugs are very distinct from each other. But we can learn the mechanisms of regeneration from sea slugs and may use such information for human medical sciences. But that is a long way, and we need much study to understand the mechanisms."
The Japanese biologists agree that there is much more work that needs to be done for their research to offer any insights for human medical science. But there is hope.
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