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Neat to know ~ Creature of the week

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Sea Slug (dorid nudibranch), Black Ray Channel, Malaysia

Photo: Bernard Dupont, CC BY-SA 2.0

Lettuce Sea Slug (elysia crispata),  Curacao

Photo: Laszlo Ilyes, CC BY 2.0

Sea Slugs


There is a creature, which separates its head from its body at will.  This cut-off head can survive without the heart, digestive system, or other vital organs until it grows a whole new body!

This hasn’t been seen anywhere in the animal kingdom, except in the sea slug.

In a study published in the journal Current Biology, scientists in Japan report on the peculiar ability of two species of the sacoglossan sea slug (elysia marginata and elysia atroviridis) to simply let go of their bodies by decapitating themselves.  One of the scientists, Sayaka Mitoh, a biologist at Nara Women’s University, said that the study was initiated after she had observed the separated head of one of her lab’s sea slugs moving around on its own, immediately after separating from the body.  The head then started feeding on algae.  Within a day, the place on the neck where the break had taken place had healed, and in just under three weeks, the head had grown a perfectly healthy and functioning new body.  Meanwhile, the left-behind body started  to decompose.  For the study, the scientists observed serveral sea slugs in their lab doing the same thing. 

Other animals self-amputate, too, in a process called “autonomy.”  (If a predator has a grip on a lizard’s tail, for example, the lizard can detach itself from its doomed appendage and make a quick escape.  Eventually, a new tail grows.)  But nowhere has such a dramatic example been observed, as in the sea slug, where the entire body is cut off.  Researchers believe that the detached heads survive without the rest of their bodies partly because they are living off of the energy coming from photosynthesis taking place in cells from algae they had previously eaten.

So why would an animal decide to get rid of its body and grow a new one?  The researchers think that sea slugs do this when their bodies contain parasites.  Consider it the ultimate cleanse.

Read on to find out more about sea slugs – the anything-but-ordinary creatures with the deceptively plain name.


– The sea slugs observed in the recent study are sacoglossa, or “sap-sucking” slugs.  They have a thin, delicate shell and live in both salt and fresh water.  They are herbivores, meaning, they eat plants (mainly algae).  Their name comes from the fact that they pierce the cells in plants and suck out the cytoplasm.

Nudibranchs (say NOO-di-branks) – another type of sea slug

– The name of this sea slug means “naked gills.”  Some nudibranchs have structures along their backs called cerata.  These help them breathe and, in some species, aid in self-defense, as you’ll read later.  Other nudibranchs have a crest of gills that help them breathe.

– Nudibranchs live for about 1 year in the wild.  They are mostly found in warm, coastal waters.  But they live at all ocean depths and can be found in the tropics, at the poles, and everywhere in between.

– There are upwards of 2000 species of this type of sea slug, and scientists are discovering and naming new ones almost every day.  They can be anywhere from 1/4 inch to 12 inches in length.  Pretty much all nudibranchs are oblong.  But they can be whispy and floaty or plump and spiky, outrageously colorful or elegantly monochromatic.  The jaw-dropping variation in their looks make them endlessly fascinating.

– Unlike the sacoglossa, nudibranchs are carnivorous.  They eat whatever they can find while skidding along the bottom of their ocean floor habitats.  This might include anemones, fish eggs, corals, sponges, or jellyfish.

– Nudibranchs have club-shaped tentacles on their heads that they use to find prey.  These stalk-like structures are called rhinophores and are able to sense the proximity of food in the water using keen smell receptors.

– This type of sea slug has no shell.  But it is far from defenseless.  The exuberant coloring of lots of nudibranchs actually comes from the colors of the prey they consume.  At the same time, nudibranchs often absorb toxins that some of their prey use as protection from predators and store those toxins, ready to be unleashed through their own skin as self-defense.  The bad-tasting poisons that the nudibranchs secrete repel predators.  What we consider to be the other-worldly beauty of nudibranchs’ hues and patterns are seen as danger signals to predators.  Those colors tell attackers that they will regret taking a bite!

– Some nudibranchs also use other tools from their prey in self-defense.  Lots of nudibranchs eat animals like jellyfish that protect themselves using nematocysts – venom-containing, stinging cells.  These shoot out small barbs of toxins when disturbed.  Some lucky species of nudibranch are unbothered by the nematocysts.  They can eat away without feeling a thing.  Moreover, they are able to hold onto these tiny weapons in their cerata, which are structures that project out of the sea slug.  One of several jobs the cerata do is to serve as extensions of the digestive system.  When a nudibranch eats an anemone, for example, it lets the prey’s stinging cells move undigested and intact to the tips of the cerata.  Then, if the sea slug is attacked, it can release the stingers to protect itself.  Another way the cerata help some species of nudibranch protect themselves is by acting as a distraction.  When in danger, the sea slug will detach a cerata (autonomize).  This separated piece of the slug’s body will move around wildly and even secrete a sticky substance for a while after it is dropped.  Scientists think this is a way to divert the predator’s attention and give the nudibranch a chance to make an escape.

– While a shell offers an animal great protection, it also uses up lots of energy to develop and keep in good condition.  Nudibranchs can conserve the energy needed for shell maintenance, since they are shell-less.  (Only in the larvae state do these sea slugs briefly have shells, which they then ditch before reaching adulthood.)  Additionally, they are unencumbered by a hard and weighty structure on their backs and can thus enjoy a bigger range of motion and more flexibility to move where and how they want.

– Scientists have found these sea slugs to be helpful in certain areas of medical research.  Some species of nudibranchs fend off aggressors using a chemical called latrunculin A (sea slugs get this from the sponges they eat).  This toxin has been found to kill cancer cells.

– Environmental scientists look to sea slugs for critical information about the health of marine ecosystems.  Researchers call nudibranchs an “indicator species.”  Their presence – or lack thereof – signals whether an ecosystem is sound and flourishing or dwindling.

– As is the case with every creature on the planet, climate change is affecting sea slugs.  The warming of the oceans is causing changes in the overall distribution of nudibranchs.  Their numbers may suddenly get very high during warm water events, thereby stressing the existing ecosystem by disrupting the balance of organisms.  In other areas, particularly where oceans are becoming acidified, nudibranch larvae are weaker and less likely to survive. 

– The usual threats from humans – like buildings, factories, waste dumps,  and pollution – are rapidly destroying the habitats of these remarkable creatures.  They may be resilient and clever, but they are no match for humans.  Scientists are still in the process of discovering new species of nudibranch.  The fear is that as marine habitats disintegrate, sea slugs will start to disappear.  Already now, researchers report that certain species, particularly in tropical areas, are becoming harder to find.  Also, as coral reefs are declining, some types of nudibranchs that depend on the coral will die off, too.

Sources: Readfearn, Graham, The Guardian, “Keep your head: the self-decapitating sea slugs that regrow their bodies – hearts and all,” https://www.theguardian.com/sea-slugs, March 8, 2021; Roth, Annie, The New York Times, “Meet the Sea Slugs That Chop Off Their Heads and Grow New Bodies,” https://www.nytimes.com/decapitated-sea-slugs.html, March 8, 2021; The Living World of Molluscs, “Sap-sucking Slugs (Sacoglossa),” http://www.molluscs.at/gastropoda/sacoglossa.html; Silen, Andrea, National Geographic Kids, “Nudibranch,” https://kids.nationalgeographic.com/nudibranchNational Geographic, “Nudibranchs,” https://www.nationalgeographic.com/nudibranchs-1; Kingdon, Amorina, Hakai Mazaine, “Nudibranchs: Armed and Fabulous,” https://www.hakaimagazine.com/nudibranchs/, September 19, 2018; Smithsonian, “A Collage of Nudibranch Colors,” https://ocean.si.edu/nudibranch-colors.