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Half Animal, Half Plant: The Solar-Powered Sea Slugs

Can eating lots of vegetables turn you into a vegetable? Not really, unless you are a sea slug.

There are several species of sea slugs that have chlorophyll in their bodies which they utilize to make food from sunlight through the process of photosynthesis, just like plants do. But these sea slugs weren’t born with chlorophyll. They acquired them during their lifetime by eating too many veggies. They are known as sacoglossans or the "sap-sucking sea slugs” because they live by literally sucking out the cell contents from strands of algae, as if they are straws. But instead of digesting the food, like normal animals do, they keep the chloroplasts from the algae and then incorporate them into their own cells. Now all they have to do is lie in the sun. This unusual phenomenon is known as kleptoplasty, and this strange ability has earned these sacoglossans the title of "solar-powered sea slugs".


Elysia chlorotica feeding on Vaucheria litorea, a yellow-green algae. Photo credit

One of the best example of this behavior is found in a type of green sea slug called elysia chlorotica. They are most commonly found in salt marshes, pools and shallow creeks, along the east coast of the United States, but also as far north as Canada. They can grow up to 60 mm in length but are more commonly found between 20 mm to 30 mm.

A young elysia chlorotica is reddish or greyish in color. Once they start feeding and retaining chloroplasts in the cells, they turn bright green. The green color also helps the animal to camouflage against predators, since the slug does not have a protective shell or any other means of protection. By taking on the green color from the chloroplasts of the algal cells, the slugs are able to blend in with the green sea bed beneath them, helping them improve their chances of survival and fitness. An elysia chlorotica can live off photosynthesis without feeding for up to a year. By spending less energy on activities such as finding food, the slugs can invest this precious energy into other important activities allowing them to longer.

However, a chloroplast is only one part of the cellular machinery that’s necessary to turn sunlight into energy. Merely possessing the chloroplast without the necessary genes responsible for photosynthesis won’t allow the animal to synthesize food. It’s like taking the blades off a blender and expecting it to make carrot juice without the motor.

Researchers have found that elysia chlorotica not only steals the chloroplast from the algae, but also their genes which it incorporates into its own DNA. This is a fascinating and rather unique instance of horizontal gene transfer.


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Costasiella kuroshimae is a another species of sacoglossan sea slug that is known to exhibit the phenomenon of kleptoplasty. This species is found in Ryukyu Islands, in Japan.


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Sources: Wikipedia / Science News

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