The Elysia chlorotica is a sea slug that looks like a leaf and eats by sucking the insides out of strands of algae. (Yum!) These algae, like plants, get their food by using sunlight to help make sugar i.e. photosynthesis. Pierce and his colleagues already knew that a slug has chloroplasts inside its cells, it can use photosynthesis to make food — which means it may not even have to eat for the rest of its life (about a year). Surprisingly, now he’s learned that the sea slugs aren’t simply stealing what they need to do this from the algae. They’ve also stolen the recipe for how to make chlorophyll, a chemical that is vital to the process, and can make chlorophyll themselves. This happens because when Pierce’s slug eats algae, it separates out the chloroplasts. Instead of digesting and excreting the chloroplasts, the sea slug absorbs them inside its own cells.
But the chloroplasts use up the chlorophyll during photosynthesis, and a fresh supply is needed. Where does it come from? Pierce and his colleagues found that unlike other animals, sea slugs can make their own chlorophyll — which means that they have stolen more than just the chloroplasts i.e. they also have genes for making chlorophyll.
So sea slugs not only ingest the chloroplasts — they’ve also “adopted” part of these genetic instructions from their food (algae). In other words, these sea slugs are truly becoming what they eat. This is the first time the worlds of algae and animals have seemed to overlap like this.
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