Organ(elle) Snatching Slugs

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Elysia chlorotica: sea slug, organelle thief, and species extraordinaire. Image from Patrick Krug.

A host of spineless thieves reside off the east coast of North America. In the shallow waters of salt marshes and tidal pools lurk small, brilliant green marine slugs that are the organ snatchers of the natural world. Like many other sea slugs, the eastern emerald elysia, Elysia chlorotica, feeds on algae. However, instead of digesting the algae completely, the slugs suck out their chloroplasts and keep them for their own. Chloroplasts are a specialized chlorophyll pigment-containing photosynthetic structure or organelle found in plant and algal cells. They use light energy to turn carbon dioxide into simple sugars. These simple sugars, like sucrose, are food for the cells they are made in, or for the animal that eats and digests that cell. Elysia chlorotica is unique in that the cells lining its digestive tract can incorporate the chloroplasts, and the slugs owe their brilliant green colour to the chlorophyll pigment in these stolen chloroplasts.

But what exactly the slugs do with their ill-gotten gains? The matter is up for debate among scientists. When sunlight shines on these slugs, they act like the leaf of a plant and begin to produce sugar and oxygen, supporting the argument that the slugs are harnessing the solar-powered sugar-producing abilities of the chloroplasts they steal. Considering that only 10% of the genes necessary for photosynthesis are encoded in chloroplasts themselves, it seems a minor miracle that the chloroplasts are active at all. The secret to this success? A bit of genetic thievery. Genome analysis shows that, along with chloroplasts, Elysia has also nicked specific photosynthesis-related genes from its algal dinner through a process known as horizontal gene transfer. Baby sea slugs hatch with some of the genes to control photosynthesis and produce key enzymes already in their genetic makeup. However, the eastern emerald elysia’s kleptomaniac habits aren’t enough to make it a self-sufficient animal with no need to eat, raising the question of whether they really do rely on their stolen chloroplasts for sugar. Transcriptome experiments show that both nuclear and chloroplast genes for photosynthesis are expressed only at very low levels and when the sea slugs are starved, they gradually shrink in size, lose their brilliant green colour, and eventually die. Starving Elysia will begin to break down both their own tissues, as well as their stolen chloroplasts in order to get the nutrients and energy needed to survive. This has led to an alternative theory that the slugs are just saving the chloroplasts as snacks for later when food is not so easy to come by. Regardless of how they might be using their stolen chloroplasts, if you are a sea slug, thievery might pay off in the short term, but it won’t keep you going forever.

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