The photosynthetic slug

Along the East coast of North America lives a small mollusc, the sea-slug Elysia chlorotica, which is able to embody chloroplasts belonging to the alga Vaucheria litorea, on which it feeds. This gastropod can use these organelles to survive for long periods without food, using sugars produced by the photosynthesis process. During the juvenile period this slug needs to feed on the alga, before chloroplasts are stably incorporated in his cells.

However, the most interesting fact about Elysia chlorotica is its ability to maintain chloroplasts alive for several months. This is due to the fact that it doesn't only embody the organelles, but it also integrates in the genome the alga's genes that support the chloroplasts in their photosynthetic function. This process is usually called "horizontal gene transfer", since it implies that an organism incorporates some genes of another organism without being part of its offspring, and is common in different bacteria species.