Photosynthesis is a process by which green plants synthesize food(glucose) By capturing sunlight with the help of Chlorophyll.Since they are not able to synthesize their own chlorophyll molecules , they steal these chlorophyll molecules from an algal species Vaucheria litorea. by a phenomenon known as kleptoplasty. In this phenomenon Elysia feeds on the algae vaucheria by puncturing a hole in its cell wall through its radula, then it holds the algal strand firmly in its mouth and starts sucking out the sap like a straw. Instead of digesting everything it ingests, it stores the chloroplasts in its own cell (Phagocytosis) through a specialized digestive system. The chloroplasts (chlorophyll containing bodies) now inside elysia's cells allow it to capture sunlight and perform photosynthesis. Juvenile elysia's are reddish-brown in colour but after acquiring chloroplasts in their cells they turn green just like plants and this green colour helps them camouflage. it has been found that they can retain these chloroplasts in their cells for 8-10 months and when kept in dark they start turning yellow like plant leaves do! If you are still with us, you might be wondering: Why is this organism so special? and Why is it able to retain the chloroplast and perform photosynthesis? Well, the interesting thing here is that having chloroplasts is just about 10% of the story. To actually be able to perform photosynthesis is a whole different story. There are specific genes in plants DNA that control Photosynthesis and as elysia is able to perform photosynthesis so it must mean that it also has the genes for photosynthesis. But how did it get them? Scientists think that it is due to a process known as horizontal Gene Transfer and the proof of it is the gene psbO which is very similar to the algal gene. Due to this elysia is able to maintain the chloroplasts in its functional state for such a long period of time. So why don't we all just use this horizontal gene transfer thing and become self-dependent like elysia? The answer is because we cannot. Horizontal gene transfer is due to evolution and evolution doesn't always go the way we want it to. The gene transfer thing could have been an accident, we would never know. In horizontal gene transfer part or whole DNA of one organism gets transferred and encoded in another organisms DNA and replicates along with it so that the offsprings contain both the characteristics encoded in both the DNAs. This is usually a method of reproduction in single-celled organism and is fairly common. Think of it like Tobey Maguire's Spiderman, only a lot more complicated. horizontal gene transfer usually involves bacteriophages and plasmids and is the reason for bacteria becoming resistant to antibiotics. Elysia timida and Plankobranchus ocellatus are also found to display this phenomenon.
The basic need of every living organism is food, water and shelter. We humans still think that we are the at the pinncale of food chain! But a mollusc called Elysia chlorotica has surpassed us. Elysia is a marine opisthobranch gastropod mollusc. It is also known as sap-sucking sea slug or eastern emerald elysia. This organism is capable of producing its own food by using the same pathway the plants use - Photosynthesis . For those who might have been living under a rock during their high school biology class, here's a quick def: