One of a Kind: The Animal That Can Photosynthesise
If you were asked which organisms can photosynthesise, plants and algae would probably be the first things that come to mind. After all, photosynthesis is one of the defining features of plant life, allowing organisms to capture energy from sunlight and convert it into a form they can use. Animals, meanwhile, obtain their energy by eating other organisms.
Or at least, that is usually the case.
Hidden among seagrass beds and algae-covered habitats are a group of sea slugs that have developed a remarkable way of blurring the line between plant and animal. Rather than simply digesting the algae they feed on, these slugs are able to steal part of the algae's photosynthetic machinery and keep it functioning inside their own bodies.
Stealing Chloroplasts
Species such as Costasiella kuroshimae and Elysia chlorotica belong to a group of sea slugs known as sacoglossans. When these sea slugs feed, they pierce algal cells and suck out their contents. Most animals would digest everything they consume, but some sacoglossans do something rather different. Instead of breaking down the chloroplasts, which are responsible for photosynthesis, they retain them within their own tissues.
The stolen chloroplasts can remain functional for weeks and, in some species, even months after they have been removed from the algae. This process is known as kleptoplasty, which literally means "stealing plastids".
Borrowing the Machinery of Photosynthesis
Chloroplasts are the structures that allow plants and algae to carry out photosynthesis, using energy from sunlight to produce sugars that can be used as a source of energy.
The remarkable thing is that these chloroplasts can continue functioning even after they have been incorporated into the sea slug's tissues. As long as they remain active, they continue capturing energy from sunlight inside an animal's body. Although this has led to sea slugs sometimes being described as "solar-powered", the reality is a little more complicated.
Not Quite a Plant
Despite making use of stolen chloroplasts, these sea slugs are still entirely animals. They do not produce their own chloroplasts and cannot survive indefinitely on sunlight alone. Eventually the stolen chloroplasts stop functioning and must be replaced through feeding.
Some species, particularly Elysia chlorotica, can retain functional chloroplasts for surprisingly long periods, allowing them to survive for extended lengths of time with very little food. However, what makes them so unusual is not that they have become plants, but that they can keep part of another organism's cellular machinery working long after it has been eaten.
