Hank: Hello, this is Hank Green, and welcome to SciShow. You might have noticed that I talk a lot about how great plants are, and why shouldn't I? They're autotrophs. They can harness the raw, untamed power of the sun and use it to make food for themselves. Pretty impressive. So why can't animals do it?

Well, oddly enough, some of them do. And in just the past few years, researchers have been discovering animals that seem to be able to harvest sunlight for energy.

Take, for instance, the Eastern Emerald Elysia, a sea slug that looks like a big floating leaf that lives along the east coast of the United States, where it just hangs out eating a specific kind of yellow-green algae. It's been known for a long time that this slug has a special relationship with the algae, but until recently, nobody knew that the slug was actually using the algae's genes for photosynthesis.

See, the slug eats the algae to absorb its chloroplasts, the organelles in the cells that actually do the photosynthesizing. But in order to turn the sunlight into chemical energy, the chloroplasts need a whole set of specialized proteins to help. So, the slug has lifted genes from the algae that allow it to make these proteins ITSELF. Nobody's sure exactly how this happened but we do know that these slugs pass those algal genes onto their little baby slugs, which only need to eat about two weeks of the year. The rest of the time, they're just soakin' in the rays.

Another photosynthesizing animal is the pea aphid, which, depending on environmental conditions at birth, can be either white, orange, or green. Now, aphids are notoriously weird; some of them can be born pregnant, some males are born with no mouths and die of starvation right after mating... So maybe it's not a huge surprise that pea aphids have been found to manufacture their own carotenoids, colorful compounds that plants, fungi, and some algae use to help with photosynthesis. Carotenoids help plants absorb more light and protect their chlorophyll molecules, which make photosynthesis possible.

Animals need carotenoids too. Beta-Carotene, a precursor of Vitamin A, is one you've probably heard of. But we can only obtain them by eating plants that make them. But since pea aphids have somehow acquired the genes to make their own carotenoids, scientists have found that some aphids, depending on their color, can use sunlight to make adenosine triphosphate, or ATP, the same energy-storing molecule that plants create through photosynthesis, and that we make by metabolizing food.

Researchers have found that aphids with green carotenoids make a whole lot more ATP than white aphids do, while orange aphids make more ATP when exposed to sunlight than when kept in the dark.

Weirder and weirder, right? Well, hang onto your hats because the spotted salamander, common to the United States and Canada is the first and, so far, only known photosynthetic vertebrate. In 2010, scientists discovered that these salamanders have chlorophyll-containing algae IN THEIR CELLS, all over their bodies, and the cells with algae in them seem to help power other nearby cells. Again, no one's sure how this relationship came about, but the algae has been found to first appear in the salamanders' fertilized eggs, possibly, when the embryos are just big enough to feed the algae with their waste, creating a kind of algal bloom inside the eggs. The algae then enter the embryo's body, probably when the nervous system first forms, and they spend the rest of their lives together! All of this is totally surprising because vertebrates immune systems generally destroy foreign stuff that tries to live inside their cells, but this salamander and the algae seem to be a match made in Heaven.

So, it's good to know that there is, INDEED, an exception to every rule. Now that we know about at least three good examples of photosynthesizing animals, and we can start to wrap our minds around how exactly that works, who knows? Humans could be next. Take that, plants! You think you're so great...

Thank you for watching this SciShow Dose. If you have any questions, or comments, or ideas, we're on Facebook and Twitter, and of course, down in the comments below. And if you want to keep getting smarter with us here at SciShow, you can go to YouTube.com/SciShow and subscribe!