In the animal world, the ability to live off of solar power is virtually unheard of. Sure, most would say corals do it, but the truth is that the corals themselves aren't the ones soaking up the sun - it's tiny dinoflagellates called 'zooxanthellae' that actually perform the photosynthesis. In fact, that is the case with most other arguments for solar-powered animals. Most have some form of plant-like symbiont doing the hard work for them.
Not so with the green sea slug Elysia chlorotica, claims biologist James Manhart and his colleagues at Texas A&M University who have been studying the slug's plant-like ability. Along with other institutions, the research group believes it has found the source of the slug's unique ability, and their findings have been published in the current issue of Proceedings of National Academy of Sciences (the one with an image of a green slug on the cover). There are no plants living inside the slugs - though, there are some plant parts.
In plants, the organelles responsible for photosynthesis are called plastids. The sea slug uses plastids to photosynthesize, but not ones it makes by itself. No, the slug eats algae and digests the majority of the algae, save the precious plastids which it relocates for its own use.
While this sounds simple, it overlooks one major problem: in plants, the majority of the proteins that are required to keep plastids functioning are made separately by the nuclear genome - proteins which don't survive the slug's digestion. It is estimated that photosynthesis requires upwards of 3,000 genes which animals don't have. So the real question is: how does the sea slug actually use the plasmids it injests?
The researchers have found that the sea slug has evolved the genes to photosynthesize. "The slug has at least one gene required for photosynthesis in its nuclear genome, which has never been found in any animal," says Manhart. "The critical thing is the plastids come from the alga, but the slug nucleus contains at least one, and probably more of the genes required for plastid functioning."
After the slug has ingested the plasmids it needs, it can survive for at least nine months off of the sun alone.
It also means that the slug babies are born with the ability to photosynthesize, once they gather their own plastids. The sea slug is a rare case of horizontal gene transfer between a plant - in this case its main food, the alga Vaucheria litorea - and an animal. Somehow the sea slug has stolen the genes from its food and inserted them into its own genome so that it could make use of the algae's plastids.
It's possible that if the slug survives long enough, it could transfer the genes necessary to make the plastids in the first place, and become entirely photosynthetic.
Isn't evolution fun?
M. E. Rumpho, J. M. Worful, J. Lee, K. Kannan, M. S. Tyler, D. Bhattacharya, A. Moustafa, J. R. Manhart (2008). From the Cover: Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica Proceedings of the National Academy of Sciences, 105 (46), 17867-17871 DOI: 10.1073/pnas.0804968105