Eating Sunshine

Image: NewScientist; 11, Dec. 2010 issue; "Dawn of the Plantimals"

I have read a lot recently about the photosynthesizing capabilities of animals, and why most animals simply don't do it. I personally thought this luxury was awarded only to plants - you know, almost as a pity: "Hey, plants, sorry you're stuck in one place, but at least you can obtain energy directly from sunlight!" As it turns out, there are several small animals that use photosynthesis to live!

So let's talk about the idea of photosynthesis in animals. This means that animals (animals is actually a euphemism here for aquatic invertebrates - 99% of the time, anyway) have an evolutionary adaptation that allows them to use light as an energy source. Look at the sea slug Elysia chlorotica pictured at right, for example. Its diet consists of algae, and upon reaching adulthood, steals a chloroplast from its food. As a result of this theft, the sea slug doesn't have to eat anything for the rest of its life. Seriously. Once the chloroplast is embedded in the slug's genes, it is able to convert sunlight into food forever. Think about how much money I'd save if I could eat sunshine and feel full! No more Oreo cookies for me!

Anyway, there are a bunch of conditionals that must happen for animals to be able to photosynthesize. The first is that light must be present. Duh. Many of the animals that currently photosynthesize have lifestyles that allow them to easily obtain sunlight (jellyfish that float on top of the water, hydras, coral, etc.) This winter has prevented me from getting much sunlight, I'll tell you that much. It's way too cold, even for the southwest. Another requirement is body shape. The E. chlorotica has a flat, leafy shape, giving sunlight a large surface area to penetrate. Some animals have a branching body shape that looks a lot like plants (coral and anemones).
Why don't a lot of animals photosynthesize, then? Many animals live in dark, cool areas where they are least likely to be spotted by a predator (I hide in the dark to prevent the Oreo cookies from getting me - I totally understand). In terms of evolution, this explains why most animals don't photosynthesize - the risks far outweigh the benefits. Let's see, easier access to energy, or not getting eaten by a predator? I think the choice seems clear.

Getting a chloroplast is also quite the feat, unless an animal is good at internal thievery (maybe they could get some pointers from the sea slug). I liked this description: "For most animals, the genetic and physiological costs of having photosynthetic symbionts or their own chloroplasts may simply be too high. Given that animals need to eat to acquire other essential nutrients anyway, the more efficient strategy for most creatures is simply to eat green plants, which have a billion years' experience with photosynthesizing efficiently." (John Rennie, "Why Animals so Rarely Photosynthesize")

With a bunch of calculations I won't go into provided by NewScientist, they have found that large fish such as carp could actually survive as a photosynthesizer. They would be able to obtain the energy they need for maintenance and survival. However, most fish eat the plants and algae that do undergo this process, and the fish are obtaining the necessary proteins and nutrients to survive anyway. There really seems to be no evolutionary need for bigger fish to photosynthesize since they seem to be doing just fine with their eating patterns right now.

The link to the NewScientist free article has expired, so I'll direct you to John Rennie's article for a similar story with some more details.

Sources:
"Dawn of Plantimals" Debora MacKenzie and Michael Le Page, NewScientist, December 2010, Volume 208, #2790
"Why Most Animals So Rarely Photosynthesize" John Rennie, The Gleaming Retort, December 20,2010