Are there any animals out there which are capable of converting sunlight and CO2 into energy on their own?
If not, why? Is there any sort of evolutionary stumbling stone which could make this unfeasable, or did things just not happen that way?
Also, would it be possible to splice in genes to allow animals to do this?
If you count bacteria as animals, there are several varieties of photosynthetic bacteria.
On a multicellular level, I’m guessing that if animals that derived energy (beyond passive heat) from sunlight were possible, plants would have evolved a larger range of motion.
Er, no. If the did, they would be, by definition, plants, or more specifically, of the kingdom Plantae. (There are some chlorophyl-using alge as well, but we’ll set them aside for the moment.) Plants utilize what is known as the Calvin-Benson Cycle, which is roughly complementary to the Krebs Cycle in animals, to generate the ATP (adenosine triphosphate) that powers the organelles in the cells. Plants alse use respiration, but to a lesser extent and with less efficiency than animals.
Well, technically speaking this is a semantic argument, since we more-or-less define plants as engaging in photosynthesis, but to expand this further, this is a very low energy reaction which requires both exposure to sunlight to maintain and relatively low power usage. An animal that used photosynthesis to generate the sugars to fuel the cycle would be much, much slower than a sloth; hence, this is why we don’t have to worry about sunflowers and ivy choking us to death when we irritate them.
Most plants have some limited degree of articulation, in the form of osmotic pressure (sunflowers pointing toward the sun, for instance) but they lack actual musculature as is seen in animals. Very few plants are capable of actually moving, though many will creep or send spores far and wide.
No doubt some mad genius could graft some kind of chlorophyl-generating capability into animal cells. Whether the cells could maintain a energy-generating cycle or utilize the products thereof is another question; the differences between plant and animals cells is fundamental.
But hey, you want to try? Be my guest. Just don’t let your Triffids loose.
Stranger
Okay, but can I borrow your Junior Mad Scientist Genetic Splicing Kit™?
And I wasn’t thinking of animals using photosynthesis for all of their energy needs, just as a supplement. Surely it wouldn’t harm the animal to have an additional energy source. It just seems to me like somewhere down the evolutionary trail there should have been at least one animal which used photosynthesis to compliment its regular diet.
Oh well, off to go create some planimals!
Bacteria are not animals. It is a common misconception that there are only two kingdoms (animals and plants). There are actually five.
I thought it was now three. Bacteria, archea, and eukaryotes with plants and animals subdivided later on.
According to my uni microbiology lecturer there are five (Monera, Protista, Fungi, Plantae and Animalia).
It depends. The “domain”-style taxonomies are becoming somewhat more popular, as described by threemae. Not everyone agrees on how to divide things, but yeah, the “traditional” division now is the five kingdom approach.
As with everything in biology though, there are exceptions.
One that comes to mind is a type of sea slug (Elysia chlorotica) that eats algae, incorporates its chloroplasts into its own cells, and uses them to generate food. It cannot synthesize the chloroplasts itself, so the chloroplasts typically seem to die off over the course of ten months, but this is truly a photosynthetic animal. I seem to remember others as well (google turned up this)
For all intents and purposes this happens with many reef-building corals, tridachna clams, and some other invertebrates. Corals contain symbiotic algae called zooxanthellae that photosynthesize and provide them with a significant part of their energy requirements. (The corals are also capable of using food they capture themselves.) The algae receive some nutrients from the coral animals and a sheltered place to grow.
Animals have had no need to develop chlorophyll. It’s been much easier just to co-opt the existing abilities of other organisms that already had photosynthetic capacity than to develop it de novo.
Damnit, man, think about what you’re saying? What would happen if almost the entire human race will go blind for almost no reason?
It’s this type of short-sighted (no pun intended) thinking that leads to getting . . . um . . . killed by . . . uh . . . Triffids.
One possible answer to that might be that the requirements for photosynthesis are resource-intensive to build… so that for an animal, with high enough energy needs that the sunlight energy wouldn’t count for much, it actually would hurt them to develop that energy source, using up nutrients that could otherwise be devoted to running faster or something like that. 
Here ya go, your very own DNA re-combiner
Speaking of sloths, I understand they’re often green from the plants (algae?) growing on their slow-moving bodies. I think I’ve seen pictures of this.
Not the same as an animal with chlorophyll, but still…
Both two-toed and three-toed sloths have specially modified hair, different from that of all other mammals, that appears to be designed as a particularly suitable growth site for algae. The animals can appear very distinctly green, particularly when wet.
Although this helps to camouflage them, they are already pretty well camouflaged. This benefit doesn’t seem to be great enough to account for the development of such specialized hair. At least one biologist has speculated that they might derive some nutritive benefit from the algae (particularly vitamins), but since they do not seem to lick their fur very often this would presumably have to be through absorbtion through the skin. More research is needed.
Two points: First, to add to Colibri’s list of symbiotic photosynthesisers, there’s the common lichen, which is a fungus and algae working together.
But more relevantly, I believe there are single celled organisms that exist in forms both with and without chlorophylls, and that a particular cell could theoretically lose its chlorophylls and continue living (gaining energy from external nutrients), thereby switching kingdoms in its own lifetime!. Of course, this just points out how the Plant/Animal concept breaks down for many microorganisms.
For multi-cellular life, I tend to agree with everyone else: photosynthesis doesn’t provide enough energy by itself to move about much, so you have to eat things. And if you’re eating things, then the return on investment for building a photosynthesis system is just too small to be worthwhile – better put that investment in moving faster, or building chemical protection so nothing eats you, or whatever.
But it’s a symbiotic relationship between an animal and a photosynthetic life form, not a direct development of photosyntheic mechanisms in animal tissue. Ditto for lichen. But it is an interesting example of symbiosis and the interdependance and coadaptation of life forms, kind of like real estate magnates and Eastern European supermodels.
Neat! I’ll have to check that out.
Now, if only we could figure out how to create an alcohol fermentation and distillation organ that could be directly implanted into the salivary tissues…that would be convenient. 
Stranger
True. I mentioned it in part as an explanation why animals don’t develop chlorophyll themselves: it’s easier just to hijack an entire photosynthetic organism (or in the case of the sea slugs, just their chloroplasts).
And it also bears mentioning that chloroplasts themselves are believed to be derived from an ancient symbiosis. Like mitochondria, they have their own separate genetic system apart from the nuclear DNA of the cells they are found in. They are probably derived from independent photosynthetic bacteria-like organisms that became symbiotic with the ancient ancestors of eucaryotic photosynthetic organism.
Therefore, algae and higher green plants in a way don’t develop chlorophyll themselves, but instead are relying on these ancient symbionts to produce it. Basically, it’s the same thing that corals do with algae, but it’s such an ancient symbiosis that for all intents and purposes the chloroplasts are now a part of the eucaryotic organism.
Neat! I’ll have to check that out.
Now, if only we could figure out how to create an alcohol fermentation and distillation organ that could be directly implanted into the salivary tissues…that would be convenient.
Stranger
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This really weirded me out when I first learned about it. “You mean they have their own DNA?”
I think learning this was the first time I realized that biology could be just as bizarre and exotic as physics and math.
Stranger
Even more suitable than polar bears?