Two weeks ago I wrote about recent evidence that Oriental hornets might draw energy from sunlight—namely, that the microscopic structure of their exoskeleton seems to have photoelectric properties, and that the hornets might use the small amount of electrical energy that resulted for some as-yet-undetermined biological purpose. In that post, I mentioned the emerald green sea slug Elysia chlorotica, one of the small number of animals (almost all of them invertebrates) that draw energy directly from sunlight, just as green plants and photosynthetic bacteria do. In truth, those animals don’t photosynthesize simply like green plants; rather, they retain microalgae and cyanobacteria in their bodies as symbionts and consume what those cells produce. The emerald green sea slug exhibits a particularly advanced form of this partnership, in that its DNA contains genes that help to sustain the chloroplasts it gets from consumed plants.
The obvious follow-up question was, “Why don’t we see more photosynthetic animals?” Subsequently, however, I’ve read a marvelous new exploration of that very topic in New Scientist, “Light diet: Animals that eat sunshine,” by Debora MacKenzie and Michael Le Page (11 December 2010 issue). (If you aren’t a subscriber, follow that link quickly because the article’s free availability is due to lapse on Thursday, Dec. 23.)
I won’t recap MacKenzie and Le Page’s article—read the real thing for details—but it appears that although vertebrates and other animals more anatomically sophisticated than sea slugs probably could be photosynthetic in principle, several factors make it a less practical evolutionary option:
- The obvious considerations are ones of structure and lifestyle. Photosynthesis works best for organisms with lots of surface area that they expose to the sun—hence the flat leaves and branched structures of trees, for example. Animals, especially big ones, typically have small surface areas relative to their volumes. Moreover, many animals spend much of their lives under cover, away from sunlight, avoiding predators. Nevertheless, even some invertebrates with only weak exposures to sunlight do photosynthesize, such as giant clams, and one might still expect animals more generally to use photosynthesis as an energy supplement. (MacKenzie and Le Page run through calculations showing that in theory, even a standard fish such as a carp has more than enough surface area to sustain itself energetically through photosynthesis.) So structure and lifestyle can’t be the complete answer.
- For many animals, the potential benefits of drawing free energy from sunlight might be offset by the considerable risks of extra UV exposure and overheating.
- The bounteous sugars that photosynthesizing symbionts might spew into their animal hosts could be too rich a diet for them, and pose health risks of their own.
- 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.
That latter point, which I suspect might be the most influential one, also seems like the one most reflective of how evolution works in general. The existence of all the sea slugs, flatworms, clams and even salamanders that rely to some degree on photosynthesis shows that nature, endlessly innovative, continues to tinker with solutions for living. Conversely, the absence of still more photosynthetic animals suggests that although this strategy may work for some, it faces constraints.
Or to put the point in more scholarly terms, consider the abstract from “Why Do so Few Animals Form Endosymbiotic Associations with Photosynthetic Microbes?” by D.C. Smith and E.A. Bernays, Phil. Trans. R. Soc. Lond. B 29 August 1991 vol. 333 no. 1267 225-230, DOI: 10.1098/rstb.1991.0071:
A survey of modern associations in which protists or invertebrates are hosts shows that very few of the many species of photosynthetic microbes are adapted to an endosymbiotic existence. None occurs as intracellular symbionts in animals structurally more complex than cnidarians and platyhelminths. Photosynthetic symbionts are not usually capable of being the sole food source for hosts because they do not provide a balanced diet; most hosts therefore retain holozoic feeding. Interactions between hosts and intracellular symbionts are complex, and have to include mechanisms for inducing release of photosynthate from symbionts as well as controlling symbiont cell division. Possession of symbionts imposes a measurable cost on hosts. For the great majority of animals, the costs of adapting to herbivory or other forms of nutrition are probably less than that of hosting photosynthetic symbionts, especially when the need for exposure of a large surface area to light is borne in mind. Once hosts become multicellular, it is virtually impossible for any photosynthetic symbionts they possess to evolve into organelles because they are restricted to specific host cell types. After the evolution of the eukaryotic ancestors of plants in the late Precambrian, the major significance of endosymbiosis to the evolution of plant-animal interactions has been the development of gut symbioses in some major groups of herbivores.
Look at it this way: All of today’s green plants and algae seem to have arisen from a permanent partnership that formed about a billion years ago between a single-cell host and its photosynthetic bacterial hitchhiker. Why hasn’t this union repeated itself over and over again in other organisms in the billion years since then? Perhaps such symbioses are uncommon, but the variety of photosynthetic animals we know about hints that may not be true. The more likely possibility is that newly photosynthetic organisms most often lose out in the race for resources against both plants (which have more refined and efficient photosynthetic systems) and animals (which can scavenge and prey without photosynthetic baggage). Maybe the new photosynthesizers can’t elbow their way into crowded ecosystems.
In any case, such reflections aside, go read “Light diet: Animals that eat sunshine” and learn more.