New Scientist has a really fun cover story this week, "Dawn of the Plantimals," by Debora MacKenzie and Michael LePage that looks at photosynthesizing animals.
A number of animals derive some of their energy from sunlight by hosting algae on or in their bodies, including giant sea clams, sea slugs, coral, anemones and jellyfish. Most interesting is a salamander that stores algae in its oviduct. The algae then grow inside the salamander's eggs as well as on them, presumably offering the embryos a meal. Other creatures use cyanobacteria–photosynthesizing bacteria–to turn sunlight into energy.
It may be possible to engineer this nifty trick into animals that today have to eat their dinner. For example, a researcher at Harvard Medical School has injected cyanobacteria into the eggs of zebrafish. Both fish and bacteria lived. Done right, this kind of thing might someday produce backyard koi that require slightly less of that expensive Japanese koi food.
Among the problems with photosynthesis in animals is that it requires damaging exposure to UV radiation and heat, which is why all photosynthesizing animals are water-dwellers. Another problem? Photosynthesis produces junk food–sugars.
Still, the fact that some animals use sunlight to generate food, as plants do, is a reminder of biologist Lynn Margulis' idea that all labels are problematic.
In Symbiotic Planet, she writes, "Our classifications blind us to the wildness of natural organization by supplying conceptual boxes to fit our preconceived ideas….We can group life into three or five or a million categories, but life itself will elude us."
Plants versus animals? Of course there are green creatures that reside in the grey areas!