The sea slugs known as Elysia chlorotica are certainly a strong contender for my favourite animal. They are born clear, and they feed on algae for nutrients and energy. But, they poke little holes in the turgid algae pieces with their tooth, and suck out the individual chloroplasts. The chloroplasts do not get digested, and instead migrate to the animal’s skin, where the animal has gained the ability to keep the chloroplasts alive inside its animal cells (not in-between, but inside). The chloroplasts then photosynthesize using sunlight, and produce energy for the slug’s cells. The chloroplasts make the slug’s skin green, and indeed the slug has evolved to take on a shape which maximizes surface area, in order to harvest as much sunlight as possible. The adults resemble leaves for this reason!
Watch this video of the little guy sucking out the chloroplasts from a piece of algae:
These slugs were successfully kept alive in excess of a year in a tank with absolutely no caloric input in the water, and with nothing but a UV “grow lamp.” Their growth was very slow during this time, but this indicates that an animal was capable of living entirely off of photosynthesis! This is the only animal known to be capable of keeping the chloroplast endosymbiont alive.
There are other pseudo-photosynthetic animals, but these others keep the plant cells in-tact inside their bodies, and the plant cells keep the chloroplast endosymbionts alive. Like Russian nesting dolls. The Elysia chlorotica actually kills the plant cells, and takes the endosymbionts out, and cultivates them as its own (the same way it does its mitochondrial endosymbionts)! This really reinforces the idea of the chloroplast and the mitochondria as endosymbionts, rather than as organelles or parts of the cells.
How did Elysia chlorotica gain the ability to keep chloroplasts alive on its own? Research by Sidney K. Pierce, Nicholas E. Curtis and Julie A. Schwartz indicates that the slugs have obtained algal genes via horizontal gene transfer, from the algae that it lives in and eats all its life. Horizontal gene transfer, or the transfer of genes from one species to another (in this case plant-to-animal) seems to appear more and more common than once thought, every time we discover a new example of it. Bacteria share DNA like this all the time, but it can still be surprising to hear about it in animals and plants.
Now the only question is… how can I get MY cells to keep chloroplasts alive? I could have lovely green skin, and save a lot of money on food by lounging in the sun.