Evolutionary branch tips merge in animal-algae combo-creature

When running with the cross country team here at school, often for an hour or more, it's essential to maintain conversation lest we die of boredom.  Frequently this is done with bizarre, acontextual questions like, "what would be your ideal superpower?"  This question always gets answers along the lines of "invisibility," "flight" and "teleportation," but recently one of the blokes proposed something original and intriguing: photosynthesis.

Imagine - you could go into the wilderness for weeks with no food supplies and be fine; you could sunbathe for dinner!  You would never have to pay for a meal again!  But let's not entertain such frivolous thoughts.  Photosynthesis has always been, and will always be, reserved for plants (and algae and some bacteria), right? 

Of course not.  Why would I be writing this article, then?

Behold, the solar-powered sea slug:

noe.futurevessel.com

Elysia chlorotica is the first animal known to perform photosynthesis. A single algal meal shortly after birth gives it the chloroplasts it needs to survive for a whole lifetime of about a year with no food at all.

But just algal chloroplasts aren't enough to keep E. chlorotica supplied with food for so long. Chloroplasts are merely machines that convert sunlight, water and CO₂ into sugar.  To do this they must be constantly supplied with “parts:” enzymes and chlorophylls manufactured by the cells that contain them.  In their native algae, these parts are synthesized by the algal nucleus and require 16 enzymes not manufactured by chloroplasts themselves.  E. chlorotica must not simply be stealing organelles, then.  

Although it doesn’t necessarily need to, E. chlorotica will eat algae regularly in the wild.  If artificially prevented from eating it eventually stops producing digestive waste and then lives on just as well – its chloroplasts remain fully functional.  When radioactively labeled amino acids were given to slugs in this condition, they turned up in protein products identified as chlorophyll-a, the most common chlorophyll found in nature, signifying that slug cells must be producing the compound on their own. 

Further investigation has revealed E. chlorotica genes that appear to be directly assimilated from algae after that initial meal.  Even embryonic slugs that have never eaten have “algal” genes, meaning at least some of those required for chlorophyll-a synthesis are now part of E. chlorotica’s heritable genome.  This is an animal well on its evolutionary way to true autotrophy.