No, this isn’t a piece of lettuce (but that was a nice guess from our
food blogger, Amanda
). This is a
sea slug, Elysia chlorotica
, and it looks like a leaf because it has acquired chloroplasts from its algal prey and stored them in its gut lining. If you or I ate those algae, the chloroplasts wouldn’t do us any good because we don’t have the necessary genes to keep the chloroplasts going (the DNA in these organelles encodes for only about 10% of the proteins the chloroplasts need). But this sea slug has acquired those genes through some sort of transfer from the algae, letting the sea slug rely on solar power for weeks at a time.