Here’s a neat video about jellyfish that have evolved to utilize photosynthesizing algae that produce sugars, in turn providing food for the belled organisms:
These jellyfish are locked in a freshwater marine lake that formed within pieces of volcanic land that ‘sprung’ up in the Pacific; the lake filled in some 12,000 years ago as rising ocean levels reached its basin. With no notable predators (sorry, sea anemone), the jellyfish have reproduced to incredible numbers (10 million by one estimate). They have faced huge die-offs over temperature differences and toxicity levels in years past, indicating that they are part of a fragile environment, but they are currently going as strong as ever.
As evolution predicts, these creatures have lost abilities no longer useful to them. Whereas many of their salt water counterparts are painful sons-of-bitches, the stingers on these guys are closer to being cute than harmful. This, luckily, makes it possible to swim alongside the jellies (and since every article and paper I’ve found on them takes care to note that 15 meters below the surface is a heavy layer of hydrogen sulfide, I suppose I’ll do the same – the stuff can kill you).
Visiting this lake is definitely on my bucket list.