Thanks to an enterprising reader, we now know that this mysterious jellyfish isn't even a jellyfish at all. Shocking, I know! It's a comb jelly, and all the stuff we saw moving around inside of it was actually the cilia it uses as its mode of propulsion. These puppies have been around for a long time, half a billion years or so, and they can shimmer with iridescence, especially under a black light. Trippy, no?
Here's an article on the comb jelly. And here's my favorite part from the article, describing the introduction on the American comb jelly into the Black Sea:
This comb is widely blamed for the collapse of the commercial fishing industry in the region. The highly adaptable comb, christened ‘The Monster’ by the locals, is said to have been transported in the ballast tank of a US Ship. Since its colonisation of the Black Sea. ‘The Monster’s’ population is estimated to weigh over a billion tonnes. Each individual comb can produce 8,000 offspring every 24 hours.