A reader from the UK sent me these lovely photos that he took of a jellyfish and asked for my help in identifying it. Unfortunately, while I'm pretty good with bacteria, plants, and tropical fish, my taxonomy skills don't go much farther unless I have a sample of DNA.
These photos were taken at La-Manga off the coast of southern Spain.

My Sciblings have guessed: Rhizostoma, Stomolophus meleagris, or Mastigas

If you know, we'd all appreciate an answer in the comments.
And if you don't know about much jellyfish, RPM at evolgen would like your help answering a different reader's question: When a manatee swims, does it get water up its up nose?
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Jeremy Jackson calls it "The Rise of Slime". Daniel Pauly sees a future in jellyfish burgers. And given that this week is the 2nd International Jellyfish Bloom Symposium--where D.
I just came across two blogs dedicated to all things jellyfish:
JellyBiologist
Jellyfish Aquarist
When Daniel Pauly first began talking about jellyfish burgers, he did so as an absurd metaphor.
I'd have to go with Rhizostoma pulmo. The look and the location are right. There's also a similar beastie that hangs around the UK called Rhizostoma octopus.
Oh, that's Steve. We met while I was diving a few years back...he's a cool dude.
Definitely a Rhizostoma pulmo. Mediterranean and maybe Atlantic. Good picture on flickr:
http://www.flickr.com/photos/arne/198961001/.
It's armless, you can touch it and swim with it.
The flicker shot is great! It definitely looks like the same kind of creature.
rhizostoma pulmo
Thats definately the one that sung me in the Mar Menor,La Manga !!
I swam into it whilst snorkling just INSIDE the safety nets (yeh right!) and I thought I'd swam into a plastic bag until my face HURT !!
I swam to shore quicker than Phelps !!