Every dive is an adventure, and for a (former) biologist like myself, the sea is a wealth of interesting and unusual creatures, behaviors, and relationships.
While snorkeling near Tabarca Island in the med, I came across a mauve stinger jellyfish (Pelagia noctiluca) that was swimming in close association with a tiny fish. If you watch through the movie clip (its a bit jerky, as I was both trying to keep the camera on the jellyfish and avoid getting too close) you'll see the little fish swimming around the bell of the jellyfish.
If you are having trouble seeing him (he was very quick) you can spot him in this still from the video, just to the left of the jellyfish. Throughout the whole encounter, he never strayed more than an inch or two from the bell of the jellyfish.
Notice something else about the jellyfish? Its missing its tentacles! (Compare with the picture on the wikipedia page). This starts to give a clue as to the relationship between these animals.
It took a bit of digging to find anything on these relationships, but I came across an old paper talking about similar symbiosis between young fish and an atlantic jellyfish species (Copeia, Vol. 1963, No. 1 (Mar. 30, 1963), pp. 40-80). The paper describes how larval fish associate with jellyfish Chrysaora quinquecirrha when they are young, using the jellyfish first as protection, then as a ready food supply, nibbling pieces of their host until they are large enough to eat them entirely and live an independent life. That paper also mentions an even earlier 1915 paper that describes how young scad seek shelter among jellyfish, using them initially for protection before consuming their gonads and tenticles as they get older. The unwillingness of the larval fish to leave the proximity of the jellyfish and the lack of tentacles on its host suggest that what I filmed is one of these symbiotic relationships, and that it won't be much longer before this larval fish is ready to leave its host for an independent life.
I would be curious if anyone else has filmed this relationship between a fish and a mauve stinger... there doesn't appear to be much in the scientific literature and given the age of the papers I found, I doubt anyone in the 60's (never mind 1915) was plunking a camera into the mediterranean to chase jellyfish. So I guess what you are seeing here is a scientific first... in my museum of curiosities.