"Jellies have likely swum and swarmed in our seas for over 600 million years. When conditions are right, jelly swarms can form quickly. They appear to do this for sexual reproduction. Since males need to release gametes (sperm) into the water, they need to be very close to females. Therefore swarming behavior is just a way for them to be close to each other. In some locations, jelly populations are increasing. But such increases are regional in nature; we do not have evidence that there are "global" increases due to global influences, but evidence is mounting that climate change may have an effect. Various types of environmental problems may promote the formation of jelly swarms. These problems include pollution, the overharvesting of fish, the introduction of non-native jelly species into new habitats, the addition of artificial substrate (like fishing reefs, and various offshore platforms) in the ocean and climate change. Higher water temperatures may speed jelly reproduction as well as extend the growing season for jellies; a longer reproduction season could result in more jellies. Climate change may also increase the amount of food available for jellies. Also, climate change may change ocean currents. Changed currents may transport jellies — which drift with currents — into new habitats. Because climate change appears to be a world-wide phenomenon, climate change may have worldwide impacts on jellies. But we really don’t know yet what the details regarding interactions between climate change and jelly populations. " --Live Science http://www.livescience.com/animals/081219-bts-jellyfish.html A great question! :D

Maybe via a communication system yet unknown to us, sort of a common consciousness in between them, like in between other swarms of animals ( ants etc). Please, should you be in the know, let me know....

Because I have a spoonful of peanut butter??? Or it is mating season:):)LOL:) Which means don't go a swimmin when the jellyfish fish are horney..you might just get your ass kicked:):)LOL:) By swarms of them:)

like in findig nemo?? lol http://www.xray-mag.com/en/content/why-do-jellyfish-swarm

1) "Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. In addition to sometimes being concentrated by ocean currents, blooms can furthermore be the result of unusually high populations in some years. The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and ambient oxygen concentrations." Source and further information: http://en.wikipedia.org/wiki/Jellyfish 2) "I had never heard the term swarm used for these wind- and current-driven creatures. Unlike bees or locusts, jellyfish don't cruise in a crowd. Mostly, they're solitary individuals that occasionally become pushed together by wind and currents." "A swarm, Webster says, is (among other things) "a large number of animate or inanimate things massed together and usually in motion." That pretty much describes what happened here in July with the box jellies. But it was interesting to me that not one of the local newspaper accounts I saw used the word swarm. Invasion, infestation and influx were the reporters' words of choice. Why do I care about the word swarm? I'm uneasy with its tone. Maybe I saw too many horror movies when I was young, but to me, a swarm usually means hundreds or thousands of creatures moving with a single, sinister purpose: swarms of angry bees delivering stings; locusts moving over crops and devouring everything in sight; ants or flies swarming over dead bodies. Jellyfish don't get together for that kind of organized carnage. The box jellies we see in Hawaii mostly drift with the currents, alone, trailing four stinging tentacles behind them to catch tiny pieces of food. Hawaii's box jellyfish usually show up in leeward waters 8 to 10 days after a full moon. When the numbers get large, all hell breaks loose, both for people, who get stung by trailing tentacles, and for the jellyfish, who die in droves on the beach. And even though the sting is accidental, the creatures become animal outlaws, reviled and feared." "OK, maybe thousands of jellyfish on one side of an island constitutes a swarm. But they don't mean to be bad. Jellyfish are victims of circumstance, just as we are when we connect with their tentacles." Source and further information: http://www.susanscott.net/OceanWatch1997/sep15-97.html