It gets worse still!!! We used to thing they drifted around aimlessly at the mercy of the tides, floating where they were driven on the currents; this not so:
"...if comes under sustained attack it responds by sending its compatriots into a super-breeding frenzy in which millions of replacements are created." ... "The box jellyfish is so packed with venom that the briefest of touches can bring agonising death within 180 seconds."
Just in case you're wondering about my first comment, well jellyfish, at least the ones they've tried to keep out of swimming areas, have a "highly complex sensory capacity and the ability to distinguish colour" - they appear to be very sensitive to colour and the only deterrent found so far, according to reports (see first link) is the colour red. Obviously the really deadly creatures are far off from many of us - both the Irukandji and Box jellyfish (in fact both are Cubozoa, not jellyfish) are in the South Pacific but they're on the move - and don't think the occurance of incidents aren't getting more common in cooler water nearer to home; reports like this one are positively scary and of course it isn't the first time and although those links refer to fish farms the danger to swimmers, even in the UK is clear!
"On the other side of the world scientists who managed to place tracking devices on the jellyfish in the Pacific proved that they were not drifting on the ocean currents but heading determinedly - and at the speed of an Olympic swimmer - towards the coast."