Thousands of holidaymakers in the Mediterranean have been stung by jellyfish as huge swarms of the creatures invade coastal waters.
The chances of encountering a jellyfish are now much higher
Some Spanish beaches have been closed, but Sicily and North Africa are also reported to be badly affected.
Researchers say at least 30,000 people have been stung since summer began.
Marine biologists blame hot dry weather for bringing jellyfish closer to the shore, and say overfishing may be increasing jellyfish numbers.
A recent survey by the Oceana environmental group found concentrations of jellyfish of more than 10 per square metre in some areas off the Spanish coast.
Francesc Peters of the Institute of Marine Science in Barcelona told the BBC World Service's Europe Today programme that coastal waters were warmer than usual, because of the hot weather, and saltier than usual because of low river flows.
He said this meant the offshore waters which jellyfish usually inhabit were being washed closer to the coast.
He added that global warming could mean that these conditions occur more frequently.
"Probably because of overfishing, populations of jellyfish offshore will increase and then these special environmental conditions... higher temperatures and higher salinity near the coast, may bring these swarms of jellyfish close to the beach," he said.
The jellyfish is at least 95% water
Overfishing meant that the jellyfish's predators and its competitors were being removed from the sea, he said.
Jellyfish are themselves voracious eaters, and experts say that because they consume fish at a very high rate it may be hard for the fish they replace to re-establish themselves.
Jellyfish numbers have grown as commercially important fish such as herring and sardines have reduced.
At the same time, populations of jellyfish predators such as tuna and turtles are diminishing.
Mr Francesc said Mediterranean jellyfish, which are at least 95% water, do not have lethal or powerful stings.
However, he added that some people could suffer a powerful allergic reaction.
He advised people who have been stung to wash the wound with salty water, and to cool it with ice inside a plastic bag.