Brainless, made mostly of water and equipped with a nasty sting, they are set to take over the world’s oceans and beachside resorts. Jellyfish populations have exploded dramatically in the last few years, thanks to overfishing and climate change, leading experts to caution that it could signal irreversible changes to marine ecosystems.
"Jellyfish are an excellent bellwether for the environment," explains Jacqueline Goy, of the Oceanographic Institute of Paris. "The more jellyfish, the stronger the signal that something has changed."
Data collected from over two centuries indicate that jellyfish populations expand over 12-year cycles, remaining stable for four to six years, and then decline. However, 2008 marks the eight consecutive year that jellyfish numbers continue to increase, thanks to warmer waters and the anthropogenic elimination of marine predators (such as turtles, sharks and tuna) through overfishing. "When vertebrates, such as fish, disappear, then invertebrates - especially jellyfish - appear," says Ricardo Aguilar, research director for the international conservation organization Oceana.
With fewer predators, there is also less competition for the jellyfish to feed on small fish and zooplankton.
"Jellyfish both compete with fish for plankton food, and predate directly on fish," says Andrew Brierley of the University of St Andrews, Scotland. "It is hard, therefore, to see a way back for fish once jellyfish have become established, even if commercial fishing is reduced."