PARIS  - The dramatic proliferation of jellyfish in oceans around the world, driven by overfishing and climate change, is a sure sign of ecosystems out of kilter, warn experts. "Jellyfish are an excellent bellwether for the environment," said Jacqueline Goy, of the Oceanographic Institute of Paris. "The more jellyfish, the stronger the signal that something has changed." Brainless creatures composed almost entirely of water, the primitive animals have quietly filled a vacuum created by the voracious human appetite for fish. And marine biologists say dislodging them will be difficult. "Jellyfish have come to occupy the place of many other species," said Ricardo Aguilar, research director for Oceana, a international conservation organisation. Nowhere is the sting of these poorly understood invertebrates felt more sharply than the Mediterranean basin, where their exploding numbers have devastated native marine species and threaten seaside tourism. And while much about the lampshade-like creatures remains unknown, scientists are in agreement: Pelagia noctiluca - whose tentacles can paralyse prey and cause burning rashes in humans - will once again besiege Mediterranean coastal waters this summer. That, in itself, is not unusual. It is the frequency and persistence of these appearances that worry scientists.