Jellyfish Booms Signal Ecosystems Out of Whack

June 18, 2008 -- 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," explains 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.

Dislodging them will be difficult, marine biologists say.

"Jellyfish have come to occupy the place of many other species," notes Ricardo Aguilar, research director for Oceana, a international conservation organization.

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 paralyze 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.

Two centuries worth of data shows that jellyfish populations naturally swell every 12 years, remain stable four or six years, and then subside.

2008, however, will be the eighth consecutive year that medusae, as they are also known, will be present in massive numbers.

The over-exploitation of ocean resources by man has helped create a near-perfect environment in which these most primitive of ocean creatures can multiply unchecked, scientists say.

"When vertebrates, such as fish, disappear, then invertebrates -- especially jellyfish -- appear," says Aguilar.

The collapse of fish populations boost this process in two important ways, he added. When predators such as tuna, sharks, and turtles vanish, not only do fewer jellyfish get eaten, they have less competition for food.

Jellyfish feed on small fish and zooplankton that get caught up in their dangling tentacles.