Jellyfish: The New Shark and Tuna of the Sea

Previously considered the ultimate marine survivors, and garnered the infamous designation of being the "cockroaches of the open waters," jellyfish are doling out the comeuppance to those who ever doubted their predatory skills. These aren't just gelatinous scavengers capable of living in overheated, acidic, and polluted seas - no, jellyfish are the new sharks.

The silent and slow-swimming gelatinous animals can punch just as much of an impact on the food web, and are taking over the top predator position in the ocean.

Even though its attack is not based on a visually coordinated strike, like that of a shark or tuna for example, jellyfish can eat their weight in crustacean prey making them predatory competitors with larger fish.

"In spite of their primitive life-style, jellyfishes exhibit similar instantaneous prey clearance and respiration rates as their fish competitors and similar potential for growth and reproduction," writes biological oceanographer José Luis Acuña of Spain and colleagues in this week's journal Science.

Indeed, in areas where overfishing and habitat destruction have depleted the large fish population, jellyfish have succeeded in taking over as top predators in the food web, the oceanographers report.

And the larger a jellyfish the more prey it can catch. Though a larger medusae, the top bell-shaped part of a jellyfish, slows the animal's swimming capabilities -- slow and large works just as well for running into unwitting prey. "Although larger bodies are less efficient for swimming, optimization analysis reveals that large collectors are advantageous if they move through the water sufficiently slowly," the authors write.

That means even the smaller ones can take over a top predator position that's vacant. "Most economically relevant fishes and trophically dominant jellyfishes are cruising predators, which hunt while swimming. Fishes have compact bodies and use their eyes to detect prey. In contrast, swimming medusae pulse their bells to create vortices that serve as a feeding current and transport prey to their tentacles and oral arms," explains the biologists.

The authors stress that part of the reason jellyfish weren't previously recognized for their predatory skills was because comparisons focused on size rather than carbon weight. If a jellyfish and a fish of equal size each entered a speed-eating contest the fish would always win. But jellyfish are made mostly of water. Judging based on body-size misses the big picture: "The competitive ability of a predator depends not only on prey capture and ingestion rates but on how efficiently the energy obtained translates into body growth and population buildup," they said.

To asses the "bioenergetic performance" of fishes, jellyfish, and their prey, the researchers based their comparisons on body carbon count. Their analysis showed that carbon-kilogram-to-carbon-kilogram jellyfish and fish come out even.

Read more at Discovery News