How jellyfish avoid bumping into things

Box jellyfish might have 24 eyes but new research says they only use a few of them to avoid collisions.

Researchers say an Australian box jellyfish Chiropsella bronzie uses these eyes to help navigate around the seabed by detecting contrasts in light intensity.

Dr Anders Garm, from Lund University in Sweden, compared the jellyfish with another box jellyfish, Tripedalia cystophora, a native of Caribbean waters.

The research, presented at the Society of Experimental Biology's conference in Glasgow at the weekend, found that T. cystophora is better at avoiding obstacles than its Australian relative, a good thing given the Puerto Rican's mangrove habitat has more snags.

Each jellyfish has two sets of camera-type eyes with fish-like lenses, called the upper and lower lens eyes.

The researchers say the lower lenses help them avoid obstacles, after observing the animals didn't respond to objects above the water's surface.

They say the lower lenses can pick up changes in light intensity, and the greater this so-called intensity contrast, the better the jellyfish is at avoiding objects.

"Contrast is important because without contrast the object cannot be detected by any eye," Garm says.

"Obstacle avoidance is governed by intensity contrast which fits with our other data which strongly suggest that the jellyfish are, in fact, colour blind."

C. bronzie is the less-harmful relative of the deadly Australian box jellyfish Chironex fleckeri.

Sunset go-slow

Dr Jamie Seymour, director of the Tropical Australian Stinger Research Unit at James Cook University in Queensland, says when the Sun goes down the box jellyfish stop operating.

"For T. cystophora once the Sun disappears or the light shaft disappears the animal sinks to the bottom, for C. bronzie we're not too sure and for C. fleckeri the data suggests that they shut down and go catatonic; we suggest that they go to sleep."

But Seymour is most excited about the clues that the eyes of jellyfish, which have no brain and only a basic nervous system, could provide into the workings of more complex eyes.

"The simpler system the easier it is to understand, once you've got an understanding then you relay that on up the line," he says.http://www.abc.net.au/science/articles/2007/04/02/1883903.htm

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