A Jellyfish’s Blurry View

Some types of jellyfish have great eyes that can't focus.

When you see a jellyfish, you might think, “looks like a blob.” But what would a jellyfish see if it could look at us? For certain types of jellyfish, the view would be decidedly fuzzy.

Researchers in Sweden have found that some of the eyes of a box jellyfish have very good lenses, but they don’t focus sharply. So, what the jellyfish sees must be blurry, the researchers conclude.

This photo of one type of box jellyfish shows the boxy shape typical of this group of creatures.

B. Hamner

Box jellyfish are named after their shape. From above, they look squarish and boxy. They’re excellent swimmers. And they have lots of eyes—typically four sets of six eyes, for a total of 24 eyes.

Two eyes in each set (a total of eight eyes) are different from the other eyes. They’re similar to the cameralike eyes of octopuses and vertebrates such as people, with a lens, iris, and retina. The remaining eyes are simpler.

Researchers analyzed a box jellyfish’s two cameralike eyes (arrows), which sit on a movable stalk (left). The iris of the lower of the two eyes contracts in bright light (right top) and opens in darkness (right bottom).

Dan E. Nilsson, Lund University

The researchers studied these special eyes in box jellyfish that live in the water around the roots of mangrove trees in Puerto Rico. The scientists took the eyes apart and tracked how light travels through their lenses.

Calculations showed that the lenses would bring light to a sharp focus. But the light-detecting retina in a jellyfish’s eye isn’t located where the focus would be. So, the animal’s view must be blurred.

A blurry view, it turns out, is good enough for the creatures to see large objects such as mangrove roots. In the environments where they live, box jellyfish don’t actually need to see any better than that. Jellyfish eyeglasses aren’t necessary.

Scientists still don’t know exactly what the other 16 eyes do. They’ve done enough work, though, to realize that box jellyfish see in a way that’s very different from the way we do. A variety of eyes may do for them what our own two eyes (plus brain) do to process images for us.—E. Sohn

Going Deeper:

Milius, Susan. 2005. Built for blurs: Jellyfish have great eyes that can’t focus. Science News 167(May 14):307-308. Available at http://www.sciencenews.org/articles/20050514/fob2.asp .

You can learn more about vision in jellyfish at www.biol.lu.se/funkmorf/vision/dan/jelly.html (Lund University, Sweden) and www.reef.crc.org.au/publications/explore/feat47.html (CRC Reef Research Centre, Australia).

Information about jellyfish is available at www.dnr.state.sc.us/marine/pub/seascience/jellyfi.html (South Carolina Department of Natural Resources).

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