I mourn the dearth of science columns by Natalie Angier in the New York Times. Maybe I’m wrong, but they seem to have been much more frequent in past years. At any rate, she had a good one on jellyfish in Tuesday’s Times, “So much more than plasma and poison.” Scientists are starting to discover that jellyfish are neurologically much more complex than previously suspected. Some even think that they have the equivalent of a “brain.”
Angier’s piece led me to a nice paper in a recent Current Biology (reference below) on the box jellyfish (Tripedalia cystophora), which lives in Caribbean mangrove swamps. Like all jellyfish, it’s carnivorous, and depends for food on copepods that live in water beneath the mangrove canopy. These jellyfish are famous for having 24 eyes of four different types: upper and lower lens eyes, pit eyes, and slit eyes. The upper and lower lens eyes have a lens, cornea, and retina, and so resemble vertebrate eyes. The other two eyes can distinguish light and dark. You can see some of the eyes in this photo:
The box jellyfish (from NYT article): photo by Anders Garn and Jan Bieckeki.
The upper lens eyes are unusual, since the eyestalk contains a heavy crystal that keeps the stalk upright at all times, even when the jellfish is sideways or upside down.
Here’s a picture of the upper lens eyes, with captions from the paper:
“In freely swimming medusae, the rhopalia maintain a constant vertical orientation. When the medusa changes its body orientation, the heavy crystal (statolith) in the distal end of the rhopalium causes the rhopalial stalk to bend such that the rhopalium remains vertically oriented. Thus, the upper lens eye (ULE) points straight upward at all times, irrespective of body orientation.” Scale bar is 5 mm.
This always-upright position of the eyestalks, plus the observation that jellyfish displaced away from areas of mangroves swam back to them, led the authors to suggest that these eyes are image-forming, and are used to guide the displaced jellyfish back to the mangroves and their food. In other words, these jellyfish might use terrestrial navigation. This idea was supported by two lines of research. First, the authors did optical modeling of the eyes (combined with simulations from cameras), and concluded that “The approximately 5 m tall mangrove canopy can be readily detected at a distance of 4 m from the lagoon edge and, with some difficulty, can be detected even at a distance of 8 m. . .”
Second, they put jellyfish in floating cylinders full of water and then moved those cylinders away from the mangrove edge. They found that there was no directional preference for swimming if the cylinder was under the mangrove canopy, but when it was displaced away in any compass direction, the jellyfish swam toward the edge of the cylinder nearest the mangroves. (This ability was lost if the cylinder was moved 12 m or farther away.) The authors conclude, “Visual detection of the mangrove canopy by the upper lens eye is the only plausible explanation for the behavioral results. Chemical or mechanical cues cannot have guided the medusae because of the enclosed experimental tank.” They can also rule out celestial cues since they can orient even when the sun is at its zenith.
It seems very likely, then, that this is the first case of a jellyfish being able to navigate using terrestrial landmarks. Ten to one further work is going to find out that jellyfish are far more complex in other ways than we ever suspected.
And here are two other notable jellyfish. First, the deadliest one—the box jellyfish or “sea wasp”, Chrionex fleckeri. It’s been called the most venomous animal on earth, since it supposedly contains enough toxin to kill 60 people. Most people who are stung do survive, but those who don’t can die within four minutes of the sting, faster than any snakebite.
And the world’s largest jellyfish, the lion’s mane jelly (Cyanea capillata) from the Arctic. As Wikipedia notes, “The largest recorded specimen found, washed up on the shore of Massachusetts Bay in 1870, had a bell (body) with a diameter of 7 feet 6 inches (2.29 m) and tentacles 120 feet (37 m) long.”
Garm, A. M. Oskarsson, and D.-E. Nilsson. 2011. Box jellyfish use terrestrial visual cues for navigation. Current Biology 21:798-803.