By Emily Tripp
While jellyfish seem like the simplest creatures, box jellies actually have a very complex visual system. They have 24 eyes of four different kinds. A new study recently published in Current Biology, has evidence that four of those eyes are always looking up out of the water. It is these eyes that allows box jellies to navigate through the mangrove swamps in which they live.
“It is a surprise that a jellyfish — an animal normally considered to be lacking both brain and advanced behavior — is able to perform visually guided navigation, which is not a trivial behavioral task,” said Anders Garm of the University of Copenhagen. “This shows that the behavioral abilities of simple animals, like jellyfish, may be underestimated.”
The fact that box jellyfish have a unique set of eyes is not new information. Scientists have known for more than a century that they rely on vision to respond to light, avoid obstacles, and control their swimming speed. The species that Garm’s team studied, Tripedalia cystophora, lives among the prop roots of Caribbean mangrove swamps, where they stay close to the surface to catch their food. They are never found out in the open where they might risk starvation. This limits them to a small area, less than two meters wide.
The researchers examined the function of one of two types of “upper lens eyes,” already known to form images, to determine just what those eyes can see. Those four eyes cover the visual field needed to see through the water’s surface. The researchers calculated that the jellies should be able to detect the mangrove canopy from a distance of at least eight meters. Behavioral experiments on the jellies in the field have supported these conclusions, proving that the box jellyfish can use those eyes to navigate based entirely on the view of the mangrove canopy. When the canopy was obscured from their field of view, they could no longer get around.
“We have shown that the box jellyfish can use vision to navigate in their habitat, and we now want to understand how their simple nervous system supports such advanced behaviors,” Garm said.
They will continue to study these jellies to detrmine if other box jellyfish species do the same thing in thier habitats.
“Instead of having a single pair of general-purpose eyes like most other animals, box jellyfish have several different types of eyes used for special purposes,” said Garm. “This means that each individual eye type is dedicated to support only a limited number of behaviors. The eyes can then be built to collect precisely the information needed, minimizing the need for further processing in a big brain. The automatic orientation of the upper lens eyes to constantly look through the water surface is a clear example of this.”
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC