Box jellyfish may lack a brain, but they still make use of two dozen eyes. Now it seems that some of these eyes serve a surprising purpose: helping the jellyfish to navigate using landmarks above the water.
Box jellyfish, Tripedalia cystophora, typically live in tropical mangrove swamps, where their crustacean prey abound. We know that their impressive suite of eyes help the animals steer clear of objects as they swim â€“ but Dan-Eric Nilsson, a biologist at Lund University in Sweden, and colleagues have just discovered that a heavy gypsum crystal embedded in the structures surrounding the eyes keeps some of them pointing directly upwards, no matter how the animal is oriented.
To try to find out why the animals constantly look up, Nilsson and his team placed box jellyfish in a clear, open-top tank, lowered it into a mangrove swamp in Puerto Rico, and monitored the animals with a video camera. When the tank was a few metres from the canopy edge, and the canopy could still be seen directly overhead, the jellies repeatedly bumped into the wall closest to the trees.
But when the tank was moved 12 metres away from the canopy edge â€“ and the trees were no longer visible from below the water’s surface â€“ the animals swam in all directions. Because the tank kept out chemicals, and jellies can’t discern much under the murky water, Nilsson says they navigate using the trees as landmarks.
“This is the first time terrestrial cues have been demonstrated to be used for navigation by jellyfish or any other invertebrate,” says Nilsson. Staying under the shelter of the canopy will be helpful to the jelly because the crustaceans they eat are found primarily in those shallow waters.
By taking pictures near the surface of the water and modelling what the jellyfish would be able to see using their upper eyes, the researchers confirmed that the animals could detect the trees up to 8 metres from the canopy edge.
The discovery of this advanced visual ability in an animal with a primitive nervous system may surprise some, says Edward Buskey, a marine scientist at the University of Texas at Austin. “We have an under-appreciation for how sensory systems in simple organisms are used for fairly sophisticated adaptations.”