Visual cues help jellyfish maneuver
Michael J. Lander
Generally, jellyfish are not known for their ability to dart around swiftly in confined areas. Rather, they are most often observed in open waters, slowly moving in a pulsating fashion. Some box jellyfish that grow up in obstacle-laden environments, however, exhibit unusually fishlike swimming patterns and make sharp turns with ease.
To assess the physiological basis of these phenomena, Anders Garm and colleagues at Lund University in Sweden studied the behavior and eye structure of two such species. They presented their results April 1 in Glasgow, UK, at the Society of Experimental Biology’s annual meeting.
A jellyfish of the species Tripedalia cystophora swims freely. Two of the organism’s four rhopalia — eye-containing stalks — appear as distinct bright dots close to the rim of the bell-shaped body. Courtesy of Anders Garm and Dan-E. Nilsson, Lund University.
Tripedalia cystophora lives amidst densely clustered mangrove roots in the Caribbean. Chiropsella bronzie is native to northern Australian beaches that contain larger potential entanglements, such as rocks and fallen trees. Both jellyfish have 24 eyes, distributed equally among four sensory structures called rhopalia. Morphological analyses revealed that the upper and lower eyes on each rhopalium — with their spherical, fishlike lenses —may yield the highest spatial resolution.
In the lab, the researchers video-recorded the swimming patterns of representative organisms in a flow tank with a 1.5-cm/s laminar flow rate. Dispersed near the end of the tank were colored and transparent barriers of varying width as well as some that were colored above, but clear below, the water’s surface.
As the animals neared the colored markers, they were seen to carry out 120° to 180° turns. Barriers with a higher contrast, as measured with an Ocean Optics spectrophotometer, elicited a turning reflex earlier than less contrasting ones. As expected, C. bronzie came closer to every color of marker before changing direction. All of the jellyfish collided with objects that were not visible underwater.
Although the researchers had hypothesized that the upper eye might collect obstacle-related data from above the surface, their data indicate otherwise. Results on the animals’ color-sensing ability were inconclusive. What became clear, however, was that these jellyfish use intensity contrast to detect, and use vision rather than other sensory modalities to avoid, hazardous objects.
Because the invertebrates also approached narrow obstacles more closely than wide ones, the team concluded that they do not measure distances, but rather the amount of space an obstacle occupies in their visual field.
“We are looking at what kinds of interactions are happening between photoreceptors directly in the retina,” Garm said in reference to one of his team’s current projects. He explained that flow receptors in jellyfish eyes seem to carry out the same kind of processing that occurs in the neurons of higher-level organisms.
With a better understanding of this activity, researchers could design robots with better optical systems and simpler brains.
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