Swim pacemakers in box jellyfish are modulated by the visual input

Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained unknown. Here we show that box-jellyfish lenses contain a finely tuned refractive index gradient producing nearly aberration-free imaging. This demonstrates that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. However, the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings indicate that tailoring of complex receptive fields might have been one of the original driving forces in the evolution of animal lenses.

Like other hydrozoan medusae, Aglantha lacks a brain, but the two marginal nerve rings function together as a central nervous system. Twelve neuronal and two excitable epithelial conduction systems are described and their interactions summarized. Aglantha differs from most medusae in having giant axons. It can swim and contract its tentacles in two distinct ways (escape and slow). Escape responses are mediated primarily by giant axons but conventional interneurons are also involved in transmission of information within the nerve rings during one form of escape behavior. Surprisingly, giant axons provide the motor pathway to the swim muscles in both escape and slow swimming. This is possible because these axons can conduct calcium spikes as well as sodium spikes and do so on an either/or basis without overlap. The synaptic and ionic bases for these responses are reviewed. During feeding, the manubrium performs highly accurate flexions to points at the margin. At the same time, the oral lips flare open. The directional flexions are conducted by FMRFamide immunoreactive nerves, the lip flaring by an excitable epithelium lining the radial canals. Inhibition of swimming during feeding is due to impulses propagated centrifugally in the same epithelium. Aglantha probably evolved from an ancestor possessing a relatively simple wiring plan, as seen in other hydromedusae. Acquisition of giant axons resulted in considerable modification of this basic plan, and required novel solutions to the problems of integrating escape with non-escape circuitry.