Ultrastructure of the retinal synapses in cubozoans.

Cubomedusae (box jellyfish) are well known for strong directional swimming, rapid responses to visual stimuli, and complex lensed eyes comparable to those of more advanced multicellular animals. They possess a total of 24 eyes that are of four morphologically different types, yet little is known about the neural organization of their eyes. The eyes are located on ganglion-like structures called rhopalia. Each of the four rhopalia contains an upper and a lower lensed eye (with a cornea, lens, and retina), two pit ocelli, and two slit ocelli. Transmission electron microscopy was used to examine the synaptic morphology of the eyes and pacemaker region of four species of cubozoans (Tamoya haplonema, Carybdea marsupialis, Tripedalia cystophora, and Chiropsalmus quadrumanus). Invaginated synapses were found in all four species, but only in the upper and lower lensed eyes. Density measurements indicated that the invaginated synapses were located close to the basal region of photoreceptor cells, and size differences of invaginated synapses were observed between the upper and lower lensed eyes, as well as between species. Four additional types of chemical synapses-clear unidirectional, dense-core unidirectional, clear bidirectional, and clear and dense-core bidirectional-were also observed in the rhopalia. The invaginated synapses of the lensed eyes may be useful as markers to help sort out the neural circuitry in the retinal region of these complex cubomedusan eyes.