Bilateral symmetric organization of neural elements in the visual system of a coelenterate, Tripedalia cystophora (Cubozoa)

Abstract

Cubozoans differ from other cnidarians by their body architecture and nervous system structure. In the medusa stage they possess the most advanced visual system within the phylum, located in sophisticated sensory structures, rhopalia. The rhopalium is a club-shaped structure with paired pit-shaped pigment cup eyes, paired slit-shaped pigment cup eyes, and two complex camera-type eyes: one small upper lens eye and one large lower lens eye. The medusa carries four rhopalia and visual processing and locomotor rhythm generation takes place in the rhopalia. We show here a bilaterally symmetric organization of neurons, with commissures connecting the two sides, in the rhopalium of the cubozoan Tripedalia cystophora. The fortuitous observation that a subset of neurons is strongly immunoreactive for a PCNA (proliferating cell nuclear antigen)-like epitope allowed us to analyze the organization of these neurons in detail. Distinct PCNA-immunoreactive (PCNA-ir) nuclei form six bilateral pairs that are associated with the slit eyes, pit eyes, upper lens eye, and the posterior wall of the rhopalium. Three commissures connect the clusters of the two sides and all clusters in the rhopalium have connections to the area around the base of the stalk. This neuronal system provides an anatomical substrate for integration of visual signals from the different eyes.