MARTIN, V.J.; Appalachian State University, North Carolina: Eye Designs in Basal Metazoans
Cnidarians exhibit a diversity of eye designs: simple eyespots, pigment cups, complex pigment cups with lenses, and camera-type eyes with a cornea, lens, and retina of ciliated photoreceptors. The complex eyes of cubomedusae are optically competent and represent the most highly evolved eyes in the phylum. In cubomedusae the opening of the eye cup constricted and a spherical lens formed in the center of curvature of the retina, forming a camera-type eye. Such eyes achieve virtually aberration-free imaging over a full 180o visual field. The photobiology of these jellyfish is similar to that of higher metazoans. Cubozoan photoreceptors are bipolar and resemble vertebrate rod cells. They have a cilium and associated structures, stacks of photosensitive membranes rich in opsin-like and rhodopsin-like proteins, and a cytoplasm filled with mitochondria and microtubules. These photoreceptors project into a vitreous space behind the lens of the eye. The lens, composed of crystallin proteins, is a graded index lens in which the refractive index gradually changes across the lens causing light to bend continuously toward the area of higher refractive index. Below the photoreceptors of the eye are second order neurons producing glutamate, serotonin, and GABA (known visual neurotransmitters in vertebrates and other invertebrates). Ocular nerves connect the eyes to the subumbrellar nerve ring, the central nervous system of the jellyfish. Cnidarian photoreceptors react to changes in light intensity with graded potentials that are proportional to the degree of these changes. The retinal photoreceptors of cubomedusae are not inverted as in the vertebrate retina. Pax genes are expressed in cnidarians and may control many developmental pathways, including eye development. Development of the camera-type eyes will be discussed.
