Fig. 4

Examples of apposition or similar types of compound eyes found in modern crustaceans. a The sessile, apposition compound eye of the beach amphipod Talorchestia longicornis. The eye is fixed to the cuticle of the animal, and each corneal facet seen in the photograph sends light to a separate photoreceptive rhabdom (See Fig. 3a for schematic). b A zoeal larval stage of the mud crab Rhithropanopeus harrisii, showing its small and spherical apposition eye. Many crustacean larvae have eyes similar to this. c The double, apposition eyes of the marine hyperiid amphipod Phronima sedentaria. Each eye has two retinas, visible as dark, roughly bean-shaped patches of pigment on each side of the lower part of the head. Photoreceptors in the upper retinas are fed by long fiber optics that lead from corneal facets on the upper, curved surfaces of the animal’s head. These retinas sample only a patch of light overhead, probably searching for objects forming shadows or silhouettes against the dim, down-welling light. The lateral retinas sample space in the remaining parts of the visual field, probably searching for bioluminescent objects. Photograph courtesy of T. Frank, Harbor Branch Oceanographic Institution. d–f The stalked apposition eyes of the fiddler crab Uca tangeri, showing how the eyes can be extended above the animal’s body for a periscopic and panoramic view. An overall view of a female of this species is illustrated in d (photograph by F. Fiol), while panels e and f show scanning electron micrographs of the ommatidial array and the arrangement of the corneal facets, respectively (photographs by J. Jordão)