THE VISUAL SYSTEM OF THE GIANT CLAM TRIDACNA: BEHAVIORAL ADAPTATIONS

¹ Department of Neurobiology, Australian National University, Canberra City, A.C.T. 2601, Australia

Giant clams in the genus Tridacna exhibit a number of visual behaviors adaptated to the hypertrophied siphonal lobes and the symbiotic zooxanthellae. The visual system consists of many eyes located along the margin of the siphon. Shadows without abrupt changes in intensity or movement elicit slow retractions of corresponding siphon regions. A similar response also occurs in isolated tissues and appears to rely on peripheral synaptic interactions. Otherwise, a shadow response, or a sight reaction to distant movements, triggers a synchronous rapid retraction of the entire siphon along with adduction of the valves. These abrupt responses are centrally coordinated and have a startling effect on would-be predators due to the water jetted from the exhalent aperture and the visual effect of movement by the large, brightly colored siphon. Animals become habituated to repetitive shadows but remain sensitive to stimuli of different spatial or temporal characteristics, thus illustrating the existence of complex central integrative mechanisms. A behavioral reflex to a light stimulus is also described whereby the surface of the mantle is directed toward the light. This `orientation response' may represent a phototropic behavior which maximizes the absorption of light by the algal symbionts.