Reference: Biol. Bull. 170: 393-408. (June, 1986) THE VISUAL SYSTEM OF THE GIANT CLAM TRIDACNA: BEHAVIORAL ADAPTATIONS LON A. WILKENS* Department of Neurobiology, Australian National University, Canberra City, A.C.T. 2601, Australia ABSTRACT 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 re-gions. 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. INTRODUCTION Giant clams of the genus Tridacna are prominent members of tropical Indo-Pacific coral reef communities. Five species with overlapping ranges are widely distributed throughout the region (Rosewater, 1965). As the largest representatives of the bivalve molluscs, they exhibit a number of specialized adaptations and these are directly re-flected by their habitat preference for relatively shallow (maximum depth, 20 m; Hardy and Hardy, 1969), clear, tropical seas. Most importantly, Tridacna hosts the photo-synthetic endosymbiotic zooxanthella Symbiodinium (Freudenthal, 1962), and has come to rely on the algal photosynthates as a source of metabolic carbon (Trench et ai, 1981). In effect, Tridacna farms the zooxanthellae in large numbers in the tissues of its hypertrophied mantle siphon, the lobes of which are deployed past the shell margin and intentionally exposed to the intense solar radiation of the tropical marine environment. In this 'farming' posture, the siphonal lobes are unprotected by the shell and, as observed by Stasek (1965), are subject to predation by reef fish. In his paper, Stasek describes generally the protective behavioral adaptations of Tridacna, including pow-erful spurts of seawater from the siphon apertures which can be directed toward the source of a tactile stimulus. A similar spurting reflex is mediated visually, presumably by the mantle eyes (Stasek, 1966), although a directional component has yet to be Received 22 May 1985; accepted 6 March 1986. * Present address: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121. 393
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