Ultraviolet sensitivity in hyperpolarizing photoreceptors of the giant clam Tridacna

Abstract

The giant clam Tridacna is a common shallow-water inhabitant of Indo-Pacific coral reefs¹. These animals contain large populations of photosynthetic zooxanthellae in hypertrophied siphonal lobes which extend well beyond the edge of the shell for maximum exposure to light^2–4 (Fig. 1). Protection for these large fleshy lobes is visually mediated and consists of a partial siphon retraction and valve adduction, a behaviour which gives rise to a forceful, startling jet of seawater⁵. Numerous eyes (up to several thousand in large animals⁶) located along the margin of the siphon comprise a visual system which presumably mediates the defensive withdrawal reflex. Here we present the first report on the physiological properties of the visual system in Tridacna. Intracellular electrophysiological recordings have revealed two distinct types of retinal cells: both are hyperpolarized by light but only one generates axonal spikes. We also describe cells with differential colour sensitivity, including receptors that are sensitive to ultraviolet (UV) light. Ultraviolet sensitivity, although not previously reported in molluscs⁷, may be an adaptation in Tridacna to the environmental requirements of a host for algal symbionts.