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DONALD KENNEDY Neural Photoreception in a Lamellibranch Mollusc

The pallial nerves of Spisula solidissima each contain a single afferent nerve fiber which responds directly to illumination of the nerve, and apparently mediates the "shadow" response of siphon retraction. These units show constant-frequency spontaneous activity in the dark; illumination abruptly inhibits this discharge, and cessation of the light stimulus then evokes a prolonged burst of impulses at high frequency (the off-response). Impulses are initiated at a point near the visceral ganglion, and propagated unidirectionally toward it. Stimulation with monochromatic light has revealed that more than one photoreceptor pigment is involved, since the discharge patterns evoked are wavelength-specific. Inhibition is relatively prominent at short wavelengths, excitation at long wavelengths. Following selective adaptation with blue light, "on" responses can be produced with red stimuli, demonstrating the unmasking of an excitatory event which takes place during illumination. The two photoreceptor pigments may be segregated in two or more cells presynaptic to the recorded unit, or,--more likely--may both be contained in the same cell. The spectral sensitivity function for inhibition shows a single maximum at 540 in#, and is probably dependent upon a carotenoid pigment. No photoreceptor function has been demonstrated for a hemoprotein, apparently identical with cytochrome h, which occurs in high concentration in Spisula nerve. I N T R O D U C T I O N It has been known since the early experiments of Nagel (1894) that the siphons of lamellibranch molluscs contain photoreceptors which mediate withdrawal of that organ upon illumination or darkening. In fact, this withdrawal response was employed by Hecht (1920, 1927) in the classic researches on Mya and Pholas which produced the first photochemical constants for any visual system. Despite this distinguished experimental history, virtually nothing is known about the electrophysiology of the receptor units involved. This work was supported in part by a grant (NSF G 4049) from the National Science Foundation. Received for publication, March 31, 1960.