Bivalve molluscs are known for shadow responses involving closure or retraction of the siphon and valve adduction. In representative genera [Spisula solidissima (Dillwyn, 1918), Mercenaria mercenaria (Linnaeus, 1758), Lima scabra (Born, 1778)], the pallial nerves contain photosensitive fibers that exhibit physiological shadow responses. These photoreceptors are inhibited by light but trigger an excitatory burst of spikes to a shadow, the off-response. Equivalent responses are seen in bivalve eyes, e.g., in optic fibers from the siphon tentacle eyes of Cardium edule (Linnaeus, 1758) and the mantle eyes of scallops (Pectinidae) and file clams (Limidae). In scallops, they form a distal retinal layer of ciliary receptors, distinct from a proximal microvillar layer that is excited by light. In off-receptors (ciliary), light inhibition is the result of a hyperpolarizing receptor potential with spikes generated on the rebound depolarization at dimming. In contrast, the proximal on-receptors are excited by light with spikes generated by depolarizing receptor potentials. The inhibitory effect is first-order, i.e., a direct response to light, as is excitation for the proximal receptors. With separate retinas and the absence of synaptic contact, these are the primary receptors. Aside from Pecten Müller, 1776 and Lima Bruguiére, 1797, the only other bivalve eyes in which receptor potentials have been investigated are those of the giant clam Tridacna maxima (Röding, 1798). Here there are two types of hyperpolarizing, light-inhibited primary receptors, one of which generates spikes at light offset, the other non-spiking. The inhibitory response is universal in bivalve photoreception, unique among the eyes of invertebrates, but similar in polarity to chordate photoreceptors although the ionic mechanisms are different. Receptor physiology is discussed relative to image formation in bivalve eyes.
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Vol. 26 • No. 1/2