Extraocular Photoreception of a Marine Gastropoda, Onchidium: Three-Dimensional Analysis of the Axons of Dermal Photoreceptor Cells in the Dorsal Mantle Examined with a High-Voltage Electron Microscope

Abstract

Extraocular photoreception exists in a variety of forms among invertebrates [1], and cannot be identified visually from a surface view. Messenger [2] defined extraocular photoreception as, “a response to light that is not mediated by an eye.” Dermal light sense is one type of extraocular photoreception. In molluscs, extraocular photoreceptors are widespread and exhibit a surprising range of complexity. The shadow responses, categorized as dermal sensitivity by Messenger [2], are widespread in both gastropods and bivalves and are mediated by cells or sensory endings that remain unidentified. The marine gastropod Onchidium is thought to have extraocular photoreception based on behavioral responses to light and shadow [1–3]. Onchidium possesses paired stalk eyes (SEs) and dorsal eyes (DEs), demonstrating that both dermal and ocular systems can coexist. Yoshida [1] has described the dominant role the dermal system plays in Onchidium. However, the dermal light sensitivity of Onchidium should be reconsidered, as a photoreceptor cell has been found in the dorsal mantle [3, 4].