A description of the nuchal organ, a possible photoreceptor, in Euprymna scolopes and other cephalopods

A type of apparent photoreceptor, the nuchal organ, is described in the sep iolid squid Euprymna scolopes. These vesicles are identified in a variety o f other cephalopod species. The nuchal organs have specific structural simi larities to the already well-described photosensitive vesicles. Both organs have a distinct vesiculate structure and possess a rind consisting of seve ral types of cells. Certain rind cells from both organs send processes with microvilli into the lumen of the vesicle forming a core area. Within the c ores of both organs numerous microvilli occupy much of the core volume. The se histological similarities provide strong evidence that the nuchal organs , like the photosensitive vesicles, are photoreceptors. Three distinct diff erences also exist between the nuchal organs and photosensitive vesicles: ( 1) the location of the nuchal organs differs from that of all known photose nsitive vesicles in cephalopods; (2) the location of the nuchal organs is h ighly conservative in decapod cephalopods compared to the varying locations of the photosensitive vesicles; (3) the degree of structural order general ly found within photosensitive vesicles is not apparent in the nuchal organ s. The function of the nuchal organ is probably different from previously d escribed photoreceptors in cephalopods.