Electrophysiological Evidence of Photoreceptors in the Epistellar Body of Eledone moschata

Abstract

IN 1936 Young¹ described a structure in several genera of octopods (cephalopods) appearing on the ventral surface of both stellate ganglia (near the base of the stellar nerve most distal to the mantle connective) which he termed the “epistellar body”. From histological considerations he hypothesized that this body might have a neurosecretory function. Nishioka et al.² reinvestigated this structure in Octopus in more detail and, by electron microscopy, made the significant observation that the epistellar body contains cells the surfaces of which display microvilli suggesting rhabdomeric structures as seen typically in molluscan and arthropod visual photoreceptors. Later Nishioka et al.³ extended their investigations to include Eledone moschata and presented not only further electron micrograph data on the rhabdomeric ultrastructure but also biochemical data indicating the presence of a rhodopsin photopigment. Thus these investigators argued that the neurone-like cells of the epistellar body could be photo-receptors. We present here electrophysiological evidence clearly indicating the presence of photoreceptor cells in the epistellar body. There is already electrophysiological evidence for extracellular photoreceptors in arthropods^4–7, molluscs^8,9 and for photoreceptors in the diencephalon of the frog^10,11.