Anterior sense organs in Sabellaria alveolata (Annelida, Sedentaria, Spionida) with special reference to ultrastructure of photoreceptor elements presumably involved in shadow reflex

Abstract

The reef-building sedentary polychaete, Sabellaria alveolata, is well known for its specialized anterior end, the operculum, which is exposed to the environment during vital activities, such as feeding or as it seals its tube when the animal withdraws. This region represents the most important sensory structure in Sabellariidae. It comprises two lobes, tentacular filaments, protective chaetae (paleae), a median organ, and a median ridge, and bears various pigmented spots. Worms swiftly withdraw into tubes triggered by abrupt light change (shadow reflex). We suspected that the pigmented spots were photoreceptive and responsible for the shadow response. To test this hypothesis, the median organ and median ridge of S. alveolata were investigated by applying light microscopy, confocal laser scanning microscopy, scanning, and transmission electron microscopy. Besides unciliated supportive cells, these organs consist of numerous multiciliated and glandular cells on their ventral surfaces. The multiciliated cells are of two types: epidermal supportive and receptor cells. The median organ is innervated directly from the brain by two longitudinal basiepithelial nerves. The presence of multiciliated cells and mucocytes suggests that the organ is sensory, involved in both feeding and tube-building behavior. Furthermore, we found two pairs of eyespots on the lateral posterior part of the median ridge, situated close to the neurite bundles anterior to the brain. These eyes are simple in structure and resemble larval-type eyes. They are composed of only two cells each, one rhabdomeric photoreceptor cell and one pigmented supportive cell. The pigmented eyes on the median ridge are clearly involved in photoreception and very likely involved in shadow reflex.