Here, There and Everywhere: The Radiolar Eyes of Fan Worms (Annelida, Sabellidae)

Research output: Contribution to journalArticle

Abstract

Fan worms (Annelida: Sabellidae) possess some of the strangest eyes in nature. Their eponymous fans are composed of two sets of radiolar tentacles that project from the head up out of the worm's protective tube into the water column. Primarily used for respiration and feeding, these radioles are also often involved in photoreception. They display a surprising diversity of eyes of varying levels of sophistication, ranging from scattered single ocelli to compound eyes with up to hundreds of facets. These photoreceptors could represent a relatively recent evolutionary development to cope with a sessile, tube-dwelling lifestyle, and the primary cerebral eyes (haplessly positioned within the tube most of the time) amount to little more than minute pigment cups with scant visual potential. The radiolar eyes on the other hand, appear to function as visual burglar alarms for detecting looming predators and eliciting a startle response for the worm to rapidly retreat within its fortified tube. Despite sometimes resembling arthropod compound eyes, the radiolar photoreceptors have many canonically vertebrate-like physiological characteristics. Considering the unusual and apparently recently evolved nature of the fan worm radiolar photoreceptors, these animals are an excellent case for examining the emergence of novel visual systems, the development of rudimentary visually guided behaviors, and the function of distributed sensory systems. Here, we review over 100 years of investigations into the anatomical diversity of sabellid radiolar photoreceptors and eyes in an evolutionary and functional context. We provide new information on radiolar eye structure in several species of fan worms, and we attempt to organize the various eye types and ocellar structures into meaningful hierarchies. We discuss the developmental, evolutionary, and functional significance of the radiolar eyes and highlight areas of future interest in deciphering their unique nature.

Details

Authors
Organisations
External organisations
  • Norwegian University of Science and Technology
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Zoology
Original languageEnglish
Pages (from-to)784-795
Number of pages12
JournalIntegrative and Comparative Biology
Volume56
Issue number5
Publication statusPublished - 2016 Nov 1
Publication categoryResearch
Peer-reviewedYes