Meeting Abstract
Fan worms (Sabellida) possess some of the strangest eyes in nature. Their eponymous fans are composed of two sets of radiolar tentacles that project 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 express a surprising diversity of eyelets of varying levels of sophistication, ranging from scattered single ocelli to compound eyes with up to 1,000 facets. These photoreceptors are probably a relatively recent evolutionary development to cope with a sessile, tube-dwelling lifestyle, and the ancestral cerebral eyes (haplessly positioned within the tube) have degenerated to minute pigment cups with scant visual potential. The branchial radiolar ocelli on the other hand, appear to function as visual burglar alarms for detecting the silhouettes of looming predators and eliciting a startle response for the worm to rapidly retreat within its fortified tube. Despite sometimes resembling arthropod compound eyes, the branchial photoreceptors have ciliary membrane elaborations and hyperpolarize in response to light. Considering the unusual and apparently recently-evolved nature of the fan worm branchial photoreceptors, these animals are an excellent case for examining the emergence of novel visual systems and the development of rudimentary visually guided behaviors. Here we present molecular, anatomical and neurobiological investigations aimed at discerning the origins of branchial eyes in this group. We find a fascinating case of evolutionary pragmatism; a novel visual circuit likely emerging from preexisting components not previously implicated in visual systems.
