Phototransduction in fan worm radiolar eyes.

Fan worms (Annelida: Sabellidae) are sessile polychaetes that spend their adult lives in tubes and project their fans, composed of radiolar tentacles, up into the water column for respiration and filter feeding. To protect the fan from predation, many species have evolved unique compound eyes on the radioles that function as shadow or motion detectors, eliciting a rapid withdrawal response in reaction to encroaching objects in the water column [1,2]. The structure of the eyes, their complexity, and their arrangements on the radioles are very diverse among sabellid genera [3] and they display many characteristics atypical of polychaete eyes, such as ciliary photoreceptors [3,4] that hyperpolarize in response to illumination [5]. Here we examine the retinal transcriptome of the radiolar eyes from the fan worm Megalomma interrupta. We find that the opsin, the protein component of light sensitive visual pigments, and other phototransduction cascade signaling proteins expressed in these eyes are related to those commonly associated with vertebrate ciliary photoreceptors, as opposed to the rhabdomeric receptors found in the primary eyes of many invertebrates. With previous anatomical and physiological observations, these results suggest that the radiolar eyes arose independently in fan worms.