A Go-type opsin mediates the shadow reflex in the annelid <italic>Platynereis dumerilii</italic>.

Background: The presence of photoreceptive molecules outside the eye is widespread among animals, yet their functions in the periphery are less well understood. Marine organisms, such as annelid worms, exhibit a 'shadow reflex', a defensive withdrawal behaviour triggered by a decrease in illumination. Herein, we examine the cellular and molecular underpinnings of this response, identifying a role for a photoreceptor molecule of the Go-opsin class in the shadow response of the marine bristle worm <italic>Platynereis dumerilii.</italic> Results: We found <italic>Pdu-Go-opsin1</italic> expression in single specialised cells located in adult <italic>Platynereis</italic> head and trunk appendages, known as cirri. Using gene knock-out technology and ablation approaches, we show that the presence of <italic>Go-opsin1</italic> and the cirri is necessary for the shadow reflex. Consistently, quantification of the shadow reflex reveals a chromatic dependence upon light of approximately 500 nm in wavelength, matching the photoexcitation characteristics of the <italic>Platynereis</italic> Go-opsin1. However, the loss of <italic>Go-opsin1</italic> does not abolish the shadow reflex completely, suggesting the existence of a compensatory mechanism, possibly acting through a ciliary-type opsin, <italic>Pdu-</italic>c-opsin2, with a Lambdamax of approximately 490 nm. Conclusions: We show that a <italic>Go-opsin</italic> is necessary for the shadow reflex in a marine annelid, describing a functional example for a peripherally expressed photoreceptor, and suggesting that, in different species, distinct opsins contribute to varying degrees to the shadow reflex. [ABSTRACT FROM AUTHOR]