Dancing in the dark: the annelid Platynereis dumerilii is re-envisaged for its climactic final night.

When sexually mature, the marine annelid Platynereis dumerilii emerges at night from its tube and swims to the surface in search of a mate. Developing from the immature atoke to the mature epitoke, each of its four eyes doubles in diameter, but it is unclear what visual capacity these eyes have and which visual tasks they support. We assessed this question by comparing the anatomy and physiology of atoke and epitoke eyes. Platynereis dumerilii has four cephalic eyes in anterior and posterior pairs, which all have an adjustable pupil with similar dynamics. As the eyes grow, photoreceptors are added to the retina and the outer segments of the photoreceptors elongate 5-fold in the emergent epitoke. All eyes had a dynamic range of approximately 3 log units covering 0.01 to 10 W sr-1 m-2 and displayed clear photoinhibition at higher intensities. We detected no difference in photoreceptor physiology between posterior and anterior eyes. Their receptors had a spectral sensitivity matching a single opsin with a peak sensitivity around 490 nm being the visual pigment. The only difference we detected was in temporal resolution, where female epitoke vision was significantly faster [flicker fusion frequency (FFF)=9.5, 95% confidence interval (CI)=7.9-11.2] than that of both male epitokes (FFF=6.0, 95% CI=4.8-7.2) and atokes (FFF=7.3, 95% CI=6.2-8.5). Our results show that on developing to the epitoke, both sexes of P. dumerilii improve spatial resolution by enlarging their cup-eyes, while female epitokes also achieve higher temporal resolution. These changes could facilitate mate recognition of male epitokes by female epitokes.