AbstractVisual systems are a major source of information about the world for several animal groups. Ranging from single light detecting photoreceptors, to complex eyes containing photoreceptor arrays, all evolutionarily tuned to meet different environmental and behavioural needs. Platynereis dumerilii has already proven a strong model for insights into photoreceptor evolution as they posses the two types of photoreceptors, rhabdomeric and ciliary, a rarity across animal phyla. Here we aim to track the early development of the eyespot in Platynereis using volume electron microscopy, in particular FIBSEM, to achieve high resolution to identify the distinct morphology of both photoreceptor types. A targeting workflow enabled higher throughput and optimisation of imaging time and data load. Across the first six days of development we observe a shift from a rhabdomeric simple eye, consisting of one photoreceptor, to a more elaborate eyespot with both ciliary and rhadomeric photoreceptors. This structure is unique, even among the multitude of eyes in the animal kingdom. Eyes containing both rhabdomeric and ciliary photoreceptors are rare, only a few have been observed and challenge long held ideas about the distinct evolutionary paths of different photoreceptor types. While the role of the eyespot remains unknown there are clues in the observed morphology. The inclusion of two cells with specific spectral sensitivities, and the considerable surface extensions of each cell indicate a system for fast integration of colour information. Future investigation, including tracing neural pathways and correlation with photoreceptor gene expression will further determine the significance of the eyespot in Platynereis.
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