Ciliary and rhabdomeric photoreceptor-cell circuits form a spectral depth gauge in marine zooplankton.

This is the final version.Available from eLife Publications via the DOI in this record.All data generated or analysed during this study are included in the manuscript and supporting files.Source data files have been provided for Figures 1, 3 and 4 and Figure 2-figure supplement 2.Ciliary and rhabdomeric photoreceptor cells represent two main lines of photoreceptor-cell evolution in animals. The two cell types coexist in some animals, however how these cells functionally integrate is unknown. We used connectomics to map synaptic paths between ciliary and rhabdomeric photoreceptors in the planktonic larva of the annelid Platynereis and found that ciliary photoreceptors are presynaptic to the rhabdomeric circuit. The behaviors mediated by the ciliary and rhabdomeric cells also interact hierarchically. The ciliary photoreceptors are UV-sensitive and mediate downward swimming in non-directional UV light, a behavior absent in ciliary-opsin knockout larvae. UV avoidance overrides positive phototaxis mediated by the rhabdomeric eyes such that vertical swimming direction is determined by the ratio of blue/UV light. Since this ratio increases with depth, Platynereis larvae may use it as a depth gauge during vertical migration. Our results revealed a functional integration of ciliary and rhabdomeric photoreceptor cells in a zooplankton larva.The research was supported by a grant from the DFG - Deutsche Forschungsgemeinschaft (Reference no. JE 777/3–1). SY was supported by the National Institutes of Health (R01EY016400) and Emory University. KTR is supported by grants from the University of Vienna (research platform “Rhythms of Life”), the FWF (http://www.fwf.ac.at/en/) research project grant (#P28970), and the European Council under the European Community’s Seventh Framework Programme (FP7/2007-2013) ERC Grant Agreement 337011