These cells have distinct morphologies and express different classes of opsins (light-sensitive proteins). Its discovery should also stimulate new ideas about the evolution of eyes and photoreceptors.īilaterian animals have two major photoreceptor cell-types, the rhabdomeric- and the ciliary-type photoreceptor cells (rPRC and cPRC, respectively) ( Arendt, 2003 Arendt et al., 2004 Eakin, 1979 Erclik et al., 2009). By enabling the larvae to swim at a preferred depth, the depth gauge influences where the worms end up as adults. This suggests that the two work together to form a depth gauge. confirm that neural circuits containing ciliary photoreceptors exchange messages with circuits containing rhabdomeric photoreceptors. Using high-powered microscopy, Verasztó, Gühmann et al. This makes the larvae swim upwards again. Though if the larvae swim too deep, cyan light inhibits the ciliary photoreceptors and activates the rhabdomeric pigmented eyes. Ciliary photoreceptors thus help Platynereis to avoid harmful ultraviolet radiation near the surface. In the ocean, ultraviolet light is most intense near the surface, while cyan light reaches greater depths. By contrast, cyan light makes the larvae swim upwards. Shining ultraviolet light onto Platynereis larvae makes the larvae swim downwards. now show that ultraviolet light activates ciliary photoreceptors, whereas cyan, or blue-green, light inhibits them.

Yet the larval brain also contains ciliary photoreceptors, the role of which was unknown. These enable the larvae to detect and swim towards light sources. The eyes contain rhabdomeric photoreceptors. Each has a transparent brain and six small, pigmented eyes. The larvae of Platynereis are free-swimming plankton. These include the marine ragworm, Platynereis dumerilii. There are also several groups of animals, mostly sea-dwellers, that inherited both types of photoreceptors from their ancestors that lived millions of years ago. Crustaceans and their relatives, including insects, have rhabdomeric photoreceptors while animals with backbones, including humans, have ciliary photoreceptors. All detect light using specialized cells called photoreceptors, of which there are two main kinds: ciliary and rhabdomeric. The animal kingdom contains many different types of eyes, but all share certain features in common. Our results revealed a functional integration of ciliary and rhabdomeric photoreceptor cells in a zooplankton larva. Since this ratio increases with depth, Platynereis larvae may use it as a depth gauge during vertical migration. UV avoidance overrides positive phototaxis mediated by the rhabdomeric eyes such that vertical swimming direction is determined by the ratio of blue/UV light. The ciliary photoreceptors are UV-sensitive and mediate downward swimming in non-directional UV light, a behavior absent in ciliary-opsin knockout larvae.

The behaviors mediated by the ciliary and rhabdomeric cells also interact hierarchically. 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 two cell types coexist in some animals, however how these cells functionally integrate is unknown. Ciliary and rhabdomeric photoreceptor cells represent two main lines of photoreceptor-cell evolution in animals.

Marine depth gauge