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Background  Opsins have been found in the majority of animals and their most apparent functions are related to vision and light-guided behaviour. As an increasing number of sequences have become available it has become clear that many opsin-like transcripts are expressed in tissues other than the eyes. Opsins can be divided into three main groups: rhabdomeric opsins (r-opsins), ciliary opsins (c-opsins) and group 4 opsins. In arthropods, the main focus has been on the r-opsins involved in vision. However, with increased sequencing it is becoming clear that arthropods also possess opsins of the c-type, group 4 opsins and the newly discovered arthropsins but the functions of these opsins are unknown in arthropods and data on their localisation is limited or absent.    Results  We identified opsins from the spider  Cupiennius salei  and the onychophoran  Euperipatoides kanangrensis  and characterised the phylogeny and localisation of these transcripts. We recovered all known visual opsins in  C. salei,  and in addition found a peropsin, a c-opsin and an opsin resembling  Daphnia pulex  arthropsin. The peropsin was expressed in all eye types except the anterior median eyes. The arthropsin and the c-opsin were expressed in the central nervous system but not the eyes. In  E. kanangrensis  we found: a c-opsin; an opsin resembling  D. pulex  arthropsins; and an r-opsin with high sequence similarity to previously published onychophoran onychopsins. The  E. kanangrensis  c-opsin and onychopsin were expressed in both the eyes and the brain but the arthropsin only in the brain.    Conclusion  Our novel finding that opsins of both the ciliary and rhabdomeric type are present in the onychophoran and a spider suggests that these two types of opsins were present in the last common ancestor of the Onychophora and Euarthropoda. The expression of the c-opsin in the eye of an onychophoran indicates that c-opsins may originally have been involved in vision in the arthropod clade. The lack of c-opsin expression in the spider retina suggests that the role for c-opsin in vision was lost in the euarthropods. Our discovery of arthropsin in onychophorans and spiders dates the emergence of arthropsin to the common ancestor of Onychophora and Euarthropoda and their expression in the brain suggests a non-visual function.

Characterisation and localisation of the opsin protein repertoire in the brain and retinas of a spider and an onychophoran