Expression of Opsins of the Box Jellyfish Tripedalia cystophora Reveals the First Photopigment in Cnidarian Ocelli and Supports the Presence of Photoisomerases

Cubomedusae, or box jellyfish, have a complex visual system comprising 24 eyes of four types. Like other cnidarians, their photoreceptor cells are ciliary in morphology, and a range of different techniques together show that at least two of the eye types-the image-forming upper and lower lens eyes-express opsin as the photopigment. The photoreceptors of these two eye types express the same opsin (Tc LEO), which belongs to the cnidarian-specific clade cnidops. Interestingly, molecular work has found a high number of opsin genes in box jellyfish, especially in the Caribbean species Tripedalia cystophora, most of which are of unknown function. In the current study, we raised antibodies against three out of five opsins identified from transcriptomic data from T. cystophora and used them to map the expression patterns. These expression patterns suggest one opsin as the photopigment in the slit eyes and another as a putative photoisomerase found in photoreceptors of all four eyes types. The last antibody stained nerve-like cells in the tentacles, in connection with nematocytes, and the radial nerve, in connection with the gonads. This is the first time photopigment expression has been localized to the outer segments of the photoreceptors in a cnidarian ocellus (simple eye). The potential presence of a photoisomerase could be another interesting convergence between box jellyfish and vertebrate photoreceptors, but it awaits final experimental proof.

Automated summary

Cubomedusae, or box jellyfish, have a complex visual system with 24 eyes of four types, at least two of which express opsin as the photopigment. Molecular work has found a high number of opsin genes in box jellyfish, with unknown functions. This study identified three opsins from transcriptomic data of Tripedalia cystophora and used antibody staining to determine their expression patterns. The results indicate that one opsin acts as the photopigment in slit eyes and another may be a photoisomerase present in all four eye types. Additionally, an antibody stained nerve-like cells in the tentacles and the radial nerve, suggesting potential connections with nematocytes and gonads. This is the first time photopigment expression has been localized to the outer segments of photoreceptors in a cnidarian ocellus. The potential presence of a photoisomerase presents an interesting convergence between box jellyfish and vertebrate photoreceptors, but requires further experimental proof.