Box jellyfish have a complex visual system comprising 24 eyes of four morphological types. The visual functions of the two large image-forming lens eyes are well-understood; however, far less is known about the two lateral pairs of smaller eyes, the pit eyes (PE) and slit eyes (SE). Here, we raised antibodies against five opsins identified from Tripedalia cystophora transcriptomic data for immunohistochemical studies. One opsin was expressed in the outer segments of the PE photoreceptors, suggesting this as the PE photopigment. Expression patterns also suggest that two other opsins are photopigments in the SE photoreceptor outer segments; remarkably, a different opsin and a putative photoisomerase were also identified in the SE in a prior study. This is the first time that multiple opsin expression has been localised to the outer segments of photoreceptors in a cnidarian eye - particularly intriguing given the unknown function of the SE. Multiple opsins expressed in the SE, and some opsins that are expressed in both the SE and the manubrium (thus potentially with both visual and non-visual functions) supports the hypothesis of functional redundancy in the many Tripedalia opsins, which phylogenetic analysis suggests are even more numerous than the 19 opsins currently known.
Irwin et al. (Sun,) studied this question.