Colour is one of the most noticeable features of any organism and is often generated by pigments. We recently described a new protein-based pigment family that is exclusive to rhizostome jellyfish, the ‘Rhizostomins’. The blubber jellyfish Catostylus mosaicus, which forms spectacular blooms along Australia’s east coast, also belongs to the order Rhizostomeae and commonly appears with an intense blue colouration. By combining transcriptomic and proteomic approaches we characterised the blue pigment of C. mosaicus. As observed in other rhizostomins this pigment also possesses a KR domain inserted within a Fz-CRD domain. It also shares similar spectroscopic and physicochemical properties, such as a high absorbance in the ultraviolet spectrum. Anecdotal observations suggest blue colour morphs of C. mosaicus are more common in clear ocean-derived waters, while white and brown variants are more prevalent in turbid inner estuaries. We hypothesise that these blue pigments may be used to protect C. mosaicus from harmful ultraviolet radiation, in a similar manner to GFP-like proteins in other cnidarians such as Acropora millepora. We are further exploring this hypothesis with differential gene expression analyses of the colour morphs and a laboratory experiment.