Like many of us AVATAR has to be one of my favorite films ever. Love it, hate it, space Pocahontas or not, the creature concepts were astounding and I would surely kill, maim or destroy who ever I have to in order to have a chance to be part of that production team. WOW HARSH MUCH?!
With that being said here is my speculative concepts to the next installment of the franchise.
The rainbow ray, not having any bone structure can virtually collapse/roll up into its light weight shell as a means of protection. Both males and Females produce the light weight shell but a female's shell will also house their eggs. A new shell will be produced throughout their lives as they will out grow them. Mainly feeding on microscopic prey they can be active hunters especially during the breeding season. The large orange organ houses vital organs, aid in detection of predator/prey and for sonar.