Rhizostomeae (Scyphozoa), a phylogenetic comparative analysis of their gastrovascular system

Jellyfish anatomy investigation methodologies have remained largely unchanged compared to other research fields, thus resulting in a slowdown in new discoveries. On the basis of the recent published research (combining resin endocasting, X-ray microtomography and stain injections) on Rhizostoma pulmo and Cotylorhiza tuberculata, which surprisingly highlighted a more complex anatomy than previously described, and, for the first time in scyphozoans, a separated double flux circulation inside the oral arm canals, we tried the application of this protocol on different species of the order Rhizostomeae to investigate if this “new” complexity is shared inside this order or if is peculiar for some species/groups/taxa only. We choose two species from the Mediterranean Sea, Phyllorhiza punctata and Cassiopea sp., which are considerably far from each other for both the genetic distance (Mastigiidae and Cassiopeidae, respectively) and the feeding habits. Our preliminary results highlighted the presence of the double constricted canal structure in Phyllorhiza punctata, while was absent in Cassiopea sp., where only a canal without constriction into the oral arm was observed. Mostly, while in R. pulmo a proximal portion of the cruciform central mouth persists, in all other taxa the central mouth has completely regressed. The double-canal system may be advantageous in sustaining uninterrupted predatory activity, especially in large-bodied species. Relying on symbionts for nutritional support, like Cassiopea sp., may have reduced pressure toward increased complexity of the gastrovascular system. These findings once again challenge the theory of a simple digestive system in scyphozoans, revealing more complex adaptations among the Rhizostomeae.