Rhizostomeae (Scyphozoa) jellyfishes are widespread in neritic waters and include species of commercial importance in Asia. This group comprises jellyfish taxa that host endosymbiotic dinoflagellates of the family Symbiodiniaceae, which provide autotrophic benefits. Despite their value, limited molecular data for Japanese rhizostome taxa has hinder accurate taxonomic classification and interpretation of novel traits. This study combines molecular methods to provide the most complete understanding of molecular phylogenetic relations of Rhizostomeae jellyfishes while assessing the number of Symbiodiniaceae taxa that can be hosted in each species at the medusa level through a new method developed herein for tandem amplification of symbionts and host, validated with microscopy. We also evaluate which rhizostomes produce cassiosomes and whether Symbiodiniaceae are found in the core. Phylogenetic analysis of host mitochondrial (16S rRNA, COI) and nuclear (28S) gene regions of 18 medusae from five genera revealed: (1) Mastigias in Japanese waters corresponds to M. albipunctata ; (2) Cassiopea from Kagoshima likely represents an undescribed species, though Cassiopea xamachana may have been introduced; (3) Two cepheid species - Cephea cephea and Netrostoma setouchianum - occur in Japan; (4) Rhopilema esculentum , a commonly harvested species, is endemic to western Japan. Symbiotic Symbiodiniaceae ITS2 analysis identified three dominant genera ( Symbiodinium , Cladocopium , and Durusdinium ). More than one genus among these was found to be hosted in samples of the genera Mastigias and Cassiopea , indicating plasticity in symbiont association at both the taxon and individual medusa level. Microscopy confirmed cassiosome production exclusively in species examined of the suborder Kolpophorae: Cassiopea sp., N. setouchianum , and M. albipunctata , though absent in a juvenile M. albipunctata sample. Conversely, R. esculentum hosts Symbiodiniaceae but appears to lack the ability to produce cassiosomes. Overall, findings support the distinctive evolution of Symbiodiniaceae–Rhizostomeae symbiosis, the monophyly of the suborder Kolpophorae, and the synapomorphy of cassiosome production in Kolpophorae with onset likely influenced by developmental stage. Broader taxon sampling, especially within Dactyliophorae, will provide further clues on the functional evolution and cellular organization underlying photoendosymbiosis and cassiosome production in these medusozoans.
Tan et al. (2026) studied this question.