Salinity regulates early life-history transitions in the box jellyfish Carybdea prototypus

Carybdea prototypus has potent venom and is responsible for serious envenomation incidents in the coastal regions of Japan. However, the natural habitat of the benthic polyp, which is a source of medusae, has not yet been identified. Previous studies on cubozoans, including polyp collection records and environmental DNA analyses, have suggested that C. prototypus polyps may inhabit low-salinity environments. However, the role of salinity in regulating the life-history processes of this species remains unclear. Therefore, in this study, we examined the effects of salinity on key developmental processes in C. prototypus , including planula settlement, polyp growth, asexual reproduction, and medusae metamorphosis. The planulae and polyps were reared under controlled salinity conditions ranging from 15 to 33. Planulae were successfully attached across all salinity treatments, although survival was reduced at 15. Metamorphosis into creeping polyps proceeds faster under moderately low-salinity conditions. After attachment, the time required to transform into settled polyps decreased with decreasing salinity. Polyp proliferation was active at low salinity, whereas polyp size increased with increasing salinity. In contrast, metamorphosis into medusae occurred only under high-salinity conditions. At salinity of 15, polyps degenerate into tissue masses. These findings indicate a clear salinity-dependent shift in life history strategy, in which low salinity promotes clonal expansion through asexual reproduction. Meanwhile, higher salinity facilitates somatic growth and metamorphosis into medusae. This study provides experimental evidence that salinity regulates key life-history transitions in C. prototypus and suggests that this polyp may inhabit low-salinity environments such as estuaries and seagrass beds of Zostera marina . Fluctuations in salinity likely are instrumental in shaping the distribution and population dynamics of this species in coastal environments.