Long-term distribution patterns and northward shift signals of the box jellyfish Carybdea brevipedalia in Korean waters

Carybdea brevipedalia is the only cubozoan species reported from Korean waters and is a major cause of jellyfish stings during summer along the southern coast and around Jeju Island. This study examined long-term spatiotemporal distribution patterns and potential signals of northward range change by integrating our researcher-led surveys (2013–2025) with nationwide monitoring records from the National Institute of Fisheries Science (NIFS; 2011–2025). An occurrence level (OL) index was applied as a common reference to evaluate relative occurrence across methods. Results showed a regionally uneven distribution pattern, with the southern coast consistently representing the main occurrence area, sporadic records from Jeju Island, and increasingly frequent records along the East Sea coast after 2020. Repeated field confirmations in the southern East Sea and a northward shift of the northernmost confirmed record from Hwajin (36°15′N) in 2020 to Hupo (36°40′N) in 2025 together suggest a possible northward expansion of the species distribution. However, the lack of field-based confirmations in the northern East Sea, despite continued observer-reported cases in the nationwide program, suggests that intensified monitoring and expert-led field verification are needed, while accounting for uneven survey effort, method-dependent detectability, and identification uncertainty. Given the diverse cubozoan taxa reported from adjacent Japanese waters, strengthened surveillance around Jeju Island and the southern coast is also warranted to assess potential introductions of previously unrecorded species. Overall, these findings support an integrated monitoring framework that combines sensitive nocturnal light attraction with underwater observation and specimen-based confirmation to improve preparedness for range changes and localized outbreaks.