Researchers at Tohoku University have discovered a new species of venomous jellyfish, Physalia mikazuki, in the northern waters of Japan. The find, published on October 30, 2025, highlights how warming ocean currents are expanding marine species ranges. DNA analysis confirmed the species as distinct from its tropical relatives.
A team of student researchers at Tohoku University stumbled upon the new species while working in Sendai Bay, in Japan's Tohoku region. Second author Yoshiki Ochiai recalled the accidental encounter: "I was working on a completely different research project around Sendai Bay in the Tohoku region, when I came across this unique jellyfish I had never seen around here before. So I scooped it up, put it in a ziplock bag, hopped on my scooter, and brought it back to the lab!"
The vibrant cobalt-blue creature was formally identified as Physalia mikazuki, or "crescent helmet man-o-war," named in honor of Sendai's feudal lord, Date Masamune, whose iconic helmet featured a crescent moon. Professor Cheryl Ames of the Graduate School of Agricultural Science and the Advanced Institute for Marine Ecosystem Change (WPI-AIMEC) led the effort. First author Chanikarn Yongstar described the classification process: "It was a very involved process recording all the unique body structures that distinguish it from the other four species of Physalia. I looked at each individual part, comparing its appearance to old tomes where scholars drew out the jellyfish anatomy by hand. A real challenge when you look at just how many tangled parts it has."
Previously, only Physalia utriculus was known in Japanese waters, from Okinawa to Sagami Bay. DNA analysis by Kei Chloe Tan revealed that P. mikazuki's range overlaps with P. utriculus, marking the first recorded sighting of a Physalia species so far north. Tan explained: "Our morphological and DNA analyses confirmed that these specimens represent a new species, distinct from its tropical relatives."
To trace its northward journey, the team used computer models simulating ocean currents. Muhammad Izzat Nugraha detailed the simulation: "I ran a particle simulation -- which is like dropping bright red beach balls in the water, then making data-based estimations to track where they will end up days or months later. We were excited to find that in our simulation, all the beach balls essentially made a trail from Sagami Bay up to right where we found the 'crescent helmet man-o-war' in the Tohoku region."
The models pointed to the Kuroshio Current, extended northward by rising sea-surface temperatures, as the likely transporter. This discovery, published in Frontiers in Marine Science, underscores climate-driven changes in marine ecosystems. Ayane Totsu noted: "These jellyfish are dangerous and perhaps a bit scary to some, but also beautiful creatures that are deserving of continued research and classification efforts."
With tentacles reaching several meters and delivering painful stings, the find emphasizes the need for coastal monitoring to protect beachgoers. The study was supported by WPI-AIMEC and made open access through Tohoku University's funding.