A student-led research group from Tohoku University has discovered a new species of the venomous Physalia (commonly known as Portuguese man-of-war) that has never been seen before in northeast Japan. This revelation suggests that warming coastal waters and shifting ocean currents are influencing the distribution of marine organisms in northeastern Japan.

The study, published in Frontiers in Marine Science (Marine Molecular Biology and Ecology) on October 30, 2025, presents the first formal description of a Physalia species from Japan.

"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," remarks second author Yoshiki Ochiai. "So I scooped it up, put it in a ziplock bag, hopped on my scooter, and brought it back to the lab!"

The curiosity sparked by this eye-catching cobalt blue creature is what lead to this remarkable discovery of a new species of Physalia. Professor Cheryl Ames of the Graduate School of Agricultural Science and the Advanced Institute for Marine Ecosystem Change (WPI-AIMEC), stated that the species was named Physalia mikazuki ("crescent helmet man-o-war") in recognition of Sendai's feudal lord Date Masamune, a samurai warrior who is known for the crescent moon adorning his helmet.

"It was a very involved process recording all the unique body structures that distinguish it from the other four species of Physalia," says first author Chanikarn Yongstar, "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."

In Japan, Physalia utriculus is found from Okinawa to Sagami Bay. It was previously assumed this was the only jellyfish of that genus in the area, but through matching DNA sequences to public reference databases, this study revealed that its distribution overlaps with the newly described P. mikazuki. In other words, there were always two species in that area - but no one noticed until it showed up in the Tohoku region.

"Our morphological and DNA analyses confirmed that these specimens represent a new species, distinct from its tropical relatives," says Kei Chloe Tan, who did the DNA analysis, "Which is an exciting finding in and of itself, but we still had questions about how it got here."

The sighting in the Tohoku region represents the first record of Physalia being spotted this far north. To answer their questions about how it drifted so far, they ran computer simulations of how objects are carried by ocean surface currents. The results suggest that warm water from the Kuroshio Current, which has recently extended much farther north along with high sea-surface temperature anomalies, may have transported the colonies to Sendai Bay. This finding provides new insight into how marine ecosystem changes can influence the movement of surface-drifting marine organisms.

"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," explains Muhammad Izzat Nugraha, "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 discovery also highlights the importance of coastal observation and public awareness. With tentacles that can extend several metres and cause painful and debilitating stings, continued monitoring can help keep beachgoers safe while also improving our ecological understanding.

"These jellyfish are dangerous and perhaps a bit scary to some, but also beautiful creatures that are deserving of continued research and classification efforts," adds Ayane Totsu.

This research was made possible through funding from WPI-AIMEC (Advanced Institute for Marine Ecosystem Change) and the paper was made open access thanks to the APC Support Project for the Promotion of Open Access at Tohoku University in 2025.