A team of student researchers at Tohoku University has identified a previously unknown species of the venomous Physalia (better known as the Portuguese man-of-war) in the waters of northeastern Japan. The finding points to how rising sea temperatures and altered ocean currents are affecting the range of marine species in the region.
Their work, published on October 30, 2025, in Frontiers in Marine Science (Marine Molecular Biology and Ecology), marks the first official description of a Physalia species native to Japan.
An 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," recalls 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!"
That unexpected encounter with the vibrant cobalt-blue creature led to the formal identification 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) explained that the new species was named Physalia mikazuki ("crescent helmet man-o-war") in honor of Sendai's feudal lord, Date Masamune, a samurai warrior whose iconic helmet featured a crescent moon.
Naming and Classifying the New Species
"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 has long been known to inhabit waters from Okinawa to Sagami Bay. Scientists once assumed it was the only member of its genus in the region. However, DNA analysis comparing genetic sequences to global databases revealed that its range overlaps with the newly identified P. mikazuki. This means two species had been coexisting unnoticed until one appeared farther north in Tohoku.
Tracking the Journey Northward
"Our morphological and DNA analyses confirmed that these specimens represent a new species, distinct from its tropical relatives," explains researcher Kei Chloe Tan, who conducted the genetic testing. "Which is an exciting finding in and of itself, but we still had questions about how it got here."
This marks the first recorded sighting of a Physalia so far north in Japan. To uncover how it traveled such a distance, the team ran computer models simulating the movement of drifting organisms on ocean currents. The results indicated that warm water from the Kuroshio Current -- now extending farther north alongside unusually high sea-surface temperatures -- likely carried the colonies into Sendai Bay. These findings shed new light on how climate-driven ocean changes can alter the paths of floating marine life.
"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."
Beauty, Danger, and the Need for Awareness
The discovery underscores the importance of ongoing coastal monitoring and public education. With tentacles capable of stretching several meters and delivering intensely painful stings, tracking these organisms helps protect swimmers and beachgoers while also improving scientific understanding of marine ecosystems.
"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 study was supported by WPI-AIMEC (Advanced Institute for Marine Ecosystem Change) and made open access through funding from the APC Support Project for the Promotion of Open Access at Tohoku University in 2025.
