Sendai, Japan – A student-led team from Tohoku University has identified a previously unknown species of venomous Portuguese man-of-war in northeastern Japan, highlighting the impact of warming waters and shifting ocean currents on marine life.

The discovery, published in Frontiers in Marine Science on October 30, 2025, marks the first formal description of a Physalia species from Japan. The jellyfish, named Physalia mikazuki or “crescent helmet man-o-war,” honors Sendai’s feudal lord Date Masamune, famous for the crescent moon on his samurai helmet.

“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,” said 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!”

First author Chanikarn Yongstar detailed the painstaking work involved: “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.”

DNA analysis confirmed the specimens are distinct from tropical relatives, overlapping in distribution with Physalia utriculus but previously unrecognized in Tohoku. Particle simulations suggest warm water from the Kuroshio Current may have carried the colonies northward, reaching Sendai Bay.

Researchers emphasize the importance of continued monitoring. With tentacles extending several metres and capable of painful stings, awareness helps protect beachgoers while improving 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,” added Ayane Totsu.

Podcast: Play in new window | Download