New Man-o'-War Species Found in Northeast Japan — Warming Seas May Be Driving a Northward Shift

Scientists identify a previously unknown man-o'-war in Sendai Bay and warn shifting ranges may threaten swimmers and ecosystems

A translucent, bluish mass that washed ashore at Gamo Beach in northeastern Japan caught the eye of researcher Yoshiki Ochiai. He collected the tangled tentacles and brought the specimen to Tohoku University, where laboratory and genetic analyses revealed it was not a common local species but an entirely new member of the man-o'-war group.

Physalia mikazuki — whose name means “crescent helmet” and honors the crescent-moon ornament on samurai Date Masamune’s helmet — was described by researchers after detailed morphological study and DNA testing. Although commonly called jellyfish, man-o'-wars are actually siphonophores: colonial cnidarians made up of many specialized individuals, or zooids, that function together as a single animal.

The newly described P. mikazuki differs from the four previously recognized Physalia species in several ways. Its gas-filled float (pneumatophore) has a distinct shape, it can carry more than one primary tentacle, and it has yellow, banana-shaped gastrozooids that capture and digest prey. Genetic analyses confirmed these differences are consistent with a separate species.

This discovery is notable because the specimen was found in Sendai Bay — the furthest north any Physalia has been recorded in Japan. Prior to this find, Physalia utriculus was thought to be the only Physalia commonly present in Japanese waters, ranging from subtropical Okinawa to Sagami Bay.

To understand how P. mikazuki arrived so far north, the team ran simulations using compiled sighting and ocean current data. The modeled trajectories show a plausible northward drift from Saganami Bay to Gamo Beach. Researchers say rising sea temperatures are likely widening the habitable zone for these floating predators, a pattern similar to the northward expansion observed for the large Nomura jellyfish that has disrupted ecosystems and fisheries in parts of Japan.

Risks and next steps

Man-o'-wars deliver painful — sometimes life-threatening — stings, posing a hazard to swimmers and beachgoers. The research team recommends increased public awareness, enhanced beach-safety measures, and regular monitoring of both P. mikazuki and P. utriculus to track migrations, mitigate human risk, and assess ecological impacts. Continued surveys may also reveal additional, previously unrecognized Physalia species.

“These jellyfish are dangerous and perhaps a bit scary to some, but also beautiful creatures that are deserving of continued research and classification efforts,” said researcher Ayane Totsu in a press release.

The full description of the species and its implications for regional biogeography were published in Frontiers in Marine Biology and Ecology.