Researchers in Japan have discovered a new species of venomous Portuguese man-of-war jellyfish.

A student-led team from Tohoku University identified the previously unknown species of marine hydrozoan, marking the first formal description of a Physalia species from the country.

The findings were published in Frontiers in Marine Science on October 30.

The jellyfish, named Physalia mikazuki, or 'crescent helmet man-of-war', pays tribute to Sendai's feudal lord Date Masamune, who was famed for the crescent moon crest on his samurai helmet.

Second author Yoshiki Ochiai said: '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!'

First author Chanikarn Yongstar described the painstaking identification process.

He said: '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.'

Footage shows the bluish, translucent jellyfish floating in the water, its long tentacles faintly illuminated.

DNA analysis confirmed that the specimens are genetically distinct from tropical relatives, overlapping in distribution with Physalia utriculus but previously unrecognised in northern Japan.

Particle simulations suggest that warm water from the Kuroshio Current may have carried the colonies northward to Sendai Bay.

Researchers stress the importance of continued monitoring. With tentacles that can extend several metres and deliver painful stings, increased awareness can help protect beachgoers while improving ecological understanding.

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