Japanese Scientists Find New “Portuguese Man-of-War” and Give It a Samurai Name

A venomous species rode a heatwave all the way to Japan.

Our “samurai” should have never made it to the sands of Gamo Beach in Sendai Bay. This beach is historically too temperate for tropical invaders, but in July 2024, locals noticed strange, vibrant blue balloons stranded along the shoreline. They looked suspiciously like the famous Portuguese man o’ war, one of the most venomous creatures on the planet.

For decades, scientists believed the genus of the Portuguese man o’ war (Physalia) contained only one or two valid species worldwide. But when researchers analyzed these animals washing up in Japan’s Tohoku region, they realized they were looking at something entirely new to science.

Meet Physalia mikazuki

“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!”

Then, in the lab, they started describing it.

The name mikazuki isn’t random. It is a nod to the local history of the Tohoku region, derived from the Japanese word for “crescent moon” — the shape of the iconic helmet worn by the legendary Samurai Date Masamune, who founded Sendai City.

The newly described creature is smaller, with a float length ranging from roughly 9mm to 72mm, whereas the known Portuguese man o’ war can reach up to 300 mm. The Japanese creature glows with deep blue to purple hues, contrasting with a translucent bluish-green membrane.

Its “fishing” tentacles (tentacular palpons) are arranged differently, and its feeding polyps (gastrozooids) are banana-shaped with balloon-like yellow tips. It wasn’t easy to describe the species in detail, but in the end, the researchers were sure it’s a new species to science.

“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.”

An Unwanted Invader

The Portuguese man o’ war, also known as the man-of-war or bluebottle, is a marine hydrozoan fond of warm waters. It looks like a jellyfish, but it’s actually a siphonophore, a colony of animals.

The colony divides the labor, using a gas-filled bladder to keep the system afloat, that capture prey and defend the colony (and sting beachgoers), as well as digesting and reproductive organs. With this stunning division of labor, the Portuguese man of war is one of the most venomous creatures on Earth.

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It’s always exciting to describe a new species, but Physalia mikazuki shouldn’t have ended up in Japan. It’s a warm water species, and historically, these creatures were never seen in the temperate waters of the Tohoku region.

Researchers used particle tracking simulations (essentially digital drift bottles) to retrace the creatures’ journey. The models suggested that these animals likely hitched a ride on abnormally hot currents created by abnormally hot weather.

The arrival of P. mikazuki brings ecological and public health concerns. Like all members of its genus, this species is carnivorous. It feeds on fish larvae and small plankton, potentially disrupting local food webs and commercial fish stocks.

There’s also a risk for humans. Like its cousins, P. mikazuki is venomous. Its tentacles can deliver painful stings to beachgoers and swimmers, raising concerns for tourism in a region that rarely had to worry about such hazards.

Ultimately, it’s a reminder that the oceans are changing; and with them, the animals are changing too.

The study was published in the journal Frontiers in Marine Science.