Warming seas bring a dangerous jellyfish to Japan’s coast

The seas around Japan are changing fast. Warm currents are pushing north, carrying creatures that once stayed far away. Among them is a new jellyfish so vivid and strange that it made scientists stop in their tracks.

The jellyfish is a blue, balloon-like drifter that turned out to be a new species of the venomous Portuguese man-of-war.

Jellyfish found by surprise

Students from Tohoku University were not looking for anything new. Yet, while studying near Sendai Bay, one of them noticed something odd floating on the water.

“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 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 impulsive decision led to the discovery of Physalia mikazuki, Japan’s first recorded Physalia species, described in the journal Frontiers in Marine Science.

Naming the new jellyfish

Professor Cheryl Ames from Tohoku University’s Graduate School of Agricultural Science and WPI-AIMEC explained that the new species’ name means “crescent helmet man-o-war.”

The name honors Sendai’s famous samurai lord, Date Masamune, known for the crescent moon on his armor.

“It was a very involved process recording all the unique body structures that distinguish it from the other four species of Physalia,” said Chanikarn Yongstar, the study’s lead author.

“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.”

Each observation added to the picture of a creature that had somehow gone unnoticed in a region so well studied.

Two species, one coast

For years, researchers believed that Physalia utriculus was the only species found in Japan. It appears often in warmer areas like Okinawa and Sagami Bay.

But when the team compared DNA sequences, they saw something unexpected. The new P. mikazuki shared the same waters, hidden in plain sight all along.

“Our morphological and DNA analyses confirmed that these specimens represent a new species, distinct from its tropical relatives,” explained study co-author Kei Chloe Tan.

“Which is an exciting finding in and of itself, but we still had questions about how it got here.”

Physalia mikazuki drifted north

The researchers used computer simulations to trace the path that P. mikazuki might have taken. The model followed drifting particles, mimicking how floating organisms travel with the currents.

“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,” said study co-author 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.”

The Kuroshio Current, already known for its warmth and power, seems to have carried the species farther north than ever before.

Rising sea-surface temperatures have pushed its reach beyond traditional limits, turning the northern coast into a temporary home for tropical life.

Changing seas and shifting currents

This movement fits a broader pattern across the Pacific. Warm-water currents are stretching into cooler zones, altering the mix of marine species.

Tiny plankton, fish schools, and drifting colonies like Physalia all respond to these changes. When currents shift, so do entire ecosystems.

In ocean studies, scientists describe mesoscale eddies – huge spinning water masses – that can pull organisms across long distances. These natural “conveyors” often explain how tropical species appear in temperate waters.

What once seemed an anomaly now looks like part of a growing trend driven by climate shifts.

The jellyfish isn’t harmless

The find is exciting, but it also comes with caution. Physalia mikazuki has long tentacles lined with venomous stingers.

Contact can cause severe pain and even paralysis. Tracking its movement is now a safety matter, not just a scientific one.

“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 study co-author Ayane Totsu.

Learning where these jellyfish drift helps both scientists and swimmers. It also deepens our understanding of how warming seas are changing life along Japan’s coast.

Climate clues and ocean discoveries

This discovery shows how science often begins with curiosity. A random scoop of a net became evidence of climate-driven change.

The Tohoku team combined fieldwork, anatomy, DNA sequencing, and simulations to reveal a story that links biology to ocean physics.

Supported by WPI-AIMEC and Tohoku University’s open access program, the research has opened a new chapter in Japan’s marine biology.

The study shows how a single unexpected find can expose the vast, unseen movements beneath the surface – proof that the ocean is never done surprising us.

The research is published in the journal Frontiers in Marine Science.

Image Credit: ©Tohoku University

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