The Portuguese man-of-war, or Physalia, is a jelly-like creature that uses a special gas-filled balloon to float on the sea surface. Physalia inhabits the area where the ocean meets the atmosphere, in contrast to the majority of jellyfish that drift deeper. Its balloon serves as a sail, allowing it to glide with the wind and currents. Large groups may occasionally be carried to coastlines where they are rarely observed due to changing winds and ocean temperatures.
Students from Tohoku University have discovered a brand-new species of the venomous Portuguese man-of-war. The name Physalia mikazuki is a nod to Date Masamune, a local samurai warrior who was infamous for wearing a crescent moon on his helmet. This is the first-ever Physalia species formally described in Japan, and the first sighting in the chilly Tohoku region, nearly 2,000 km (1,243 miles) north of the species regular habitat around Okinawa.
The new study also hints at a bigger story. How warming seas and changing currents may be steering tropical marine life into unexpected northern waters.
Chanikarn Yongstar, first author of the study, said, “It was a very involved process recording all the unique body structures that distinguish it from the other four species of Physalia. I looked at each 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.”
For years, scientists thought Physalia utriculus was the only Portuguese man-of-war drifting from Okinawa to Sagami Bay. But a recent deep-dive study, blending jellyfish anatomy, DNA, and ocean modeling, revealed a surprise: Physalia mikazuki had been there all along, quietly sharing the waves.
It took a sighting in the far north, at Tohoku’s Gamo Beach, to spark the discovery. Turns out, the two species were coasting together, hidden in plain sight until one drifted far enough to raise eyebrows.
Kei Chloe Tan, who did the DNA analysis, stated, “Our morphological and DNA analyses confirmed that these specimens represent a new species, distinct from its tropical relatives. Which is an exciting finding in and of itself, but we still had questions about how it got here.”
Researchers studied ocean temperatures and currents over time to understand shifts in the northern seas off Japan. By combining satellite data and simulations, they uncovered a striking trend. The Kuroshio Current is drifting north, and coastal waters are warming by 2 to 4 °C (3.6 to 7.2 °F).
These changes have allowed Physalia mikazuki, generally considered a tropical jellyfish, to move into cooler northern habitats. It’s a clear sign that climate-driven ocean shifts are reshaping marine migration routes and rewriting the map of biodiversity.
Using particle simulations, scientists found that Physalia mikazuki likely drifted north on the Kuroshio Current, blending in with its close cousin, P. utriculus, along the way. It took just 30 days to reach Sendai Bay, and 45 days to sail as far as Aomori.
This hidden journey shows how shifting currents and warming seas can quietly reshape the map of marine life.
“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,” explained 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.”
Physalia mikazuki‘s discovery is a wake-up call as much as a scientific milestone. This jelly needs to be closely watched along the coast to make sure beachgoers are safe because of its painful sting and tentacles, which can grow up to meters long.
But it’s also a reminder of how much we still have to learn. By blending taxonomy, DNA analysis, and ocean modeling, researchers are uncovering hidden species and tracking how climate change is redrawing the boundaries of marine life. In a shifting ocean, watching the shore can also mean getting a glimpse of the future.
The study was published in Frontiers in Marine Science.
Source: Tohoku University
