Students Discover New Portuguese Man O’War Species — Named for Samurai ‘One‑Eyed Dragon’

New Portuguese Man O’War Species Discovered in Japan

A team of university students in Japan has identified a previously undescribed species of Portuguese Man O’War, detailed in a study published in Frontiers in Marine Science. The colony’s vivid color, distinctive anatomy and potent venom inspired researchers to name it after a famous 16th‑century samurai.

Not a jellyfish, but a siphonophore

Although often mistaken for a jellyfish, the Portuguese Man O’War (traditionally referred to as Physalia physalis) is actually a siphonophore: a colony of genetically identical individuals called zooids that specialize in functions such as hunting, digestion and reproduction while operating together as a single organism. Its gas‑filled float (the pneumatophore) keeps the colony at the surface while long venomous tentacles trail beneath, sometimes reaching lengths of 100 feet and carrying neurotoxins that can immobilize prey and injure swimmers.

From one species to many

For roughly 250 years scientists treated the Man O’War as a single species. In June 2025 researchers showed the group actually comprises at least four distinct species; the newly described specimen from northern Japan raises that total to five.

How it was found

Study co‑author Yoshiki Ochiai encountered the unusual specimen while working near Sendai Bay in the Tohoku region. “I scooped it up, put it in a Ziplock bag, hopped on my scooter, and brought it back to the lab,” he recalled. Marine ecologist Chanikarn Yongstar helped document its complex morphology, noting the difficulty of untangling and comparing the colony’s many parts against historical anatomical drawings.

Yoshiki Ochiai: “I was working on a completely different research project when I came across this unique ‘jellyfish’ I had never seen around here before.”

Chanikarn Yongstar: “I examined each component, matching its features to old hand‑drawn plates — a real challenge when so many parts are intertwined.”

A new name with historical meaning

The newly described species has been named Physalia mikazuki, which translates as the “crescent‑helmet Man O’War.” The name honors Date Masamune (1567–1636 CE), the daimyō of Sendai known as the “One‑Eyed Dragon of Ōshū,” who famously bore a crescent moon on his helmet.

Where it came from and why it matters

Before this discovery, researchers believed only Physalia utriculus inhabited waters from Okinawa to Sagami Bay. The team’s analyses showed the region has hosted two distinct Physalia species all along. To investigate how P. mikazuki arrived so far north, the researchers ran particle‑tracking simulations that modelled how a colony might drift on surface currents over weeks or months, accounting for current speed, water temperature and weather. The simulations suggest the colony most likely traveled north on the warm, fast Kuroshio Current, which has been extending farther north in recent years alongside higher Pacific surface temperatures.

Besides adding to known marine biodiversity, the find highlights how shifting ocean conditions can redistribute species and increase the likelihood of unfamiliar organisms appearing in new regions — with implications for human safety, local ecosystems and fisheries.

Published: Study in Frontiers in Marine Science. Species: Physalia mikazuki (crescent‑helmet Man O’War).