A gelatinous mass with bluish reflections, on the sand of a beach in Miyagi Prefecture (Japan): this is what caught the trained eye of a biologist during an ordinary walk.
This encounter would reveal much more than just organic debris washed up by the waves. The intriguing object, about 20 centimeters (8 inches) long, had a delicate structure and an unusual coloration for the region. Its mere presence in this temperate area of northeastern Japan immediately raised questions about its origin and nature, suggesting a scientific significance that far exceeded its fragile appearance.
Central image: complete colony showing the gas-filled float and tentacles (scale 5 cm).
A) Lateral view of the float, with a folded crest and a transparent sail.
B) Close-up view of the dorsal area, showing groups of gonodendres, tentacular palpons, and gastrozooids with yellow tips.
C) Main tentacles showing their characteristic coiling.
D) Organization of zooid groups: posterior area with six groups and one protozooid, main area with tight tripartite groups.
E) Close-up of elongated gastrozooids, ending in a yellow bubble-shaped tip. This fortuitous discovery, made by Yoshiki Ochiai of Tohoku University, marks a notable advance in marine biology. The organism, initially mistaken for a jellyfish, turned out to belong to the genus Physalia, creatures often feared for their venom. The in-depth analysis published in
Frontiers confirmed that it was an entirely new species, named
Physalia mikazuki. This unprecedented identification in such northern waters indicates significant transformations within marine ecosystems, potentially linked to global environmental changes.
An unexpected scientific identification
The meticulous laboratory examination revealed unique morphological characteristics in this specimen. The Portuguese man o' war, this gas-filled float enabling buoyancy, had a distinct shape from other known species. The researchers also noted the presence of several main tentacles and yellow, banana-shaped gastrozooids, digestive structures absent in its relatives. These significant structural differences immediately alerted scientists to the uniqueness of their discovery.
Genetic analyses conducted by the team definitively confirmed the distinct species status of
Physalia mikazuki. DNA sequencing revealed unique markers that did not match any sequences referenced in international databases. This molecular approach, combined with traditional morphological study, now constitutes the standard method for identifying and classifying cryptic species—organisms that are morphologically similar but genetically distinct.
A) Map of Japan locating Miyagi.
B) Map of Miyagi with Sendai Bay, collection site at Gamo Beach.
C) Satellite image of Gamo Beach with the three sampling points (D–F).
D–F) Photos of the three beach areas (north, center, south), showing varying levels of debris.
G–I) Specimens of P. mikazuki sp. nov. washed ashore, with size indication.
J) Size distribution of the 38 collected colonies.
K–L) Excerpts from a TV report showing stranded Physalia, including one individual more likely corresponding to P. utriculus.
M) Warning sign indicating the presence of Physalia further north, in Shizugawa.
N) Specimen from Okinawa (2025) showing morphological characteristics different from P. mikazuki sp. nov.
The chosen name, which can be translated as "crescent-helmet Portuguese man o' war," pays homage to the local cultural heritage in reference to the famous samurai Date Masamune. This choice is not insignificant, as it places the discovery within the historical heritage of the Tōhoku region. This naming respects taxonomic conventions while creating a symbolic link between the new species and the territory where it was first identified.
The biogeographic implications of a migration
The presence of this Portuguese man o' war in Sendai Bay represents the northernmost observation ever recorded for this genus. Drift simulations conducted by the researchers indicate that the specimen may have been transported from Sagami Bay by the Kuroshio Current. These oceanographic models reconstruct the organism's probable journey by following the dynamics of water masses and prevailing winds. The accuracy of these simulations now allows for anticipating the future movements of these species.
The expansion of the Portuguese man o' war's range appears directly correlated with increasing ocean surface temperatures. The gradual warming of Japanese coastal waters creates favorable environmental conditions for the establishment of tropical and subtropical species in traditionally temperate regions. This phenomenon is not isolated, as it also affects other marine organisms such as the Nomura's jellyfish, whose blooms are becoming more frequent and extensive.
The ecological and societal consequences of this migration deserve particular attention. The stinging venom of Portuguese man o' war poses a risk to swimmers, requiring adaptations to safety measures on beaches. At the same time, the arrival of new predators in the local marine ecosystem could alter established biological balances, with potential repercussions on fishing and native biodiversity.
Article author: Cédric DEPOND