The discovery of a new jellyfish species by a student team from Tohoku University is shedding light on how marine life is adapting to rising sea temperatures in the Pacific. The presence of Physalia mikazuki in Sendai Bay, an area previously considered too cold for this genus, is prompting scientists to reconsider long-held beliefs about species distribution and biodiversity in Japanese waters.
Previously, Physalia utriculus, known locally in the warmer southern seas, was thought to be the only Physalia species in Japan. This changed in June 2024 when several striking blue specimens were discovered stranded on Gamo Beach in Miyagi Prefecture. Their unique morphology caught the attention of student researcher Yoshiki Ochiai, who stumbled upon one during unrelated fieldwork. “I scooped it up, put it in a ziplock bag, hopped on my scooter, and brought it back to the lab,” he recounted in an interview published in the Frontiers in Marine Science on October 30, 2025.
A Name Rooted in Local History
The newly described jellyfish is named Physalia mikazuki, or the “crescent helmet man-of-war,” in honor of the famed samurai lord Date Masamune, whose distinctive crescent-shaped helmet has long symbolized Sendai. This name not only reflects the jellyfish’s arched, sail-like float but also its geographic identity.
According to lead taxonomist Chanikarn Yongstar, the classification process was meticulous. She compared each part of the organism to historical illustrations in century-old biology manuscripts. “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,” she explained. The team analyzed morphological traits, such as tentacle thickness, zooid structure, and color patterns, and cross-referenced them with molecular data from global Physalia samples.
DNA sequencing of the 16S rRNA and COI mitochondrial gene regions confirmed the jellyfish belonged to a distinct genetic clade, separate from known Physalia species such as P. physalis, P. minuta, and P. utriculus.
These results led to its official designation as a new species, making P. mikazuki the first Physalia ever formally described from Japanese waters.
Tracing the Path Northward
The presence of this tropical cousin in northern Japan was no random drift. Oceanographic simulations conducted by the Tohoku University team traced a likely route using the OceanParcels Lagrangian model. According to researcher Muhammad Izzat Nugraha, the particles in the simulation, likened to floating “red beach balls,” drifted along the Kuroshio Current, reaching Sendai Bay in about 30 days. Some continued farther north to Aomori Prefecture within 75 days.
This trajectory aligns with recent sea-surface data from the HYCOM (Hybrid Coordinate Ocean Model), which showed that the Kuroshio Current extended about 100 km farther north between 2022 and 2024. During that period, northern waters recorded anomalous warming of 2–4°C, creating conditions that allowed tropical organisms to survive farther from the equator. These findings suggest that marine species like P. mikazuki are not only drifting farther but also enduring longer in temperate zones.
Long-term records from Sendai Bay had shown no Physalia sightings prior to 2023, reinforcing the theory that its presence is a recent, and environmentally driven, phenomenon.
Venom, Visibility, and Vigilance
While visually stunning, Physalia mikazuki brings practical concerns. Like its relatives, it is venomous. Its tentacles can extend several meters and deliver intensely painful stings to humans. Public health warnings followed the initial discovery, with alerts posted at local beaches, and coverage on regional TV stations raising awareness of the jellyfish’s unexpected appearance.
According to marine biologist Ayane Totsu, the jellyfish represents both risk and research potential. “These jellyfish are dangerous and perhaps a bit scary to some, but also beautiful creatures that are deserving of continued research and classification efforts,” she noted in the Frontiers in Marine Science article.
The announcement comes as scientists worldwide are increasingly focused on the effects of climate change on marine ecosystems. The discovery of Physalia mikazuki not only adds a new chapter to the understanding of jellyfish biodiversity but also underscores the dynamic nature of oceanic environments in the face of global warming.
As researchers continue to study this new species, they hope to gain further insights into how marine organisms are adapting to changing climates, potentially informing conservation strategies and public safety measures in the future.
