A group of students from Tohoku University has identified a new species of the venomous jellyfish known as Physalia, commonly referred to as the Portuguese man-of-war. This significant finding indicates that changes in coastal temperatures and ocean currents are affecting the distribution of marine life in northeastern Japan. The research, published in the journal Frontiers in Marine Science on October 30, 2025, marks the first formal identification of a Physalia species in Japan.
“While I was engaged in a different research project around Sendai Bay, I stumbled upon this unique jellyfish that I had not encountered in the area before,” stated second author Yoshiki Ochiai. “I collected it, placed it in a ziplock bag, hopped on my scooter, and brought it back to the lab!” The vibrant cobalt blue jellyfish sparked curiosity, leading to the discovery of this new species.
The newly identified species has been named Physalia mikazuki, which translates to “crescent helmet man-o-war.” This name pays homage to Date Masamune, the feudal lord of Sendai, known for the crescent moon design on his helmet. First author Chanikarn Yongstar remarked on the challenge of documenting the distinct physical features that set this jellyfish apart from other Physalia species, stating, “I examined each body part in detail, comparing it to historical illustrations of jellyfish anatomy.”
Previously, Physalia utriculus was thought to be the only jellyfish of its kind in Japan, found from Okinawa to Sagami Bay. However, through genetic analysis and comparisons with public DNA databases, the research revealed that the distribution of P. mikazuki overlaps with that of P. utriculus. “This means there were always two species in the region, but the presence of P. mikazuki went unnoticed until it was found in the Tohoku area,” explained Kei Chloe Tan, who conducted the DNA analysis.
The sighting of this jellyfish in northeastern Japan represents the northernmost record of the Physalia genus. To investigate how this species migrated to such a location, the research team utilized computer simulations to track how surface currents transport marine organisms. Their findings suggested that warm waters from the Kuroshio Current, which has shifted further north, likely carried the jellyfish colonies to Sendai Bay. “I created a particle simulation—essentially dropping bright red beach balls in the water and predicting their movements based on data,” described Muhammad Izzat Nugraha. “We were thrilled to discover that our simulation traced a route from Sagami Bay to the location where we found the crescent helmet man-o-war.”
This discovery underscores the necessity for coastal monitoring and increased public awareness. With tentacles that can extend several meters and deliver painful stings, ongoing surveillance can enhance safety for beachgoers and contribute to ecological understanding. Ayane Totsu emphasized, “These jellyfish can be dangerous and intimidating to some, yet they are also beautiful creatures that warrant further research and classification efforts.”
This research received support from WPI-AIMEC (Advanced Institute for Marine Ecosystem Change), and the paper is accessible due to the APC Support Project for the Promotion of Open Access at Tohoku University in 2025.
Title: Physalia mikazuki sp. nov. (Phylum Cnidaria; Class Hydrozoa) Blown into Japan”s Northeast (Tohoku) at the Whim of Marine Ecosystem Change
Authors: Chanikarn Yongstar, Yoshiki Ochiai, Muhammad Izzat Nugraha, Kei Chloe Tan, Ayane Totsu, Waka Sato-Okoshi, and Cheryl Lewis Ames
Journal: Frontiers in Marine Science; Marine Molecular Biology and Ecology
DOI: 10.3389/fmars.2025.1653958
