image: Associate Professor Toshimasa Yamamoto of Okayama University of Science enjoys a piece of “Okayama Ridai Matsukawa-garei” sushi at the Kurasushi Osaka Expo 2025 branch, where the premium flounder cultivated using the university’s innovative “Third Water” aquaculture system is now being served.
Credit: Okayama University of Science
On July 25, the premium flounder Matsukawa—land-farmed using “the Third Water,” an innovative type of water in which both seawater and freshwater species can coexist at the Education and Research Center for Biological Production of Okayama University of Science (OUS)—was added to the menu of the conveyor-belt sushi chain Kurasushi at its Osaka Expo 2025 branch. Marketed under the brand name “Okayama Ridai Matsukawa-garei,” each piece is priced at 320 yen (tax included), available in limited quantities until sold out.
Known as the “King of Flounder,” Matsukawa is valued for its thick flesh, rich fat content, firm texture, and natural sweetness—qualities that make for an exceptional dining experience. At Kurasushi, it is served as nigiri-zushi (atop an oblong bed of vinegared-rice) with a soy-sauce koji marinade.
OUS began cultivating Matsukawa juveniles (average weight approx. 2.6 g, length 6 cm) at the Center on July 12, 2023. The fish are raised in a fully closed recirculating 35-ton tank system, growing steadily to an average shipping weight of about 1.5 kg.
This is the second aquaculture product developed with the Third Water at OUS to be commercialized by the chain. In May 2023, “Okayama Ridai Unagi” (eel, 500 yen per piece) was sold nationwide.
The Osaka Expo 2025 branch of Kurasushi, the chain’s largest outlet, features a 135-meter conveyor lane and 338 seats. In line with the Expo theme, the menu also includes dishes inspired by cuisines from around the world.
A woman in her 20s from Osaka, who tried the newly launched “Okayama Ridai Matsukawa-garei” on the first day, commented: “It has a pleasantly firm and slightly crunchy texture, with an elegant sweetness and just the right amount of fat. It’s absolutely delicious.”
Associate Professor Toshimasa Yamamoto of OUS’s Faculty of Life Sciences, Department of Biosciences, who visited the restaurant on the first day, said:
“This premium flounder has been carefully raised by our students, who treated them like their own children. I can personally guarantee the taste. I hope customers from around the world will not only enjoy this exquisite flavor, but also come to appreciate the research capabilities of Japanese universities.”