Table coral found living in Yokosuka: Setting a new northernmost habitat record by 30 kilometers — Survey conducted by Rikkyo University and others

Table corals have been discovered at Sajima Fishing Port in Yokosuka City, Kanagawa Prefecture, and a survey by Rikkyo University and other institutions has found that the northernmost known habitat record for this species in Japan has been extended by approximately 30 kilometers. Previously, the northern limits for this species were the tip of the Boso Peninsula (Chiba Prefecture) on the Pacific side of the country, and Tsushima (Nagasaki Prefecture) on the Sea of Japan side. Table corals typically inhabit the waters around Taiwan and Okinawa. A researcher commented, "Individuals with high stress tolerance may be migrating northward. We want to investigate whether the symbiotic zooxanthellae are particularly resilient, or whether the corals themselves are."

Professor Nami Okubo of the Preparatory Office for the College of Environmental Studies, Rikkyo University, who specializes in coral biology, heard that "a fisherman found something that looked like coral" when she was at her regular sushi restaurant in Fujisawa City and decided to begin an investigation. The fishermen reportedly said that "coral has been increasing over the past four or five years."

Okubo examined three colonies collected from Sajima Fishing Port. Each fragment was approximately 15 to 35 centimeters in size, and the specimens were a species of table-shaped Acropora collected at depths of 3 to 5 meters. Corals normally live in symbiosis with zooxanthellae, which supply nutrients to the coral through photosynthesis. When these zooxanthellae are lost, the coral bleaches and eventually dies. All three specimens in this case were found in healthy condition, with no bleaching observed.

Coral taxonomy is currently in a state of confusion, with "different species descriptions being used by different researchers." Scientists worldwide are in the process of revising descriptions based on both morphology and DNA analysis. The Acropora corals found on this occasion belong to the species Acropora cf. solitaryensis and Acropora aff. divaricata. There is no doubt that both are species native to the warm southern seas of Taiwan and Okinawa.

Sea water temperatures at Sajima Fishing Port ranged from 13.4℃ to 28.3℃ in 2025. Since water temperatures in Okinawa rarely fall below 20℃, the specimens found here are likely "individuals with tolerance to low water temperatures." According to Okubo, Acropora corals are considered to be inherently "fragile" and generally unsuitable for tank keeping, but the fact that these individuals are growing "vigorously" at Enoshima Aquarium in Fujisawa City and in Professor Okubo's own tank suggests that they are exceptionally stress tolerant.

Until now, the northern limits for this species of Acropora were the waters off Tsushima City, Nagasaki Prefecture on the Sea of Japan side of the country, and Hasama in Tateyama City, Chiba Prefecture, on the Pacific side. The newly confirmed habitat lies at a latitude higher than either of these locations. Sea water temperatures at Hasama, Tateyama, in 2025 ranged from 14.7℃ to 27.7℃, which is about 1℃ warmer in winter than at Sajima Fishing Port, located approximately 30 kilometers to the north.

Furthermore, based on an estimated growth rate of 3 to 4 centimeters per year, it is calculated that the Acropora corals have been living in this area for more than ten years. Okubo noted: "The Kuroshio large meander has been in the news in recent years, but there is a possibility that the corals arrived and established themselves even before that. The innermost parts of Tokyo Bay have poor water quality, so these Acropora would likely not survive there. Sajima Fishing Port has clean water and a topography that makes it easy for Acropora to enter, which may be why they settled here."

Going forward, the researchers plan to conduct comparative analyses with experimental model corals currently under development to investigate the reasons for the corals' stress tolerance. The research was funded by the Ohsumi Frontier Science Foundation. The findings were published in Galaxea, Journal of Coral Reef Studies, the journal of the Japanese Coral Reef Society, on November 21, 2025, and announced by Rikkyo University and other institutions on December 23 of the same year.