A coral species found primarily in western and southern Japan has spread in Tokyo Bay and is threatening to destroy ecosystems in the warming waters around the capital.

The coral variant Acropora squarrosa, which is native to areas around the Kii Peninsula and the Shikoku region, has created a vibrant, tropics-like scene near the Boso Peninsula at the entrance to Tokyo Bay.

On Sept. 9, a diver searched the area off the Katsuyama district in Kyonan, Chiba Prefecture, in seawater that reached as high as 28.2 degrees. After a 10-minute boat ride from the port, the explorer dove into the water and spotted a 100-square-meter carpet of corals on the seafloor.

Vivid blue neon damsel and orange sea goldie were swimming above the corals.

Chiba Prefecture’s Tateyama, at the south end of the Boso Peninsula, was believed to be the northernmost area of Japan where coral reefs could form.

The diving point, however, sits 15 kilometers north of Tateyama.

Jiro Uochi, 68, who operates the Kacchama Diving Service in Katsuyama, said the corals started popping up there 30 or so years earlier.

They were initially sparsely distributed over the seabed, but the species suddenly began to expand dramatically five to six years ago.

Behind the coral invasion are changes in the marine environment.

Global warming has prevented water temperatures from dropping, causing rocky-shore denudation and declines in abalone and turban shells that feed on seaweed.

Coral variations, such as Acropora solitaryensis, have taken root in the damaged areas. The table-shaped corals block sunlight and inhibit seaweed growth, inflicting a grave impact on fisheries.

“Corals now exist everywhere likely because of the higher ocean temperatures,” Uochi said. “Although some may consider it favorable for an Okinawa-like landscape to be reproduced here, it is causing changes in the types of fish being caught. A major problem is emerging for fishermen.”

Japan Meteorological Agency statistics show global warming caused the average sea surface temperature worldwide to rise 0.61 degree over the past century. Waters around Japan recorded a 1.28-degree rise.

During the first half of this year, ocean surface temperatures off Japan were the highest since 1982, while waters near Tateyama were 1 degree higher than usual as of Sept. 9.

“Living creatures relocate their habitats in response to environmental alterations for survival,” said Nina Yasuda, a marine biology professor at the University of Tokyo. “Seawater temperatures are expected to rise further during the next 100 years.”

Yasuda also pointed out that a shift in the Kuroshio current is carrying coral eggs and larvae to Tokyo Bay from sea area off Shikoku and the Kii Peninsula.

Yasuda said the prolonged meandering of the current since 2017 has generated a warm branch flow that has transferred 40 to 50 coral species to Tateyama.

In contrast to Tokyo Bay, colonies in some southern waters are at risk of dying out amid surging ocean temperatures.

Exposed to warmer water, corals release photosynthetic cells called zooxanthellae, destroying the symbiotic relationship. The corals cannot survive without zooxanthellae and become whitish.

This phenomenon, known as coral bleaching, was broadly observed off Amami-Oshima island in Kagoshima Prefecture in August.

The Amami Marine Life Research Association found that 90 percent of corals had bleached at one of four locations marked by colonies.

The survey results also revealed a 5 meter colony of Porites australiensis had died off despite surviving a past round of bleaching.

“Seawater temperatures did not fall this year because the oceanic water was insufficiently stirred by fewer typhoons,” said Katsuki Oki, chair of the research association.

He said coral bleaching is reportedly increasingly, so monitoring must continue.

“One possibility is that corals may rise in number in Tokyo Bay but mostly vanish off Okinawa,” Yasuda said.

“Countermeasures must be implemented, such as cutting carbon dioxide emissions, to slow the speed of maritime warming.”

(This article was written by Hiroyuki Kobayashi, Daiki Koga and Eiji Zakoda.)