In recent years, coral in the seas around Okinawa Prefecture has been turning black and dying off due to a particular variety of coral-killing sponge called Terpios hoshinota.

The sponge is like a sheet of paper of about 1 millimeter thick and attaches itself to coral’s calcareous skeleton. It expands by 1 to 2 millimeters every day, covering the coral in black material.

Blackened coral in Okinawa was first confirmed in the 1980s, but the ecology behind the phenomenon was unclear.

Yamashiro’s team began its research in full scale two years ago. It has confirmed blackened coral damage due to Terpios hoshinota on seven islands — Kouri, Minna, Aguni, Shimoji, Kurima and Yonaguni in Okinawa Prefecture, and Kikaijima in Kagoshima Prefecture.

Much of the blackened coral lies in shallow shoals only a few meters deep, but the damage has also spread to a reef spanning a kilometer off the west coast of Okinawa’s main island from the villages of Nakajin and Ogimi.