Okinawa's fragile coral threatened by 'Black Devil' killer sponge

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.

“It’s otherwise known as the ‘Black Devil.’ The way the damage is spreading poses a new threat to coral,” says Hideyuki Yamashiro, a professor at the University of the Ryukyus’ Tropical Biosphere Research Center. “I want to develop research and stop the damage.”

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.

The blackened coral is rough on the surface and doesn’t feel like a coral-killer is sticking to it. Yamashiro points out that that Terpios hoshinota is impossible to remove by hand.

“It doesn’t choose to stick itself to a particular coral but sticks to any live coral. We’ve seen particular damage on a type called finger coral,” he said.

Yamashiro is worried that if the coral keeps on dying, it will affect the biodiversity in the ocean.

Studies are being conducted on coral ecology in the center’s aquarium, but raising coral in an aquarium setting is difficult.

“We’ve tried a number of different experiments (to get the coral-killer to grow), with the tide and the food, but we still haven’t figured out the ecology,” Yamashiro says.

“It’s interesting to see there are many mysteries surrounding these creatures but in order to rescue the coral, we need further research,” he said.

This section features topics and issues from Okinawa covered by The Okinawa Times, a major newspaper in the prefecture. The original article was published July 20.