Upper thermal tolerance of hermatypic coral Acropora digitifera collected from Sesoko Island, southern Japan, based on a laboratory experiment

Clones of hermatypic coral Acropora digitifera from Sesoko Island, Okinawa, were exposed to temperatures of 29 °C, 30 °C, 31 °C, and 32 °C for 14 days in a laboratory to identify their response to high seawater temperature. Coral colour was evaluated by RGB (R, red; G, green; B, blue) values and a 6-point brightness/saturation scale coral colour reference card. The colour of A. digitifera shifted slightly toward the black end of the scale in the 29 °C and 30 °C treatments throughout the experiment. The maximum effective quantum yield (ΔF/Fm′) was stable at these temperatures treatments. By the end of the experiment at 31 °C, coral colour and ΔF/Fm′ were slightly altered in the direction of bleaching, but the coral was not bleached. This result indicates that A. digitifera can resist bleaching for at least 2 weeks at 31 °C which is 2 °C above the highest summertime seawater temperature in Okinawa. In the 32 °C treatment, while bleaching occurred approximately on day 3, the ΔF/Fm′ was reduced but increased again with fluctuations. The succession of ΔF/Fm′ after exposure to 32 °C differed from that in corals exposed to herbicides. Taken together, these results suggest that corals may keep the platform for primary production after bleaching under high temperature conditions.