This is what the process of coral bleaching looks like
Temporal sequence of pulsed inflation by Heliofungia actiniformis to remove Symbiodinium. a Acclimated coral at 26 °C. b Inflation maximum before contraction. c Contraction and expulsion of zooxanthellae. d Even after expulsion the corals often maintain a level of inflation. Scale bar 5 cm
Recent research shows free-living plate corals such as Heliofungia sp. and Herpolitha sp. can use pulsed inflation (a series of rhythmic inflation and deflation) to unbury themselves from sand or as a limited form of locomotion. New research now shows that LPS corals also use pulsed inflation to expel zooxanthellae, AKA coral bleaching. These corals can control expulsion in a rapid but controlled manner, resulting in non-lethal bleaching to help cope with acute environmental stresses.
The open-access Coral Reefs journal paper describes the process:
Heliofungia actiniformis specimens were exposed to elevated water temperatures in a 10-L aquarium system for up to 8 d to encourage bleaching. An image was collected every 3 s. Temperatures were raised from 26 to 32 °C in the first 12 h and maintained at 32 °C (±0.5 °C) for the duration of the experiment. Within the first 2 h of exposure, H. actiniformis began expelling Symbiodinium from the tissue. A green plume of symbiont cells was expelled via the mouth during tissue inflation and contraction. The intensity and magnitude of the pulses increased over a 55-h period, with coral tissue expanding up to 340% of its original state (Fig. 1b). Corals survived between 2 and 8 d under elevated temperature conditions. There was some degradation of the epidermal tissue during the later stages of the experiments.