Convict surgeonfish (Acanthurus triostegus) shoaling over bleached coral (white). Coral bleaching occurs when the symbiotic zooxanthellae that inhabit the coral's tissues are expelled by the coral. The zooxanthellae are brightly-coloured algae that photosynthesise within the coral, providing it with nutrients. The loss of zooxanthellae is thought to be due to environmental stresses, such as an elevated water temperature, and recent bleaching events have been linked to global warming. Bleached corals die within a few months if conditions do not improve. Convict surgeonfish, or tangs, inhabit reefs throughout the tropical Indo-Pacific. Photographed in the Maldives.