Black triggerfish are one of the most common fish on the reef, at least in certain areas. They are unremarkable in appearance being mostly black with bright, pale blue lines at the base of the dorsal and anal fins (top photo).
They are usually seen in large groups, moving through the water as they feed on plankton and algae (middle photo).
But when they’re agitated, bright blue lines radiate from around the eye. The more agitated, the more coloration, until they can appear like the fish in the bottom photo, with blue-green lines over the whole body.