Grey Triggerfish

They feed on crustaceans, benthic invertebrates, and molluscs.
Sea Urchins are their favourite food, they eat them by blowing water at them to overturn them and then attacking the soft area underneath.

These are the workers of the reef, often busy turning over rocks, stirring up the sand and biting off pieces of branching coral. This is why one often sees other smaller fish species around it who feed from the left overs.
Triggerfish have a hard spine dorsal fin that can be locked.
When sleeping this spine is used to wedge them into place in a crevasse and so deter predators from pulling them out of their bed!
The spine is also held erect as a warning to other fish to stay away.
Several large Titans blow shallow depressions in the sand for nesting and if approached will raise this spine as a warning, if ignored they may charge, even divers! especially the Titan Triggerfish who will take a tasty bite. Beware.