Marty’s photo of the day #4948: This is a flagtail triggerfish. There are about 40 species of triggerfish. They get their name because they can lock their main dorsal spine into an upright position using a second, smaller spine, allowing them to wedge themselves into tight, rocky crevices to prevent predators from pulling them out. Also, they frequently use their top and bottom fins to move rather than their tails, allowing them to swim backward, forward, or hover in place like a drone.
(Photographed in the Raja Ampat of Indonesian New Guinea.)