Large aggregation of Humpback Red Snapper

Humpback Red Snapper, aka Paddletail Snapper, often aggregate in large schools, staying completely stationary or drifting slowly along sloping reefs as one unit.
 
Scientific Name: Lutjanus gibbus
 
Under certain conditions periodic population explosions of crown-of-thorns starfish can occur on reefs, generating alarm due to the extensive damage they can unleash to the corals and the reef ecosystem.
 
Scientific Name: Acanthaster planci