This particular Surgeon Fish is the Blue-lined Surgeon Fish (Acanthurus lineatus).  It lives in a harem on the shallow reef flat in a small territory the size of your kitchen.  They graze on the algae that grows like a fine moss across much of the reef's surface and play a key role, as do other herbivorous fish, in stopping seaweeds from overgrowing the corals on a reef.

Corals build reefs.  Reefs are home to fish.  Corals need fish.

Another cog in the giant wheel of the reef where the whole is greater than the sum of the parts.