Surgeon fishes (ACANTHURIDAE) have received the name due to the sharp bone "scalpels" at the base of tail fin. Usually thorns are clasped to a body, but in danger they rise perpendicularly to a body and can injure the enemy. The surgeon fishes are very aggressive and defend their home-ranges. Their diet consists of algae.
The hermit crabs (Dardanus sp.) (DIOGENIDAE) live in empty shells of gastropods, hiding inside their long and soft abdomens. In danger they move into a shell completely and close the entrance with nipping claws. As hermit crab grows, its old "apartment" becomes tight, and the crab seeks a new, more spacious one.
Hermit crabs are omnivorous, they eat everything, from algae to carrion; they also can catch a small fish. In an aquarium they feed on food remnants and carry out sanitary functions.
Surgeon fishes (ACANTHURIDAE) have received the name due to the sharp bone "scalpels" 
at the base of tail fin. Usually thorns are clasped to a body, but in danger they rise perpendicularly to a body and can injure the enemy. The surgeon fishes are very aggressive and defend their home-ranges. Their diet consists of algae.
in empty shells of gastropods, hiding inside their long and soft abdomens. In danger they move into a shell completely and close the entrance with nipping claws. As hermit crab grows, its old "apartment" becomes tight, and the crab seeks a new, more spacious one.
Hermit crabs are omnivorous, they eat everything, from algae to carrion; they also can catch a small fish. In an aquarium they feed on food remnants and carry out sanitary functions.