Butterfly fish are bony fish (Class Osteichthyes). They live around coral reefs and their bodies are very compressed for maneuverability. As you can see from maneuvering this fish around they use their whole lower body to create thrust. They have extensions on both dorsal and ventral fins that also provide thrust.

This particular variety is the spotfin butterflyfish Chaetodon ocellatus. It demonstrates two interesting points of cryptic coloration. On their soft dorsal fin they have a false Eye spot which serves to fool predators. Predator may momentarily be confused over which end of the fish is its head giving the fish a chance to escape. Aiding in this deception is the stripe running through the fish's eye. This stripe breaks up the outline of the eye making it harder to see.

Although native to the Caribbean occasionally juveniles are swept into the Gulf Stream. When this happens they can be brought as far north as Cape Cod. Young fish are occasionally spotted around Cape Cod in August and September. This story does not have a happy ending as the young fish ultimately perish when the water cools off in the late fall and early winter.