Yellow boxfish (Ostracion cubicus)

The yellow boxfish (Ostracion cubicus) is a species of boxfish. It can be found in reefs throughout the Pacific Ocean and Indian Ocean as well as the south eastern Atlantic Ocean. It reaches a maximum length of 45 centimetres (18 in).
As the name suggests, it is box-shaped. When juvenile, it is bright yellow in color. As it ages, the brightness fades and very old specimens will have blue-grey coloration with faded yellow. It feeds mainly on algae, but will also feed on sponges, crustaceans and mollusks.
When stressed or injured it releases poisonous proteins from its skin that may prove lethal to any fish in the surrounding waters. The bright yellow color and black spots are a form of warning coloration (Aposematism) to any potential predators.
(from Wikipedia)