The moon wrasse is a species of wrasse native to the Indian Ocean and the western Pacific Ocean. It is an inhabitant of coral reefs and surrounding areas at depths from 1 to 20 m (3.3 to 65.6 ft).

Moon wrasses are carnivorous and tend to prey on fish eggs and small sea-floor dwelling invertebrates. This species can reach 45 cm (18 in) in total length. It is of minor importance to local commercial fisheries and can also be found in the aquarium trade.

The juvenile is blue on the lower half of its body, with a black spot in the middle of the dorsal fin and a black blotch on the caudal fin base. As it matures, the spot turns into a yellow crescent, hence the name.

The body is green, with prominently marked scales. Coloration of the head ranges from blue to magenta, with a broken checkerboard pattern. Moon wrasses are active fish, said to be moving all day long. They are also territorial, nipping, chasing, and otherwise harassing fish that get in their way.

Wrasses have strong vision, although they also have a decent sense of smell. At night, they rest in niches often under rocks or other such structures. If needed, a moon wrasse may dig out space under a rock by repeatedly swimming through it until it fits without a struggle.

The alpha male is more brightly colored, and at every low tide hour, changes from green to blue, and goes into a show of attacking and nipping all the other wrasses.

More Info: en.wikipedia.org