The halfmoon (Medialuna californiensis), also known as the blue perch, is a species of marine ray-finned fish, a sea chub from the subfamily Scorpidinae, part of the family Kyphosidae. It is native to the coasts of the eastern Pacific Ocean off western North America. It is fished for using hook and line and it is a desirable food fish. Description The halfmoon has an elongate, compressed, oval body with a small pointed head. It has a small, horizontal mouth in which the upper jaw is partially hidden by the orbital bone when the mouth has closed. Each jaw has a single row of pointed teeth. It has a continuous dorsal fin which has 9-11 spines in the anterior portion which fold down into a scaly furrow, the posterior portion has 22-27 soft rays and these are higher than the spines. The anal fin has 3 spines and 17-21 soft rays. The caudal fin is marginally concave. Most of the head and body is covered in small, thick and rough scales except foe the area forward of the eyes. The colour is dark bluish grey to grey dorsally, paler ventrally sometimes with faint, narrow, vertical bars along the flanks. It has a dark blotch on the upper corner of the gill cover. The margins of the dorsal, anal and caudal fins are dark and the outer edge of each lobe of the caudal fin is white. Juvenile fish are blue above and silvery below. The maximum recorded total length of the halfmoon is . The shape of the caudal fin is said to resemble a crescent moon, leading to the common name. Distribution The Halfmoon is native to the eastern Pacific Ocean along the western coast of North America from Vancouver Island in the north to the Gulf of California, although it is rare north of Point Conception in California. Habitat and biology The halfmoon is frequently observed high in the water column over rocks and at middle depths over kelp. Their preferred habitats are rocky reefs, kelp forests, floating kelp pads, and oil rigs. They are occasionally recorded near the surface of the water, but they are commonly found at depths between . This is an omnivorous species with kelp, red algae and green algae making up the bulk of its diet but it also feeds on a diversity of small invertebrates such as crustaceans, sponges and sea anemones. They