Halfmoon, Medialuna californiensis
The Halfmoon, Medialuna californiensis, whose common Spanish name is chopa medialuna, is a species in the Sea Chub or Kyphosidae Family, known collectively as chopas in Mexico. Globally, there are only two species in the genus Medialluna, one of which is found in Mexican waters, this species from the Pacific.
The Halfmoons have oval elongated compressed bodies with a depth that is 36 – 40% of standard length. They are dark blue dorsally and transition to paler blue ventrally. Some have indistinct short bars on their sides. They have a dark blotch on the upper corner of their gill covers, a key to identification. Their anal, caudal, and dorsal fins are gray with dark gray outer margins. They have a small rounded head, a blunt nose, and a small mouth that opens at the front with one row of pointed teeth. Their anal fins have 3 elevated spines and 17 to 21 rays; their caudal fin (after which they are named) is lunate; their dorsal fin is continuous with 9 or 10 spines and 22 to 27 rays; and their pectoral and pelvic fins are short with the pelvic fins originating behind the pectoral fins. Their body is covered with small, thick, and rough scales.
The Halfmoons are found either demersal or mid-water in small loose schools with other fish at depths up to 145 feet. They reach a maximum length of 48 cm (19 inches). They consume algae, sponges, seaweed, shrimp, mussels, squid, and small fish. In turn they are preyed upon by sea lions, seals, larger fish, and marine birds. Many live in kelp forests and are considered to be important for controlling kelp growth, however, they can also become a problem for commercial kelp farmers as they can destroy crops. Reproduction is external with the release of pelagic eggs. They have lifespans of up to eight years.
In Mexican waters the Halfmoon have a limited distribution being found along the entire west coast of the Baja and along the east coast of Baja from Loreto south to Cabo San Lucas.
The Halfmoon is similar in size and shape to the Brassy Chub, Kyphosus vaigiensis, the Cortez Sea Chub, Kyphosus elegans, the Opaleye, Girella nigricans, the Gulf Opaleye, Girella simplicidens, and the Zebraperch, Hermosilla azurea, but none of these have the blue coloration with the dark spot over the gill cover.
The Halfmoons are a target of recreational fishermen providing a good foe when hooked. They are caught as a by-catch of commercial fishermen and marketed fresh as a mild-flavored fish. They are commonly taken off Southern California piers adjacent to rocks, reefs, and kelp structures on fresh mussels or small crabs and provide a good fight.
Halfmoon, Medialuna californiensis. Fish caught out from coastal waters off Catalina Island, California, October 2015. Length: 35 cm (14 inches). Catch, photo and identification courtesy of Chris Wheaton, Loreto, Baja California Sur.
Halfmoon, Medialuna californiensis. Fish caught from coastal waters off Catalina Island, California, October 2015. Length: 33 cm (13 inches). Catch, photo and identification courtesy of Ben Cantrell, Peoria, IL.