California Butterfly Ray, Gymnura marmorata
The California Butterfly Ray, Gymnura marmorata, whose common Spanish name is raya mariposa californiana, is a species in the family Gymnuridae, the Butterfly Rays, known as rayas mariposa in Mexico. Globally, there are sixteen species in the genus Gymnura, of which three are found in Mexican waters, one in the Atlantic and two in the Pacific.
The California Butterfly Rays have extremely broad rhomboidal disc-shaped bodies that are approximately 1.5 times wider than they are long. Their pectoral fins are not separated from the head giving rise to their triangularly-shaped body. They are light brown in color and covered with dense pale brown spots overlaid by scattered blackish spots. They become a uniform darker brown upon death. Their head has a bluntly pointed snout, eyes and spiracles on top, and an arched mouth equipped with numerous small teeth in bands. The front margin of the disc is slightly concave and the rear margin is rounded. A key to identification is the distance between the eyes which is greater than the distance from the eyes to the tip of the snout. Their tail is very short and approximately one-half the length of the disc with one or two large serrated venomous spines used for defense. They have no caudal or dorsal fins. Their skin is smooth and without denticles.
The California Butterfly Rays are bottom-dwellers found in shallow coastal waters in bays and along beaches and within silty and muddy channels and estuaries at depths up to 310 feet. They reach a maximum width of 1.5 meters (4 feet 11 inches) and length of 90 cm (35 inches). Females are approximately twice the size of males. They feed on buried bivalves, crabs, mollusks, and crustaceans. Reproduction is via aplacental viviparity with embryos nourished by small yolk sacs and young born live. They have a high fecundity level, with four to sixteen pups per litter ranging in disc width from 21 to 26 cm and having a fairly rapid growth rate. In Mexican waters they are found along the entire West coast of Baja and throughout the Sea of Cortez.
Due to its wide narrow disc and short tail, the California Butterfly Ray is easy to identify and cannot be confused with any other species. The Longsnout Butterfly Ray, Gymnura crebripunctata (tip of snout greater than distance between eyes) found from Mazatlán south along the coast of the mainland to Guatemala is considered by some to be the same species.
They are also caught on occasion by recreational fishermen in Southern California. They show up in abundance in April and May during the breeding season and become a focus species. They are sold primarily dried in local markets. If you have a “fish taco” in the Baja there is a high probability that you will eat this fish. Based on the large decline of guitarfish populations attributed to intense fishing pressure within this range it is assumed that the populations of California Butterfly Rays are also in decline. They are also commonly taken as a by-catch of shrimp trawlers, beach seines, and gillnets and are normally discarded with a high mortality rate. One positive is that heavy fishing pressure has removed the majority of predatory sharks and upper trophic level teleosts from the environment decreasing both fishing pressure (fewer elasmobranch fishermen) and predatory pressure on this species. The catch of this species is mostly unregulated and the limited number of seasonal bands put in place are not enforced.