Taxonomy

Rarity

Bat rays are common along the entire California coast. A diver in a bay or over a nearshore sand flat will see them on almost any warm-water day, most often as a single ray gliding just above the bottom or resting motionless in the sand. In some bays and estuaries the rays gather in schools reported to number in the thousands during the spring and summer mating season, and California Sea Grant lists the state population as stable and increasing. The IUCN has assessed the species as Least Concern. The bat ray has never supported a commercial fishery in the United States, and it is not managed by the Pacific Fishery Management Council. Recreational take is legal under the general finfish limits, and among rod-and-reel anglers the fish carries the nickname mud marlin for the long, hard fight it puts up on tackle.

Seasonality

Location

The bat ray lives in the eastern Pacific from central Oregon south through the Gulf of California, with a population also present around the Galapagos Islands. It is common in the sandy or muddy sloughs and estuaries of the California coast, and it uses rocky reefs and kelp beds as well. The depth range runs from the intertidal down to about 350 feet, with most sightings in water shallower than 100 feet.

The rays sit or swim just above the bottom for most of the day. In the enclosed bays where they gather, individuals also shift between shallow warm patches during the day and deeper water at night, which biologists have described as behavioral thermoregulation, the same warmth-seeking strategy the leopard shark uses in the surf zone off La Jolla, San Diego. Adults are strong swimmers and range widely, and larger females roam farther than smaller males.

The wing shape reads as a diamond from above. A bat ray moves through the water by flapping its pectoral fins in slow, symmetrical strokes, closer to the wingbeat of a bird than the side-to-side sweep of most fish. That wingbeat is the reason for the common name. The species belongs to the eagle ray family, and eagle rays as a group swim, feed, and mate in almost the same fashion.

Deep Dive

The bat ray is the largest common ray on the California coast. Adult females reach a wingspan close to 6 feet and weights of about 200 pounds, and the largest individuals live up to 23 years. Males stay smaller, topping out around 3 feet across. Females also mature later than males: 5 to 6 years of age for a female, compared with 2 to 3 for a male. On the back the ray is dark brown or almost black, and the underside is chalk white. A raised head sits above the front edge of the disc, and a thin whip-like tail runs behind, longer than the disc itself is wide. The base of the tail carries 1 or more venomous spines. The spine is flexible and sheathed at birth, and the ray uses it only when a diver or wader steps on it or grabs it. The standard California beach advice is to shuffle the feet along the sand instead of stepping in and out of the water, so a ray at rest feels the pressure and moves off before a foot lands on it.

The bat ray feeds mostly on hard-shelled animals along the bottom: clams, oysters, marine snails, crabs, shrimp, echiuran worms (the fat innkeeper worm and its relatives, chunky burrow-dwellers of the mudflats), sea cucumbers, and small bony fishes. The teeth are flat plates arranged in tightly-packed rows, and the ray uses them to grind and crush shell. The hunt is more excavation than pursuit. The ray glides along the bottom sensing water currents from buried clams, then lowers itself and flaps its pectoral fins to move away the sand covering the prey. For deeper prey the ray uses its rounded snout to dig a trench, and pits from feeding bat rays have been measured up to 13 feet long and 8 inches deep. Those pits are important beyond the ray itself. Smaller fish move in behind the feeding ray and pick out the worms and small crustaceans the digging turns up.

There is an old commercial-oyster grudge against the bat ray on the California coast. Growers in the early 20th century blamed the rays for oyster losses in the growing beds. Later research pointed the finger the other way. Bat rays eat mostly the crabs that eat oysters, so the presence of bat rays around a bed is on balance protective rather than damaging. The species is more useful to the shellfish industry than harmful to it.

Reproduction is live-bearing. The bat ray is ovoviviparous, which means the young develop inside the mother, first drawing nutrition from a yolk sac and then absorbing a milk-like fluid produced by the uterus wall. Gestation runs about 9 to 12 months, and litters are small, from 2 to 10 pups. The pups are born tail-first with the pectoral fins wrapped around the body, so the sharp spine passes out of the mother folded flat. Newborns are 9 to 12 inches across. Mating pairs swim in close synchrony, the male tucked below the female with their wingbeats matched, and the male grips a pectoral fin in his teeth as he inserts a clasper into her cloaca. The reproductive biology carries an oddity: a bat ray female has a single functional ovary but 2 functional uteri, and her overall pup production per year is low compared to bony fish of similar size. That low fecundity is the reason a locally reduced population takes a long time to bounce back.

Documented predators include the California sea lion, the great white shark, and the broadnose sevengill shark. The ray's defense in open water is to bury itself in loose sand until only the eyes and spiracles show, and the pattern on its back matches the sand well enough that a diver can pass within a few feet and never notice.

The bat ray is a legal take in California under the general finfish limits, and the wings are the eaten portion. Cooked bat ray meat is described as tender and meaty, close to white fish or scallops in texture, and is prepared sautéed, deep-fried, or poached. Handling matters more with rays and sharks than with bony fish: the tissue holds urea, which converts to ammonia within hours of death and produces an off odor if the animal is not bled and iced promptly. Prehistoric coastal peoples of California, likely the Ohlone, fished bat rays in numbers along the central coast for food, and shell mounds along San Francisco Bay preserve bat ray remains in the record.

Where to Find Them

Beaches whose habitats match this species

Matched by habitat: a beach is listed when its biome tags overlap this species' biomes.