Bat Rays
Mylobatis californica
When the mudflats around Alameda and across the bay at Millbrae were divided up into parcels for spreading out and fattening up the Eastern oysters brought west by the new transcontinental railway in the 1870s, 80s, and 90s, the oyster farmers built fences of closely-spaced stakes around the parcels
View of oyster bed at low tide (top) and high tide (bottom), Millbrae, California
These fences were not only to mark the farm limits of a particular oyster proprietor, thus privatizing what had been common spaces open to all, they were also necessary to keep out a prime oyster predator, the bat ray.Nineteenth-century oystermen reported that rays could destroy several acres of their oysters within a short time.
Bat rays are common in San Francisco Bay today. They enter protected bays in large numbers in early spring to bear their young—a mature female can give birth to 10-12 pups—and they remain until fall. The adults are large, often 3 to 4 feet wide, and they weigh as much as 150 pounds. Oysters, if available, can be a chief element to a bat ray’s diet. Bat rays have heavy, flat teeth arranged in a sort of pavement in each jaw. With these teeth they crush the oyster shells and devour the meat.
Bat rays, despite their stinging ray and relationship to sharks are considered by many as adorable. They fly along the bottom, disturbing the sand or mud with their wings to feed on clams, worms, shrimp, and even crabs, leaving a tell-tale trail of discarded shells. But a bat ray’s flattened anatomy, with their eyes bulging out of their grey topsides and their mouth on the white undersides, necessitates alternative sensing abilities to locate food. A heightened network of specialized electroreceptors called ampullae of Lorenzini on the bottom side of the ray’s head can detect weak electrical fields in the environment especially in sand or murky water. These jelly-filled pores allow the ray to find oysters, clams, and other mollusks, essentially seeing their surroundings through electricity.
Close up of bat ray mouth
Rays are common in Tomales Bay and Elkhorn Slough as well as San Francisco Bay, but oyster farmers today have different techniques for avoiding bat ray predation on their farms. In Tomales Bay, oysters are put in floating mesh bags that rays cannot get into. There are plenty of other food sources for bat rays; their populations in all three estuaries are healthy.
