Scientists battling voracious bay snail
Making effort to restore the native oysters decimated by an alien whelk
Wading into the water along the rocky shore off Aramburu Island in Richardson Bay, Brian Cheng reached 3 feet under water and pulled up an algae-covered rock.
“Here we go,” he yelled as he splashed ashore soaking wet and pointed out a tiny snail on the underside of the rock. “And, we got a bonus,” he said, gesturing toward a cluster of yellow gelatinous egg capsules.
The harmless-looking creature was a whelk snail, also known as an Atlantic oyster drill, one of the most voracious alien species in San Francisco Bay — a predator that has decimated native oysters and so far spoiled all efforts to restore their once thriving population.
The grim reality of the snail’s rampage was made clear to Cheng, a biologist with the Smithsonian Environmental Research Center, who just completed an oyster restoration study of Bay Area shorelines that showed the infestation in Richardson Bay is worse than researchers thought.
Richardson Bay, where the drills seem to be multiplying, is a testing ground for efforts all along the West Coast to get rid of the insatiable pests so that California’s celebrated native Olympia oysters can be reintroduced.
“It’s a very difficult problem,” said Cheng, a San Rafael native who is also assistant professor of environmental conservation at the University of Massachusetts. “There are drills all along the West Coast, including San Francisco Bay, Tomales Bay, Humboldt Bay and Willapa Bay in Washington. And they don’t just eat oysters. They eat clams, barnacles and
other species. They can even eat each other.”
The joint study by the Smithsonian and Richardson Bay Audubon Center was part of a Bay Area-wide effort to bring back the long-lost Olympia oysters, which blanketed subtidal regions from Southern California to southeastern Alaska and were once a staple of the American Indian diet until they were ravaged by human excess.
The problem is, the oysters are being devoured by whelks as fast as they can be reintroduced and nobody has figured out a way to stop the slaughter, except by hand-plucking the snails en masse.
That’s why the Smithsonian and the Audubon centers, with help from researchers at San Francisco State, Tiburon’s Estuary and Ocean Sciences Center and the San Francisco Bay National Estuarine Research Reserve, deployed 300 volunteers and student groups who plucked 29,000 drills off the shoreline rocks over the past two years.
Chela Zabin, an ecologist with the Smithsonian center, said oyster farms in Tomales Bay had been successful removing drills by hand, so she figured it was worth a try in Richardson Bay.
To test the effectiveness, Zabin and Cheng affixed juvenile Olympia oysters to 192 ceramic tiles and placed them in tidal areas off Aramburu and Lani’s Beach, next to the Audubon Center in Tiburon. As a control, one-third of the tiles were encased in netting that the invasive snails could not breach. Another third had netting around them with holes that would allow limited access and the rest had no protective netting.
The results were disturbing. Almost all of the oysters that weren’t completely encased in netting were devoured by the gluttonous invaders. Zabin said the survival rate of those not fully protected was between zero and 10 percent maximum. Between 80 and 90 percent of the oysters encased in netting survived.
“At this point, given these results, it doesn’t make sense to do oyster restoration here,” Zabin said. “We would just be feeding the drills.”
It is one of the many problems conservation groups and the aquaculture industry are facing in their 20-year effort to bring back Olympias, California’s only native oyster.
As many as half of the Olympias in Tomales Bay are also being devoured by the otherworldly snails, which use a raspy, chain saw-like tongue that secretes a type of acid to drill into the shells before inserting a proboscis to suck out the insides.
With the two Richardson Bay sites off the list, Cheng and Zabin are now monitoring eight other sites in the North Bay to determine whether there are any suitable locations for oyster restoration programs. Researchers are testing a variety of methods, including placement of shell-mound reefs to mimic the once-abundant oyster beds that historically provided habitat for aquatic life.
The reefs are important because the filter-feeding bivalves float in the water in their embryonic state, searching instinctively for hard, rocky structures to attach to and establish colonies.
The shells of Olympia oysters, known scientifically as Ostrea lurida, were abundant in the many Indian middens discovered around the bay, some dating back 4,000 years. The mollusks, about the size of a 50-cent piece, were also a delicacy among the Forty-Niners during the Gold Rush, who said they had a “coppery” flavor and creamy texture. The Hangtown fry was created, according to legend, by a condemned man who ordered the two most expensive items he knew of at the time — Olympia oysters and eggs — for his last meal.
In 1893, Olympia oyster beds covered a total of 8,033 acres in Newport Bay, Elkhorn Slough, San Francisco Bay and Humboldt Bay, according to a study published a few years ago in the scientific journal Proceedings of the Royal Society B. Almost a half-million oysters per acre once crowded together along the bay floor, according to the report.
By 1911, the native oyster beds in the Bay Area were gone, scoured clean by ravenous San Franciscans. Today, the oysters that people mostly eat along the West Coast, including those grown commercially in Tomales Bay, are Pacific oysters, natives of Japan that are incapable of reproducing naturally in this climate.
The oyster drills were accidentally introduced to San Francisco Bay when Atlantic oysters were brought from the East Coast on the transcontinental railroad to replace the native oysters, which were all but gone by then and presumed extinct. Attempts to grow the eastern oyster species in the bay failed — the water is too cold in the summer — but the drills survived.
Zabin said the alien snails are now concentrated locally not only in Richardson Bay, but in Tomales Bay and south of the Bay Bridge, the locations where people once tried to grow Atlantic oysters. Olympias still grow unmolested in other places, like China Camp in San Rafael, but warmer water and changes in salinity triggered by global warming are jeopardizing even those populations, she said.
The snails usually stop laying eggs between November and April, but Zabin found drill egg sacks throughout the past winter, which was warmer than usual. She now plans to study ways of introducing predator-friendly habitat that might draw Dungeness, red, rock and brown rock crabs to eat the snails.
“It’s really important to make sure they don’t spread because it’s so difficult to control their population,” Zabin said. “If humans can drive fish species to near extinction, we should be able to do the same thing with the drill.”