There are no plans to move Corbicula intentionally into habitats in which it could reproduce for the sake of improving water quality — despite its seemingly positive, ecosystem-scale impacts in the Potomac. "Wherever it has already exploited, yes, it is OK in its competitive abilities in that habitat," says Rochelle Seitz, a benthic ecologist at VIMS. Corbicula, however, is still a non-native species, she warns, and may behave unpredictably in a new environment.

On the other side of the country, in the tidal freshwater areas of San Francisco Bay, Corbicula did cause major ecological disturbances, according to Jim Cloern, a biologist with the U.S. Geological Survey. Unlike in the Chesapeake, algae grow in limited supply in that estuary where Corbicula competes for food with native algae-eating zooplankton. Worse yet, the clam actually eats certain zooplankton (rotifers) outright, further decreasing their abundance in the middle level of the food web and reducing the food available for fish.

Another invasive clam, Corbula amurensis, proved even more disruptive than Corbicula in San Francisco Bay, engineering major ecosystem changes in the brackish regions. "Resource managers here think about these two invasive clams in the same way people think about nitrogen and phosphorus in Chesapeake Bay," says Cloern.

Back in the Chesapeake, non-native species simultaneously provoke concern and signal opportunity. The capture this year of a third invasive Chinese mitten crab incited much worry. But, for some, a non-native oyster inspires hope for clearing the Bay's waters and bringing back a collapsed industry. Scientists, resource managers, and politicians are currently debating the pros and cons of Crassostrea ariakensi, an Asian oyster that could prove a powerful filter feeder and, if it thrives, boost commercial harvests.

Introducing a non-native species is an option that should be approached with caution, says Cloern. "Whenever we do these kind of biological interventions, there are always surprises," he warns. That's why it's critical to take an ecosystem-level perspective, he says. "We are never smart enough to predict what all of the consequences might be."

The Chesapeake's murky water needs all the filter power it can get. The Bay has one species of oyster, but it's home to dozens of native species of clams, mussels, worms, crustaceans, anemones, and sea squirts — just to name a few (see Sampling Life at the Bottom of the Bay). These species are neither commercially desirable nor susceptible to disease. And each plays a key role in cycling nutrients through the Bay's food web. Perhaps it's time to focus on the filter power and other benefits of these little-noticed species, says ecologist Danielle Kreeger.

How might managers maximize restoration opportunities with these less charismatic creatures? Restoring oysters could be the key, according to scientists, but only if they are restored as large, vertical reefs — and those reefs are then protected from harvest. If we rebuild the three-dimensional structure of the oyster reef, they say, and leave it alone, these other species will come.

"One could argue that the most important role for the oyster is making the structure for other animals to settle on."

There's only one oyster living on this concrete block used as an artificial reef, but there's a veritable smorgasbord of sea squirts, hooked mussels, and barnacles. Photograph by Kennedy Paynter.

And they will come in mind-boggling numbers. In a recent study, University of Maryland scientists William Rodney and Kennedy Paynter found that restored plots in sanctuary oyster bars held ten times as many filter feeders as degraded, unrestored areas. The sheer numbers of organisms proved even more impressive. The hooked mussel, an important filter feeder that grows in association with oysters, was more than 200 times more abundant in the restored plots. Rodney and Paynter counted over 11,000 hooked mussels in these areas, compared to just over 50 in the unrestored areas.

At these numbers, Paynter says, the hooked mussels could be filtering even more water than oysters on a restored reef. Though no one has yet done calculations that include the filtration rate of the hooked mussel, he says, the combined impact of the two together could be quite significant.

Moreover, these restored reefs attract more than mussels and oysters, according to the study. The structural complexity of a mature oyster reef provides both surface area for other fouling organisms — such as sea squirts, anemones, and barnacles — and a spatial refuge from predation for species such as the insect-like amphipods and small fish, explains Paynter.

"One could argue, then," he says, "that the most important role for the oyster is making the structure for other animals to settle on."

These lesser-known species not only help clear the water, they also redirect nutrients to other parts of the food web. The abundance and diversity of species like amphipods, that eat the feces and pseudofeces produced by filter feeders like oysters and mussels, creates "a huge catalytic flow" into other levels of the food web, says Paynter, sequestering nutrients away from the water column.

Oyster reefs also grow dynamically over time, like coral reefs, he says. So if the oysters can remain relatively disease-free, structure will beget even more structure and even more surface area as oysters increase in size and their offspring settle on top of existing shell.

Artificial reefs, a recent innovation, might serve similar functions, according to another recent study. Researchers Romauld Lipcius and Russell Burke at VIMS measured how mussels and oysters colonized an artificial concrete reef set out in the Rappahannock River. In four-and-a-half years the concrete reef, a design not suited for conventional harvest techniques, attracted the equivalent of nearly 10,000 filter feeders per square meter of river bottom. These are among the highest densities ever recorded for natural and restored oyster reefs. And the vast majority of the filter feeders were mussels, not oysters. Hooked mussels, in fact, outnumbered oysters by more than eight times.

Three-dimensional, vertical structure, like that offered by the restored oyster reef above, encourages so-called "fouling organisms" to settle and grow in large numbers, adding more filter power than oysters alone can provide. Photograph by Jake Goodwin.

For oyster reefs, with all their fellow inhabitants, to work best, they need to remain undisturbed — safe from dredges, tongs, and boat anchors. Biologist Roger Newell suggests that restoring oysters to improve water quality may simply not be compatible with a wild-bottom commercial fishery. He predicts that in the next ten years, we'll start to see a partial moratorium on harvesting and more and more oyster bars protected as sanctuaries for their ecosystem services — in low salinity areas especially, he asserts, where recruitment of larvae is poor.

The promise of ecosystem services provided by oyster reef communities holds great appeal back on the Magothy River. After the dark false mussels first appeared in 2004, the Magothy River Association, under the leadership of dive master Richard Carey, mounted a large-scale community science initiative to survey the size of populations and to calculate how much water they could filter. They counted the mussels and did the math: more than 400 million mussels in one creek (Cattail Creek) could filter all the water in 46 hours. The dark false mussels clearly had an impact on the health of their river. Now that the bivalves have gone, local citizens remain convinced that the right filter feeder could clean up the Magothy.

"If the mussels could clean up the creeks, a big enough biomass of some filter feeder could clean up the whole river," says Carey, who established the protocol and organized teams of kayakers and divers for the 2004 dark false mussel survey.

The Association looked at some of the other possibilities — clams, other types of mussels. But ultimately the group decided that oysters and their related communities would probably have the biggest impact on water quality. They also figured that the oysters would naturally resist disease because of the low salinity in the Magothy.

In June, the Magothy River Association published an ambitious oyster restoration plan. Based on their calculations of filtration rate and river area, they are hoping to plant some 250 million oysters on stone base material. If they can get the oysters — which won't be easy — the Association hopes to plant 25 million a year for the next 10 years. Planning for no harvest pressure and 50 percent survival rate for the oysters, they believe that 125 million living oysters in the river will lead to a significant boost in water clarity. Even if the oysters live just long enough to allow underwater grasses to come back, says Carey, it could make a difference. And if other organisms come along for the ride, the restoration effort may bring even more more filter power than they bargain for.

Gliding alongside a green wedge of lawn in Severna Park's Magothy waterfront, Peter Bergstrom maneuvers his kayak until he's lined up beside a riprapped bulkhead. Just then, raindrops begin to fall and he pulls on a windbreaker and wipes off his round, wire-rimmed spectacles. He turns over a large, triangular rock and sets it gently back in place. Methodically, he reaches down and picks up another one.

There, clustered on the rock's undersurface, clings a small cluster of dark false mussels. Their shells are clamped tight, a potential sign of life. Bergstrom pries one off the rock. Quickly, its shell gapes open ...empty. The mussel is dead. He didn't really expect to find it alive, but for a second, his face flashed a brief sign of hope. Just a tease, really, but those few dead mussels reminded him that for a brief interlude in the creek's history, the waters had cleared, underwater grasses had come back, and crabs had flourished. Could it happen again?

This page was last modified September 15, 2018