Mussels. Aside from some marine species appearing occasionally on menus, what do we really know about them? Freshwater members of this group of bivalves are perhaps the most obscure members of the Chesapeake Bay area’s waterways: camouflaged filter feeders dependent on whatever flows their way.
Logging trucks and even casual hikers unknowingly crush them as they splash over shallow streams. Mussels look just like rocks and generally behave as such. Immobile and silent, they are as much a part of the streambed as the cold stones they resemble, but their critical ecological role is often overlooked. And they’re increasingly endangered by our own actions.
“The most interesting aspect to me for freshwater mussels is the complexity of their life history,” said Brian Watson, a biologist with the Virginia Department of Game & Inland Fisheries. “Here we have a critter that many people have no idea exists. An invertebrate living on stream bottoms [that] would be equated to basically a living rock.”
According to the U.S. Geological Survey, “Because they have been traditionally found in large abundances that rival all other benthic organisms, freshwater mussels have the ability to modify ecosystems via their filtration and nutrient excretion activities,” thus benefiting local algae and macroinvertebrate communities, which are the primary natural components of upland Bay restoration efforts.
Most freshwater mussels require a host fish to complete their life cycle. Things can get pretty weird, like that scene in Alien where the explorers stumble upon a hidden host of enchanting but ultimately predatory eggs. “I mean, how did mussels evolve to develop a vast array of lures that mimic things like crayfish, worms, leeches and fish?” Watson asked. “How did some evolve to behave like a Venus flytrap and temporarily capture their hosts, so they can directly pump their larvae onto the fish? How did some evolve to package their larvae into structures that mimic larval fish and aquatic insects?”
What we are certain of is that the Appalachian region has long been known as a hot spot for mussels. (North America has the highest diversity of freshwater mussels on Earth.) But these fragile animals are facing increasing man-made threats.
Their ecological roles as purifiers of waters flowing into the Bay are crucial. A single adult mussel can filter up to 15 gallons of water daily, according to the U.S. Environmental Protection Agency, and in 2014, Stanford University researchers found that a batch of native California floater mussels and invasive Asian clams could remove up to 80 percent of some hard-to-filter contaminants in wastewater.
One representative species, the subject of intensive multi-agency conservation efforts to prevent its extinction, is the James spinymussel (Pleurobema collina), a tiny, shiny brown animal, slightly less than 3 inches in length that occasionally displays short thorny spines on each valve, or shell — bivalve means two shells connected at a hinge on one end. It is currently restricted to small and scattered riverine populations in the Upper James River watershed.
The James spinymussel is so rare, and its population and range so constricted, that it’s considered endangered both in Virginia and nationwide. It has suffered from “a long series of habitat degradation including dams, water quality issues, increased sedimentation and rising water temperatures,” according to Rachel Mair, a biologist and mussel expert with the U.S. Fish and Wildlife Service at the Harrison Lake National Fish Hatchery near Charles City, VA.
Like most adult bivalves, the James spinymussel is sessile, or unable to move, Mair said, and thus has no means of escaping whatever toxins or choking sediments are coming from upstream.
The mussel relies on other species to complete its reproductive cycle. Newly spawned larvae, known as glochidia, are released by female mussels into the water to find a fish host, often smallish chubs, shiners and stonerollers. Once sufficiently developed, the juvenile mussel drops off the fish host and settles on the stream bottom.
As with many freshwater mussel species, the James spinymussel employs a tricky means of landing an appropriate host for its young. When released into the water, the larvae are tucked into the outer layer of a “conglutinate,” a type of tissue that resembles bait to potential host fish.
Once an unsuspecting fish eats the tissue, the hitchhiking larvae immediately fall off and clamp shut their bivalves, harmlessly attaching themselves to the gills, fins or body of the host.
Federal and state biologists have been working to reintroduce the James spinymussel to places where it has disappeared. This year, they expect to release about 1,000 back into the wild, the collaborative propagation program’s largest crop of this beleaguered bivalve.
With the increase of extreme storms and other weather events brought on by climate change, this species is having a difficult time producing viable larvae. “I have been working with this species consistently since 2008 and have observed that a high level of storms and high-water events reduces reproductive success and larval maturity and quality,” Mair said.
“Water quality would be my guess as a main cause” of the James spinymussel’s decline, she said, adding that mussels tend to be more sensitive than other animals to certain toxic substances.
Urban runoff also poses a threat, as does sediment from eroded streambanks, and pollutants dumped into streams by unfenced livestock. Other threats, Mair said, include ammonia, which can be discharged from sewage treatment plants; heavy metals, which can be a byproduct of mining operations, petrochemicals from leaking wells and pipelines, and even hormones and pharmaceuticals that pass through treatment plants.
“Oddly enough,” she said, “the mussels are the creatures that can filter the water and clean our rivers, just like the oysters in the Chesapeake Bay.”
Most people don’t realize that their drinking water comes from rivers, lakes or impoundments that are contaminated with the same toxins that are causing declines in and imperilment of mussels.
But Mair said, “When I do outreach events, I’ll set up two water tanks, both with muddy water — one has mussels in it and the other doesn’t. The mussels will usually have the water clean in a few hours. I’ll ask my audience which water they would rather drink. People seem to really get that.” Mussels are “the canary in the coal mine” for freshwater ecosystems, she said.
The restoration of these freshwater filters, important food sources for muskrats, otters, raccoon, geese, ducks, and even some fish, will play a critical role in the health of the Bay watershed’s inland waterways, and subsequently the Chesapeake itself.