Down to Earth with Naila Moreira: Native mussels, ‘ecosystem engineers,’ return to rivers

On the muddy bank of the Ware River, University of Massachusetts graduate student Ayla Skorupa bends over a tray of fingernail-sized mussels.

These yearling specimens are too tiny for the numbered tags she uses to identify her 2-year-old mussels. So she marks them with that most millennial of substances — glitter.

“Black, pink, gold, turquoise,” she calls out. “Bottom row, blue, gold, red.”

“You’ve got two gold,” points out her supervisor, Allison Roy, who is on note-taking duty. Roy works for the MA Cooperative Fish and Wildlife Research Unit at the U.S. Geological Survey. She’s also a research assistant professor of environmental conservation at UMass.

Skorupa grimaces. One more challenge of fieldwork. “Yeah. It was silver, but apparently underwater, silver weathers to gold.”

The mussels are brook floaters, a species of native mussel now listed by the state as endangered. For three years, these little brown wedges have been Skorupa’s charges, raised in an array of aquaria at the Richard Cronin Aquatic Resource Center in Sunderland.

She’s changed their water daily. She’s given them different types of muddy substrate. She’s tested what conditions promote their survival and growth. She’s fed them a careful diet of instant algae from the hatchery supply company Reed Mariculture.

This summer, for the first time, she’s placed them in rivers in carefully designed stone silos, hoping that the mussels stay alive and, with luck, thrive.

I’m lucky enough to have been invited along to watch Skorupa and the Roy research team at work. Today, they’re measuring the specimens’ growth. They’re also tracking water quality and habitat conditions to understand what factors promote success.

Restoration efforts

Across the United States, efforts to restore native mussels to their original river habitats are underway. Nationally, 213 of 300 total species are listed as endangered, threatened, or a species of concern.

In Massachusetts, six of 12 existing species are state listed, with one of those, the dwarf wedge mussel, also federally listed.

Mussels improve habitat quality by filtering water and by providing food for other animals. While non-native mussels can alter ecosystems in undesirable ways — such as the famously problematic zebra mussels in the Great Lakes — native mussels provide important services to habitats adapted to their presence.

“They are ecosystem engineers,” Skorupa says, “so they actually change the environment around them to benefit other species.”

Native and indigenous people once used brook floaters as a food source, and later colonists used their shells for pearl buttons.

The seemingly primitive construction of freshwater mussels — foot, gut and shell — masks a fascinatingly complex life history.

These mollusks require fish to breed, with their larvae spending two weeks to a month attached to a fish’s gills. The larvae then drop off and settle to the river bottom to grow to full adulthood.

Called “glochidia,” the larvae look like tiny sets of toothed jaws, miniature Audrey II heads out of the “Little Shop of Horrors.” They normally don’t kill fish, but, like blood-sucking Audrey, they do take nutrients from their hosts.

To attract fish, some mussels dangle a lure, a fake “prey” fish made out of the mussels’ tissue. Others possess structures that fling larvae onto host fish. Certain mussels are specific to a single fish species.

While most mussels have a lifespan of eight to 20 years, some can live up to 100 years, rivaling humans and even beating out most of us.

Human-caused challenges

A raft of human-caused challenges now faces these animals. Sedimentation and toxins from urban or agricultural runoff can damage the water quality mussels need to survive. Dams may fragment their habitat.

Climate change and loss of streambank vegetation can make rivers overheat or experience extreme flooding or drought, impacting mussel survival.

“It’s not like they can run away from any disturbances,” Skorupa points out.

Populations of brook floaters in Massachusetts have declined even over just the last decade, Roy says. Compared to populations of the mussels in Maine, where a single site boasted 300 individuals, the largest population found by Roy’s research team contained just 17 brook floaters.

Historically, the species used to populate 10 state watersheds, but now survives in only four locations — the Ware, Farmington, and Nissitissit Rivers, and Bachelor Brook.

The team aiming to restore the mussels represents a collaborative effort across agencies and organizations. The Connecticut River Conservancy, U.S. Fish and Wildlife Service and MA Division of Fisheries and Wildlife Natural Heritage and Endangered Species Program take part alongside the USGS.

An army of citizen scientists has also helped out. These volunteers surveyed native mussel populations and documented habitat conditions in rivers across the state.

The wide involvement has built up local enthusiasm. An event over the summer at the Richard Cronin Aquatic Resource Center, reporting on research progress to date, attracted an audience of over 30 people, making the mussels a kind of ambassador for river health.

‘Too precious to lose’

For the glitter-decorated mussel specimens, the day I visit the Ware field site is one of their last days in the wild, at least for now. The Roy team worries the mussels could struggle to survive the winter, so they’ll return them to the lab until next summer.

Being endangered, the animals are too precious to lose.

The silos are one way to protect them. These rounded blocks are handmade by pouring concrete into a salad bowl. A small cage in the central cavity — a hole created using a stovepipe — holds the mussels themselves.

The heavy concrete stays in place even in fast-moving water. Without the silos, the mussels would be impossible to find again to monitor and measure.

The structures create lift like “an airplane wing,” says Skorupa, encouraging water to flow through the cage and over the mussels.

Also, “one concern is tampering,” says Sam Rode, an undergraduate research intern with the team. “These designs are really low profile, and if you don’t know what you’re looking for, they look like rocks.”

The team’s mussels have had unexpectedly high survival rates this summer. The project also offered up a few other surprises. While Bachelor Brook had the lowest native population, the site actually saw the highest survival and growth rates.

That’s a hopeful sign for repopulation efforts, since it suggests these ecosystems may have the capacity to support more mussels. When population numbers are too low, it’s hard for wild mussels to reproduce successfully, Roy said.

For now, Skorupa slogs to the bank with another set of specimens to measure. “Got my bucket of mussels!” she calls.

She places six tiny animals in the measurement tray, a humble group carrying big hopes on their mud-brown shells.

One of them extends its bright cantaloupe-colored foot, like a salutation.

Naila Moreira is a writer and poet who often focuses on science, nature and the environment. She teaches science writing at Smith College and is the writer in residence at Forbes Library. She’s on Twitter @nailamoreira.

Author: Going Green

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