Hidden among the roots and stems of  in Ocracoke's marshes live clusters of a little-known bivalve known as Atlantic ribbed mussels.
Ribbed mussels drawing by Pat Garber.

By Pat Garber

Hidden among the roots and stems of Spartina Alterniflora (cordgrass) in Ocracoke’s marshes live clusters of a little-known bivalve.

Atlantic ribbed mussels (Gaukensia demissa) are similar to the delectable blue mussels often served in restaurants, but ribbed mussels are not so tasty.

They do, however, provide an even more important service. They filter bacteria, heavy metals and toxins from the waters of Pamlico Sound, stabilize the shoreline, and help stem erosion.

“They are the perfect indigents to improve water quality and keep wetlands happy and healthy,” said Joe Reynolds, executive director of Save Coastal Wildlife, a New Jersey nonprofit dedicated to education and restoration along the Jersey shore.

Atlantic ribbed mussels are yellowish-brown to black mollusks with ribbed shells and iridescent blue to silvery white interiors. They live in regularly flooded tidal marshes and mud flats, where they attach to the base of grasses and to each other using mucus strands secreted by byssal glands.

Found from Nova Scotia to the Gulf of Mexico, they are seldom seen except by boat and at low tide. Two to four inches long, they can live more than 15 years. They grow a new rib each year, so their age can be determined by counting the ribs. Their sex can be determined by the color of their mantle–yellowish in males; chocolate-brown in females.

Ribbed mussels are filter feeders. They open their shells slightly at high tide, extending two syphons to bring in water. Gills lined with cilia remove oxygen and trap plankton and organic matter.  The organic nutrients are processed into inorganic matter, which is recycled back into the mud, helping to enrich the marsh. One mussel can filter up to 1.8 gallons of water in one hour. After they close at low tide, they may oxidize sulphates to synthesize ATP needed for respiration.

Reproduction takes place in summer when larvae are released into the water. They settle near the parent mussels and develop into juveniles, reaching reproductive age between two and four years.

Worldwide, many kinds of mussels are declining, and here in North Carolina at least eight freshwater species are on the Endangered Species list.

Ribbed mussels, however, are extremely resilient. They can endure drought and extreme fluctuations in temperature and salinity. They are “photosensitive,” which means they can detect the presence of predators such as blue crabs or raccoons and quickly close their shells. As a result of these survival adaptations, their populations are listed as “stable.”

According to Junda Lin of the Institute of Marine Sciences at UNC, who conducted a study in 1989, these mussels are “a major component of the macrofauna of the marshes” all along the Atlantic coast.

Reynolds noted that due to sea level rise, ribbed mussels are losing habitat and scientists are trying to find solutions.

He described how NOAA Fisheries Service researchers conducted a pilot project from 2011 to 2013 using ribbed mussels in an industrial area in the South Bronx, not far from a sewage treatment plant. The waters were closed to shellfish harvesting because of bacterial contamination.  Scientists monitored the condition of the ribbed mussels and water quality over time to see how each responded.

They found that after two years “the mussels were largely healthy and still living, and that they had removed an estimated 138 pounds of nitrogen from the Bronx River,” he said.

The researchers estimated “that a fully populated 20 x 20-foot mussel raft similar to the one used in this study would clean an average of 3 million gallons of water and remove about 350 pounds of particulate matter, like dust and soot, daily.”

The study has implications for waters all along the east coast, including Ocracoke.

When I first moved to Ocracoke in 1984, I was determined to collect and eat my own seafood. Using my little sailing Snark I followed the creeks through the marshes, where at low tide I spied clumps of ribbed mussels clinging to marsh grasses in the mud.

Curious, I began reading up on them. I learned that they were seldom eaten by human beings, as they were said to be tough and muddy tasting. I wanted to try them anyway, so I collected a few. I left them in a bucket of sound water for a day to filter out some of the mud, and then tried them steamed and fried with cocktail sauce. I found them edible but far from tasty.  

Now I read that eating ribbed mussels harvested at low tide (which I did) can be dangerous since this is when they might contain toxins and bacteria. I suffered no bad effects but am not recommending them to anyone for eating. Leave them in the marsh to do their work.

I exalt these humble-looking, seldom-seen mollusks, silently and diligently filtering away to keep Ocracoke’s ecosystem healthy for all.

This story was originally published in the September 2020 issue of the Ocracoke Observer.

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