Explosion of false dark mussels revives hopes for clearer waters and concern for dogs in Anne Arundel County

Paul Spadaro talks about an explosion of dark false mussel growing in Cattail Creek in Severna Park on Wednesday, July 11, 2018. Video by Jen Rynda / BSMG

Paul Spadaro grabbed the line to pull in his floating dock last weekend and stopped. The soggy rope was crusted with something tiny, dark and solid, like BBs.

He looked closer: mussels.


Swarms of small mussels coated the line and his oyster cages submerged in Cattail Creek off the Magothy River. The dark false mussels, though familiar, haven’t been seen in such quantities in more than a decade, said Spadaro, president of the nonprofit Magothy River Association.

“I picked up the line and said, ‘Oh, my God,’ ” Spadaro said Wednesday on his pier. “We’ve noticed an explosion.”


Considered native to the Chesapeake Bay, the mussels — commonly called “dark false mussels” — grow to about the size of a fingernail. They’re food for ducks and crabs in the bay and its rivers.

Out of the water, they sparkle in the sunlight as the shells open and close for tiny breaths.

“This is abnormal,” Spadaro said. “We’re seeing mussels in massive numbers, sufficient enough to have a cumulative effect to start filtering the water.”

Spadaro wrote his friends and neighbors Tuesday with his surprising discovery, but he also warned them.


The mussels have been suspected of causing liver failure in dogs that ate them.

During the winter of 2014, about 20 dogs suffered liver failure and half of them died, Dr. Carl Rogge, of Arnold Veterinary Hospital, said at the time.

Families reported their dogs falling sick after munching on clumps of mussels. Still, Rogge and other researchers couldn’t say for certain what in the mussels would sicken the dogs.

Mussels are natural filters and clean local waters where they proliferate. This filtering causes the mussels to collect pollutants, including human pathogens, such as those from a sewage spill. And yet, the mussels can purge themselves just as quickly.

During a surge in the mussel population in 2004, there were reports of dogs growing sick and dying after eating the shellfish. In November 2004, three dogs were sickened after eating mussels from Spa Creek. Two of them died.

A decade later, in November 2014, mussels in Dividing Creek in Severna Park resulted in the death of two dogs. A third dog nearly died the next month when its liver enzymes, normally between 10 and 100, jumped to almost 7,600 before he recovered.

Researchers with the state Department of the Environment tested samples of mussels back then and found no toxins. One possible, though unproven, explanation was that dogs ate rotting mussels with a deadly bacteria.

Another possibility: Mussels trigger a chocolate syndrome, meaning that a food usually harmless proves toxic only to dogs. Biologists had no reports last year of dead ducks, otters, raccoons or other animals that eat mussels.

Researchers have also considered whether the sharp mussels cut dogs' mouths and throats, opening lanes for pathogens.

The mystery continues today.

“There has been no cause and effect established between the false dark mussels and the dog illness,” said Jay Apperson, spokesman for the state Department of the Environment.

Nonetheless, Spadaro repeated the warning in his email Tuesday to neighbors on the Magothy.

“Please be mindful for your pet,” he wrote.

Still, the mussels bring hopes for clear waters in the Magothy this summer.

He said his team of researchers have already noticed the waters clearing in the creeks. They have found clumps of the tiny mussels covering bulkheads, lines and crab pots.

The mussels were so thick on an oyster cage submerged off the pier of his neighbor, Lise Crafton, that she could hardly extract the oysters.

“One or two mussels won’t do anything,” Spadaro said, “but millions and billions of them working together do the trick.”

Clearer waters allow sunlight to reach the depths, helping grow the aquatic grasses that have all but vanished from the creek. The grasses, in turn, shelter crabs and fish. These plants feed on nutrients that pollute and cloud the waters.

Spadaro said the dark false mussels thrive in fresher waters. The rainy spring flushed the creeks and rivers and salinity levels are unusually low.

Back in 2004, during the last big spawn, the mussels filtered Cattail Creek every 48 hours, Spadaro said.

He said neighbors could see maybe 6 inches underwater before the mussels, but 10 feet down after.

“You could drop The Capital newspaper and read it underwater,” he said.

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