A New Invasive To Worry About: Manila Clams

CHATHAM – The town has the distinction of being the first in Massachusetts to observe the presence of a new invasive in coastal waters — the Manila clam.

 The clam, a native of the northwest Pacific Ocean region, was first observed in Chatham waters in 2023. Since then, live Manila clams have been found in other areas of the Cape and as far north as Boston Harbor and Salem Sound. 

 Just last week, “somebody found them in Pleasant Bay,” according to Shellfish Constable Renee Gagne. 

 “It’s on everybody’s radar now, so that’s another reason why we’re seeing more, because everybody’s looking for them,” she said.

 Manila clams resemble quahogs in size, shape and color but have crosshatched, raised ridges not seen in the native bivalve. According to a paper put out by the Massachusetts Office of Coastal Zone Management, Manila clam shells are roughly oval shaped, cream or gray colored and can have dark lines, chevrons, streaks or other patterns. One tell-tale difference between Manila clams and quahogs is that the lunule (the depression where the two sides of the shell come together) of a quahog is heart-shaped, while the Manila clam lunule is oval-shaped, according to the CZM paper. Like quahogs, Manila clams burrow in intertidal areas.

 According to a research paper in the journal Biological Invasions published in July — which lists Gagne as a contributor — Manila clams have a worldwide range and are “one of the world’s most widely introduced bivalves,” though its appearance is unlike any other bivalve found in New England waters, which makes it easy to recognize.

 There were intermittent reports of dead Manila clam shells found in New England and New York area waters as far back as the 1980s, according to the Biological Invasions article, while the Chatham discovery was the first time living Manila clams were reliably identified. The article provides evidence that the bivalve is now established in Massachusetts and the Northwest Atlantic, “representing the completion of its journey across the global north.”

 According to the paper, possible vectors for introducing Manila clams to local waters include ballast water from ships and the live shellfish trade. There is no evidence that the species was intentionally introduced here. However, that remains a possibility since Manila clams are available in supermarkets, bait stores and online where they are sold for both human consumption and use as bait. 

 On the West Coast, where Manila clams were first reported in the early 1900s, the species is commercially harvested and grown in aquaculture operations. They were intentionally introduced in France in the 1970s to diversify the country’s aquaculture industry. The species was also intentionally introduced in Hawaii and Sweden and is widespread in European coastal waters. According to the CZM paper, the species is grown in aquaculture operations in its native range along Japan, China, Korea and Russia and in non-native locations

 “It is one of the most economically productive clam aquaculture species globally, surpassing $8 billion in value and four million tons in 2022,” the report reads.

 Manila clams were first found on the flats between South Beach and Monomoy, where shellfishers were digging for steamers, Gagne said. The clams were sent to the Barnstable County Extension Program where they were identified. 

 “We are seeing more of them recently,” she said. That may be due to the fact that quahogs have taken over the flats between South Beach and Monomoy, and quahog raking turns them up more easily than clam raking, she suggested.

 Gagne said while Manila clams are “good to eat,” there is no commercial market here because they are considered invasive and they must be culled from commercial catches. Recreational shellfishers are free to harvest them, she added.

 “If people harvest them, they can take them home and eat them,” she said. “Just don’t put them back.” The invasive has yet to be reported in local embayments like Stage Harbor, where much of the town’s recreational shellfish occurs, she added.

 Commercial shellfishers, however, are encouraged to remove Manila clams out of concern for the impact on native shellfish. It is unknown if the invasive clams carry diseases that could impact local species or how aggressive they are in replacing natives.

 “Are they going to impact our native species? Are they better equipped at reproducing? Are they going to take over certain areas?” Those are the questions Gagne said concern scientists and shellfish officials right now. CZM has set up a Manila clam working group that meets monthly to review ongoing research, monitoring, sightings and to discuss management implications.

 “The best thing to do with invasives is to take them out if you find them,” Gagne said.