Daily Invader

Chesapeake Bay Clams, Oysters, Mussels, and Shipworms

Asian Freshwater Clam (Corbicula fluminea)


Asian Freshwater Clam (Corbicula fluminea) are small clams with far reaching impacts.




The Asian Freshwater Clam is a freshwater clam native to Asia, Indonesia, Philippines and probably Africa and Australia. It was introduced in western North America by Chinese immigrants as food before 1924, and soon could be found throughout the Pacific drainages, the Mississippi and Gulf drainages, Great Lakes, and Atlantic drainages. Its rapid spread suggests that there were multiple introduction pathways and entry points including ballast water, canals, fisheries, and the aquarium fish-bait trades. It arrived in Chesapeake Bay drainages in the 1970s and is now very common in most freshwater tributaries. The introduction of such a rapidly reproducing filter-feeder has increased water clarity and provided a new food resource, but may also have far-reaching effects on food webs, nutrient and organic material transport to lower parts of the Bay, and on migratory birds and fishes feeding or breeding in the region. In areas with high population densities they compete for food and space with native freshwater mussels. Complete Record


Chinese River Oyster (Crassostrea ariakensis)


Chinese River Oysters almost became the most recent intentional introduction in the Bay.






The Chinese River Oyster is native to the coasts of Japan, China, India, and Pakistan. In the 1970s a few West Coast hatcheries started raising the River Oyster and marketing it as the new summer oyster; replacing the Pacific Oyster (Crassostrea gigas), which isn’t as palatable during the summer months. At the same time, the Eastern Oyster (C. virginica) was in decline on the East Coast due to overharvest and disease. Virginia aquaculturists started experimenting with sterile River Oysters in 1998 with the aim to replace the native with a disease-resistant oyster. These early experiments showed that the River Oyster was both disease-resistant and faster growing than the native Eastern Oyster. Soon a contentious debate began about whether fertile oysters should be stocked in the Bay. From 2000 to 2002 the Virginia Seafood Council and the Virginia Marine Resources Commission stocked over a million experimental sterile oysters to look at the feasibility of an introduction. But after years of study and debate, the Army Corps of Engineers and the states of Maryland and Virginia prohibited the introduction of fertile River Oysters and ended cultivation of sterile oysters in open waters. Complete Record


Pacific Oyster (Crassostrea gigas)


Pacific Oyster (Crassostrea gigas) was considered for introduction into Chesapeake Bay in the 1990s.






The Pacific Oyster is native to the Indo-West Pacific and is the most widely transplanted shellfish in the world, introduced to at least 42 countries. Incredibly there are no established populations in western Atlantic waters, in spite of illegal or unofficial introductions in Atlantic waters near Chesapeake Bay and Delaware Bay. But the introduction of Pacific Oysters was considered as a possible means of replacing or supplementing native stocks of the Eastern Oyster (Crassostrea virginica), which has been devastated by diseases. Both the Pacific Oyster and the Chinese River Oyster (C. ariakensis) were investigated for possible introduction until 1998 following research in the Bay, which including the stocking of sterile oysters. But initial studies found that the Chinese River Oyster had better growth and survival under East Coast conditions than the Pacific Oyster, and so further research and political interest shifted to the Chinese River Oyster. Ironically, early plantings of the Pacific Oyster in the 1950s in Delaware Bay are one of the possible means of introduction of MSX (Haplosporidium nelsoni), one of the diseases that lead to the decline of Eastern Oysters. The Pacific Oyster was, however, successfully introduced to Puget Sound WA in 1902 to replace the Olympic Oyster (Ostreola conchaphila), which was devastated by overfishing. Complete Record 


Florida Marsh Clam (Cyrenoida floridana)


Florida Marsh Clam (Cyrenoida floridana) came north but didn’t like the weather.




Florida Marsh Clams live in decaying marsh vegetation. This habitat, and their small size (19mm max), means that they are easily overlooked and uncommon in shell collections. They were first described in Florida in 1889 and there range was thought to extend from Brunswick, GA south to the Everglades, and northward to Charlotte Harbor, FL on the Gulf coast. In the 1950s and 1960s the clam was discovered in the North, Chesapeake Bay, Delaware, and Beaufort NC. But due to their small size and habitat there was some uncertainty as to whether or not these northern populations were introduced. In 1982 a study was conducted that found that these northern populations were poorly adapted to the northern climate. Their reproductive cycle was out of sync and they have high winter mortality, suggesting that this clam was a recent immigrant to the mid-Atlantic region. We believe these clams were transported in rafted marsh grass or mud on the decks of barges or coastal ships, or in mud on canal dredges coming through the Intracoastal Waterway. Complete Record


Zebra Mussels (Dreissena polymorpha)


Zebra Mussels (Dreissena polymorpha) are the most infamous of the aquatic invaders. 








Zebra Mussels are often the first thing that people think about when they hear about aquatic introductions or ballast water. Zebra Mussels were brought to the US from Europe in the 1980s in ballast water. The damaged cause to the Great Lake following their introduction pushed us toward much of the ballast water regulations in place today. Zebra Mussels are native to the Caspian and Aral Seas, and low-salinity lagoons of the Black Sea and adjacent rivers. From their initial discovery in Lake St. Clair the mussels spread into all five of the Great Lakes, the freshwater portions of St. Lawrence River, the Mississippi River Drainage, and the Hudson River Drainage. In 1991 larvae of the mussels were found in the Susquehanna River near Binghamton, NY, but an established population wasn’t found in the watershed until 2001. One population, found in a quarry in Potomac watershed, near Manassas, VA, was successfully eradicated, but many populations are now known in the New York and Pennsylvania drainages of the upper Susquehanna. In 2008 they were found at an intake at the Conowingo Dam, just above the head of tide of Chesapeake Bay. It is now thought that they are established in the lower Susquehanna and are expected to colonize tidal fresh regions of upper Chesapeake Bay. Because there are multiple means of introduction including, ballast water, fouling of boat hulls, disposal of bait, etc, their invasion of Chesapeake Bay seems inevitable. Complete Record 


Plain Pocket Book Mussel (Lampsilis cardium)

Plain Pocket Book Mussels, large enough to fit in your pocket (4-7"), but named in the 1800s after pocketbooks carried by women of that era.


Plain Pocket Book Mussels are large enough to fit in your pocket like a wallet (4-7 inches), but were named in the 1800s because they are shaped like the small purse (pocketbook) used by women of that era. They are native to the Winnipeg and Nelson River systems of central Canada, the Great Lakes, the St. Lawrence River, and the Ohio Rivers. In the late 1800s they were accidentally introduced to the Shenandoah River (1889) and the mainstem of the Potomac (1894), as glochidia (parasitic) larvae attached to fish that were stocked by the United States Fish Commission and state agencies. The larvae have a wide variety of fish hosts and include many sport fish stocked at the time such as Largemouth Bass (Micropterus salmoides), Smallmouth Bass (Micropterus dolomieu), Yellow Perch (Perca flavescens) and Walleye (Stizostedion vitreum). The abundance of the mussels in introduced areas like the Potomac is not well known, but they probably do compete with native species for food and habitat, and hybridization has been reported between the Pocket Mussel and Yellow Lampmussel (L. cariosa) in the nontidal Potomac. The glochidia larvae attach to fish for only a few days and usually don’t harm adult fish, but larval fish and early juveniles can be killed by heavy infestations. Complete Record


Gulf Wedge Clams (Rangia cuneata)


Gulf Wedge Clams (Rangia cuneata) are newcomers to a historic land. 



Gulf Wedge Clams, as the name might suggest, are native to the Gulf of Mexico from northern Florida to Texas. Back before the Ice Age (Pleistocene) they were found from New Jersey southward through the entire northern Gulf coast and northern South America, but no living specimens were reported from the East Coast, north of Florida, until about 1955. By the 1960s they were abundant in Chesapeake Bay, and by 1988, had colonized the Hudson River estuary. Even though these clams show up in the Bay’s fossil record, this new surge in population growth is the result of a recent introduction. There are several ways in which these mussels were introduced, including being transplanted with oyster shipments, transplanted as larvae though ballast water. They are now abundant in the Bay. They also provide food for waterfowl and the native Blue Crab but they also compete with native clams for space and food. Complete Record  


Forked Shipworm (Teredo furcifera)


Forked Shipworm (Teredo furcifera) are burrowing bivalves that travel around the world on wooden ships.  


Shipworms may look like worms with their long tubular bodies, but they are actually bivalves like clams. Instead of living in their shells, they use them to burrow into wood to create a home. This tropical species was first described in Indonesia in 1894 and is now widely distributed. Like many shipworms, it moved around the world in the hulls of wooden sailing ships. The first report of the Forked Shipworm on the East Coast was from North Carolina in the 1940s. Because it is so widespread, there is some uncertainty as to its introduced status south of Cape Hatteras. It was introduced into the Chesapeake Bay region in 1988 where it was discovered in submerged pine planks in Wachapreague VA. Although it is unlikely to survive winters in this region, it is one of several tropical species that is transported north by boats travelling up the coast. In 1960 it arrived in New York on the Bounty II, a wooden sailboat built for the movie “Mutiny on the Bounty”, which had been in Tahiti. Complete Record 


Naval Shipworm (Teredo navalis)


Naval Shipworm (Teredo navalis), the world traveling bivalves.  


Naval Shipworms are bivalves (like clams) that look like worms. They don’t use their shell for protection; rather they use it as a tool to burrow into wood. They live in the burrow they create in the wood, poking their heads out to feed. Historically these shipworms made their homes in the hulls of wooden ships and traveled the world. Because wooden ships have moved these species around the world for so long, it is difficult to say where they originated and where they were introduced. We believe Naval Shipworms are introduced to the East Coast because reports of this species were confined to ships and shipwrecks, but were absent in natural areas and in wood of a 5,000 yr-old fishweir in Boston, through a similar native species (Bankia gouldi) was found. But because this species has been transported by ships for so many centuries, its native region remains a mystery. Naval Shipworms were first seen in the Elizabeth River in 1878 under in debris from a wharf. In the late 1940s and early 1950s, they were reported in Chincoteague Bay, Ocean City MD, and Hampton Roads, Norfolk and Portsmouth VA. Complete Record. Click here for more on their biology.


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