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In its native waters, a hungry green crab feeds on marine animals that its ancestors may have been feeding on there for millions of years. Together, those animals help make a balanced ecological system. But when that same green crab travels to a new locale, it suddenly can become a party crasher­ and the party it crashes can be an entire marine ecosystem.

On land or in water, an ecosystem is a complex community of organisms that functions as a whole. The introduction of a non-native species (also known as an exotic or bioinvader) does not necessarily spell doom for a habitat, but it can easily disrupt a delicate balance. In a marine environment, for instance, an exotic fish or invertebrate may find few or no natural predators. This allows it to swiftly multiply, monopolize food sources, crowd out native inhabitants, and potentially spread disease to local species.

Green crabs are just one example. Since making their way from Europe to the Eastern Coast of the United States in the 18th century, and more recently to the West Coast, they have caused widespread problems.

In the Northeast, marine invaders include the Japanese shore crab (Hemigrapsus sanguineus), the common periwinkle (Littorina littorea), several species of sea squirts, an algae known as green fleece (Codium fragile tomentosoides), the green crab (Carcinus maenus), and mute swans.

New introductions are sometimes intentional but most often unintentional. While we know how they occur, predicting the environmental and economic consequences of an invader is tricky. Here are a few ways that exotic marine species find themselves in a new home.

Ballast Water
Let's say a ship starts out from X, headed for Y, to pick up a load of iron or apples or computer parts. On that first part of the voyage, the ship may carry millions of gallons of water —known as ballast water— for better maneuvering and stability. When the ship reaches its destination, it will release much of the water to make way for the cargo. Because that water, taken from another sea, estuary, lake, or river, also contains living plants and animals, those organisms are also released into waters that may be similar to their home environment.

In the United States, the most notorious freshwater invader to arrive in ballast water is the zebra mussel. Since its introduction to the Great Lakes in the mid-1980s, it has caused immense environmental woes, with a price tag of billions of dollars.

Hitchhikers in marine ballast water include plants such as algae and animals such as snails, sea squirts, comb jellies, crabs, and fish.

Recreational Boats
Boats don't have to use ballast water to transport species. The smallest motorboat or sailboat, when moved from one body of water to another, can also carry a creature or plant on its hull, shaft, or propellor.

And Baits and Trailers and...
Boat trailers, motors, anchors and axles can also transport exotic species. And bait buckets can transport non-native baitfish species, snails, clams, aquatic weeds, and tiny larvae. In New England, marine worms used as bait are regularly shipped around the world, often in algae that also carries other hearty organisms.

Aquaculture —or the farming of fish, shellfish, and algae— supplies increasing amounts of seafood worldwide. As marine aquaculture expands globally, the potential for aquaculture to be a culprit in marine bioinvasions increases as well. For example, shrimp imported for aquaculture may carry a virus that can infect native species. Also, an imported species that escapes into the wild may threaten indigenous animals or plants by preying on them or winning the competition for food. For instance, on the West Coast, Atlantic salmon have escaped from aquaculture pens and may pose threats to the native Pacific salmon.

Also known as Hemigrapsus sanguineus, this Japanese import was released from ballast water in New Jersey around 1987 and has since pushed its way north into Massachusetts and south into North Carolina. An omnivore with an appetite for young clams, scallops, oysters, algae, fish larvae, and many other species, these crabs may well pose a threat to New England ecosystems and aquaculture operations.

The Japanese shore crab measures 2 to 3 inches across and has a square-shaped shell with three spines on each side. Its claws have red spots, and its legs have alternating light and dark bands. Depending on its location, this crab can range from a mottled, deep brown to purple, orange, green, and pink.

Mneniopsis leidyi is a comb —a jellyfish-like animal that feeds on zooplankton and is native to New England. Since its export to Europe's Black and Azov Seas, this creature's population has grown so much that it has caused a significant decline in stocks of anchovies, which also consume zooplankton.

If you fish, be sure not to release bait, fish, shellfish —or the materials in which they are stored or shipped— into new environments. Always empty your bait bucket on land and discard unused bait in trash.

If you boat, be sure to remove any plants or animals from your boat, trailer and other equipment. Drain all the water from the motor, livewell, bilge and transom well away from the water. Wash your boat, tackle, trailer and other equipment with 104° water, a high-pressure spray, or at the very least, with tap water. Then dry everything for at least five days to make sure that none of the exotics have survived.

If you have an aquarium, don't dump fish or plants into a waterway. Dispose of them in the trash.

Scientists can't map new range expansions or new discoveries until new finds are verified. If you see a plant or marine creature that looks like a new invader, you can help out by contacting the Sea Grant program in your state, the local natural history museum, a University field station or a local U.S. Fish & Wildlife Office. If you're concerned about ballast water invasions, contact your state and federal regulators and find out how they are addressing this problem.

MIT Sea Grant's exotic species web pages

Nonindigenous aquatic species information from the United States Geological Survey

Sea Grant nonindigenous species site

Northeast aquatic exotic news

Northeast Sea Grant's Feature on Marine Bioinvasion