While Homeland Security officials monitor the Maine coast for unwanted terrorists, another threat is also under surveillance — the intrusion of aquatic invasive species.

The National Sea Grant Office reports that aquatic invasive species (AIS) are “the largest single threat to our coastal ecosystem, our coastal economy, and human health in the coastal region.” They define AIS as “species not native to an ecosystem, and whose introduction to that aquatic ecosystem can harm the environment, public health or welfare.”

“Alien,” “bioinvader,” “non-indigenous,” “introduced,” “non-native” and “exotic” are some of the terms used to describe plants, animals and microorganisms living outside their native territory. The use of the term “invasive,” however, is generally reserved for those species “which may cause harm” by displacing or consuming native organisms or by competing for resources necessary for their survival.

The introduction of aquatic invasive species may result in loss of diversity, as the invasive species overtake an area. ASI may even contribute to the eventual extinction of some native species, as they crowd out and feed on native species.

According to Maine Department of Marine Resources (DMR) scientist Pete Thayer, ASI that have already invaded Maine waters include European green crabs, Asian crabs, shipworms, tunicates (sea squirts), MSX (single cell parasites which are lethal to oysters), Orthomyxovirus (a virus that causes infectious salmon anemia), and Codium (a bushy plant also known as “oyster thief” and “dead man’s fingers”).

Thayer said that the European green crabs were the first major AIS to invade Maine and were the first the DMR tried — unsuccessfully — to handle.

“Once they get into a stretch of the coast, as long as the temperature is right, they’ll multiply and establish themselves,” explained Thayer. “With 3,000 miles of coastline that’s a daunting task to prevent that.”

Thayer added that Asian crabs are currently a big threat to Maine’s economy.

“We’re concerned about what effect these might have on juvenile lobsters,” said Thayer.

Asian crabs were first spotted in the U.S. off the New Jersey coast in 1988. They were discovered in Maine about four and a half years ago, when a marine biologist from Cornell University found one in Casco Bay. Since then, Asian crabs have been reported in Penobscot Bay (off Owls Head), around Isle au Haut and at Schoodic Point.

No one knows for certain how Asian crabs first arrived in the U.S. However, some have speculated that they were transported in ballast water discharged from ships traveling from Asia.

Ballast water is one of many means by which ASI may be introduced to a new environment. In addition, some species may attach themselves to ship hulls; educators and science supply companies provide schools with biological and research specimens that may escape or be released into the water; non-native species are sold at pet stores and over the Internet; public aquariums and labs may pump seawater into tanks containing different species of animals and then back into the ocean; seaweed used for packing may transport non-native species from one state to another; fishing gear or marine equipment may transport various species to other locations, if the organisms become attached to gear or equipment; and natural forces such as wind and currents may also carry eggs or larvae to new areas.

Thayer identified several ASI as “potential invaders”: Chinese mitten crabs, Pacific oysters, whelks from the mid-Atlantic and other tunicates.

“They may decide to require hull cleaning or ballast water testing,” said Thayer. “Right now we’re involved in risk assessment to get a handle on the highest priority.”

For further information visit the National Sea Grant website: www.seagrant.noaa.gov/themesnpa/aquaticinvasivespecies.html.