Location: Washington State
Title: Invasive Marine Species
Description
The deleterious effects of non-native species in the United States drew considerable attention in the late 1980s when the population of European zebra mussel (Dreissena polymorpha) exploded in the Great Lakes, fouling power plants, dams, and locks and causing more than $70 million in damages. At the same time, a European crab was gaining a foothold out on the West Coast.
As with the zebra mussel, the European green shore crab (Carcinus maenas) was carried to American shores as larvae in the ballast of ocean-going vessels. Upon entering U.S. ports, the ballast from the ships was discharged, releasing the non-native species into new territory. Although some exotics may not survive the journey, those with flexible life requirements and rapid reproductive rates- such as the above-mentioned mussel and crab-may not only survive, but flourish and spread. First seen in San Francisco Bay in 1989, the European green shore crab has migrated southward into southern California and northward to Willapa Bay, Washington.
Although similar in size and appearance to the benign native green (or "yellow") shorecrab (Hemigrapsus oregonensis), the European variety is dramatically different in regard to diet and behavior. Not content to feed on algae and carrion like the West Coast native, C. maenas fuels a voracious appetite consuming local bivalves and crabs. In Maine, the European green shore crab was implicated for the collapse of the softshell clam fishery. One adult crab reportedly consumed forty half-inch clams in a day, and is capable of devouring crabs equal to its own size. (In addition, C. maenas may compete for important food sources with numerous native fish and birds, as well as carry a parasite that can infect local shorebirds.) In Washington, the state's oyster, clam, mussel, and Dungeness crab fisheries are in jeopardy due to potential consumption by the European crab.
Responses to this marine invasion start with educating the general public and scientific community. Fundamental to this effort is clarification as to the identity of this alien species compared to the native green shorecrab. In many cases, the European green shore crab is not actually green. The carapace may vary from dark, mottled green to orange or red, with yellowish patches on the dorsal surface. The European variety has a rounded carapace as compared to the squareish shape for H. oregonensis. Finally, close examination of the carapace also reveals five sharp spines on either side of the eyes in the European crab, while the West Coast native lacks such projections (see Figure 1).
Eradication methods of established non-native species is usually unsuccessful. In Willapa Bay, Washington, oyster growers have been fighting a losing battle against a species of intertidal cordgrass introduced from the Atlantic Ocean early in the 20th century. The non-native grass (Spartina alterniflora) aggressively replaces areas with vast swaths of grass that are productive for shellfish rearing. Handpicking, mowing, and herbicides have not significantly reduced the invasion, threatening the future of the oyster industry.
Preventative approaches offer the best hope against bio-invaders, starting with simply banning the release of non-native species of plants and animals. The more difficult and expensive measures include forbidding the discharge of foreign ballasts into coastal waters or requiring various treatments such as ozonation, chlorination, etc. on ballast water to "sterilize" it before discharge. Only time will tell if these preventative steps by local, state, and federal agencies will reverse the disturbing spread of this latest "biological pollutant" introduced into American waters.
Key Principles
Mark Plunkett
Science Division
Bellevue Community College
Bellevue, WA 98007
mplunket@bcc.ctc.edu