Location: Washington State
Oceanic Region: Eastern Pacific Ocean

Invasive Marine Species

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 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 40 half-inch clams in a day. These crabs are capable of devouring crabs equal to their 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. Clarification of the identity of this alien species, compared to the native green shorecrab, is fundamental. 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 squarish shape for H. oregonensis Finally, close examination of the carapace also reveals five sharp spines either side of the eyes in the European crab, while the West Coast native lacks such projections (see Fig. 1).

Figure 1
Comparison of several non-native crab species.

Species	Distinguishing Features	Description
(1) European green shore crab	Five spines or teeth on each side of the shell (carapace) Three rounded lobes between eyes (frontal area) Last pair of legs somewhat flattened	Carcinus maenas European green crab Moeeled, dark green to orange or red. Up to 3.5" across the shell (carapace). Carapace broader than long. Habitat: Intertidal and shallow subtidal areas of estuaries, usually at depths ‹ 20 ft. Can be associated with cordgras (spartina or eelgrasses).
(2) Helmet or horse crab	Large jagged teeth on each side of carapace Frontal area protrudes past eyes Entire body covered with stiff bristly hairs Long, bristly antennae Spiny, short front claws	Telmessus cheiragonus Helmet crab: Predominantly yellowish green, tips of claws dark. Up to 4". Carapace wider than long with jagged triangular teeth, hairy. Habitat: Intertidal to 360 feet. Prefers subtidal areas on sandy or muddy tideflats, among eelgrass or algae. In early spring sometimes found among algae-covered rocks.
(3) Yellow or hairy shore crab	Three spines or teeth on each side of carapace Two lobes on frontal area Hairy legs	Hemigrapsus oregonensis Yellow shore crab: Varies from dull brownish-green to mottled gray, grayish green or muddy yellow. Up to 1.5". Carapace rectangular, wider than long, smooth surface. Habitat: Intertidal, under rocks on sheltered mud or gravel beaches, sometimes burrows into mud banks of estuaries. The most abundant and widespread crab in Puget Sound. H. nudus is similar in shape, but generally reddish-purple or dark purple.
(4) Red rock crab	Ten teeth each side, somewhat broad and rounded Frontal area protrudes beyond eyes Claws are dark at the tips Reddish color	Cancer productus Red rock crab: Adults generally uniform reddish color, claws large with black tips. Up to 5"-6", sometimes larger. Carapace broadly oval, uneven, slightly convex. Habitat: Middle intertidal to 260 ft. Can be found on a wide variety of substrates, but most commonly on gravel, rock, and among eelgrass.
(5) Dungeness crab	Broadly oval, uneven, carapace with ten teeth, widest at the tenth and final tooth Narrow frontal area with five unequal teeth Light-colored leg tips Large size of adults	Cancer magister Dungeness crab: Generally light reddish-brown, often light orange below; sometimes gray-purple. Little variation in color or pattern. Can be 9" or larger, but generally to about 7". Habitat: Low intertidal to 750 ft. in substrates from mud to sand. Most common on sandy bottoms, eelgrass beds, usually buried during the day. Found in Northwest estuaries and offshore waters; near shore and bays in summers.

Eradication methods of established non-native species are 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 productive for shellfish rearing with vast swaths of grass. Handpicking, mowing, and herbicide use have not significantly reduced the invasion, which threatens 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

1. Exotic species
2. Water legislation
3. Water mitigation

Ethical Considerations

1. Who is responsible for the spread of exotic species?
2. Should mitigation steps be taken, or should we let nature take its course?
3. What preventative steps should be taken, and who will pay for those steps?

Author

Mark Plunkett
Science Division
Bellevue Community College
Bellevue, WA 98007
mplunket@bcc.ctc.edu