The highly successful marine invasive species, the European green crab (Carcinus maenas) is native to the northeast Atlantic Ocean and the Baltic Sea. However, it has made its way into other areas where it is not welcomed. When you think of a crab, you may think of one that gets along with its neighbors and makes for a good meal. That, unfortunately, does not describe the European green crab.

There is much to learn about this crab. You need to understand why it is such a nuisance and what is being done to ensure it does not completely take over beaches on the east and west coasts of the United States.

The Invasion 

The European Green Crab

The European Green Crab (Source: http://steamboatisland.org/wp-content/uploads/2015/07/green_crab_image.jpg)

This marine predator was thought to have been introduced to the U.S. Atlantic coast in 1817. The crab had spread to the north all the way along the coast of Maine and up into Nova Scotia by the mid-1900s. By the 1980s, the crab had reached San Francisco Bay and started to spread along the west coast, and in 1996, the crab inhabited over 500 km in California.

Although the European green crab has successfully spread to various locations, it has not been able to permanently establish itself anywhere. The reason is that aggressive control efforts and immediate response to their introduction work to keep populations low.

Introduction

The exact means of introduction of the European green crab to the United States is unknown. The general agreement among researchers in the field is that the crabs were introduced via ballast water. Ballast water is the water a vessel picks up and then disperses at port. It can carry many foreign species into the water at port. This can be regulated, though. Many ports have specific areas for ballast water to prevent such introductions.

They may have also been carried into the area via fishing vessels. They may get entangled with lobster or shrimp catches. However, these days, fishing is highly regulated to prevent such issues.

They also may come into the area by people who buy them online. Online sales are not well managed so that anyone can buy them. People often use them for bait and end up just tossing any unused crabs out, which leads to the invasion issues.

Lastly, they do spread naturally. The larvae move along in the waters and are carried all over. This is impossible to prevent.

Appearance

This image of the underside of the European green crab shows the unique sets of spines on either side of the eyes.

This image of the underside of the crab shows the unique sets of spines on either side of the eyes (Source: http://www.exoticsguide.org/sites/default/files/species_images/c_maenas_lg_c.jpg)

The most distinctive morphologic feature of the European green crab is its five spines on either side of its shell. This feature can be used to distinguish it from other small crabs that it could be confused with, such as helmet crabs or hairy shore crabs. Another feature that can be used to distinguish the crab is three rounded lobes that can be found between its eyes.

Despite its name, the crab is not always green. The top carapace, or shell, can vary from brown to dark, spotted green, with yellow patches. The bottom surface of the crab often changes color during molting, switching from green to orange to red. The carapace is wider than it is long and rarely exceeds four inches across. The crab’s last pair of hind walking legs is flatter than its other pairs.

Habitat and Feeding

European green crabs have been so successful as invasive species due to their feeding habits and the fact that they tolerate almost any environment.

It can be found in a variety of beach locations, including protected rocky shores, cobble beaches, sandflats, and tidal marshes. They also have the ability to tolerate a fairly wide range of salinities and temperatures.

A European green crab feasting on a clam.

A European green crab feasting on a clam (Source: https://ferrebeekeeper.files.wordpress.com/2013/12/shore-crab-eating-clam_paul-naylor_fre.jpg)

Many researchers have studied this voracious predator’s devastating impact upon various species in the marine environment. The European green crab has the ability to learn and modify its prey-handling skills during the process of foraging for its meal. It is also more dexterous and quicker than other similar-sized species of crabs.

They have been shown to alter the populations of other smaller shore crabs, clams, and small oysters. They get the blame for harming the soft-shelled crab industry in New England and Canada in the 1950s.and also devastate oyster populations. Although they aren’t able to break the shell of a mature oyster, they can easily rip apart the shells of young oysters.

Lab studies have shown that these crabs can also actively prey upon Dungeness crabs (Cancer magister) of equal or smaller size. In addition, they often harm native fish and bird populations because they come in and eat the available food sources. Although the populations of these aforementioned species are decreased by the presence of European green crabs, the crabs can also increase the populations of some species by consuming their predators.

Reproduction

A pregnant European green crab

A pregnant European green crab (Source: http://www.rimeis.org/species/images/cmaenas3.jpg)

Mating usually occurs between the spring and fall, though it can occur outside that window as well. In order to reproduce, a larger male holds a smaller female underneath him, carrying her around until she molts. After molting occurs, the mating takes place. The female then carries the mass of eggs underneath her abdomen. With favorable conditions, females are capable of spawning up to 200,000 eggs at a time!

Preventing European Green Crab Invasions

In order to minimize the potential for this shore crab to spread further, the Washington Department of Fish and Wildlife has put into place several regulations controlling the movement of shellfish and other aquatic invertebrates within the state.

One of the many measures enlisted was to place more restrictions on out-of-state imports, including hour-long chlorine dips for shellfish coming from European green crab infested areas. Once these crabs were discovered on the Washington coast, a stricter restriction was enacted, declaring it illegal to transport any live crabs within the state without a permit.

Although, Washington state is not the only area being bothered by this crab. In 1998, the Federal Aquatic Nuisance Species Task Force declared this species an official ANS. Both California and Oregan have also recognized it as a prohibited species.

The Bottom Line on the European Green Crab

While you may not think of a crab as a bad thing, when it comes to this species, too many are very bad. Once this species moves into an area, it takes over and can be devastating to the natural population. By enacting regulations, the U.S. west coast is taking a stand against these predators and trying to reduce their numbers.

Quiz Time!

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