European Green Crab

About the Species

Common Name:European Green Crab / Shore Crab
Scientific Name: Carcinus maenas
Lifespan: 5 Years
Origin: European to North African Coasts
Identification: Color varies from green to brown, gray or red. Approx. 5cm and 12cm wide. Has five short teeth along the rim behind each eye, and three undulations between the eyes.

Classification:

Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Family: Portunidae

First Introduction

The European green crab was first discovered in the 1800s when the species was brought to Cape Cod, Massachusetts, and is now located in the eastern U.S. from the Gulf of St. Lawrence to Delaware. In 1989, European green crabs were discovered in San Francisco Bay, and now they inhabit the West Coast up to British Columbia. Green crabs have also been recorded in Australia, Sri Lanka, South Africa, and Hawaii.

The likely dispersal was in result of the transportation of ballast water in ships, or in seaweed that was used to pack live marine organisms. Ballast water is water that ships take from one body of water for increased stability and maneuverability, then released in another body of water. The disbursement to other locations is unknown but is credited to rare long-distance events, possibly caused by human actions.

C. maenas are impressive with their reproductive abilities; females can produce up to 185,000 eggs at one time. Young crabs will live among seaweeds and seagrasses in the intertidal zone and eat until they reach adulthood. They are an omnivore predator, feeding on a wide range of organisms, particularly bivalve mollusks (such as clams, oysters, and mussels), polychaetes, and small crustaceans (including their own).

Reasons of Establishment

C. maenas can live in many extreme situations, and their anatomy makes them a perfect candidate for an invasive species. They can live in both salt and brackish water (at least semi-protected, and even though soft bottoms are preferred, they can establish in bottoms with sand, mud, or rocky substrates. Since this crab is euryhaline, they can withstand a wide range of salinities and temperatures. Their wide tolerance of tolerance (4%-52%) allows them to live in lower salinity bodies of water such as estuaries, or higher such as the ocean. Similarly, their temperature tolerance (0°C-30°C) allows them to survive in colder climates, beneath the ice in the winter, for example.

New Ecological Role

In general ecological and economical terms, there is no doubt that C. maenas will affect marine biodiversity. Although it is hard to objectively quantify impacts, introduced areas are is constantly changing as the crabs are disrupting the natural fauna. Since this species is a fierce and opportunistic predator, it is affecting organisms on various trophic levels. It eats at least 104 families, 158 genera in 5 plant and protist and 14 animal phyla, it could possibly cause extinctions, extirpations and reductions in abundance of native species. It is fighting its way to become a dominant species and removing those who crucial to the ecosystem and those who are important to the economy.

Effects

Decreased native clams (Nutricola spp.) in California, allow invasive clam to populate

Decreased eelgrass (Zostera marina) in Southern Gulf of St. Lawrence, Canada

Decreased lobster and crabs in various locations gloabally

The consumption-ability of C. maenas, along with the high population rate, has reduced many populations of native species in areas across the globe, thus affecting the fauna.

On the South African Coast

P.J Le Roux, G.M Branch, and M.A.P Joska explore C. maenas in the Cape Peninsula (the discovery place) and 117km north and 10km south where they disbursed. The methods they used to retrieve the crabs are handpicking via snorkeling, and by setting up rectangular traps (60 x 40 x 20 cm), covered with stretched-mesh nylon that were retrieved within 48 hours. The gut contents of 50 crabs collected (21 males and 29 females, ranging in size from 28 to 71 mm) were analyzed for their gut contents, stomach volume, and percentage fullness. In this study, they looked at:

Size Frequency and distribution
Diet
Predation

Table 1. Mean number of prey items eaten by Carcinus maenas, their percentage occurrence and percentage volume.

Figure 1. Histogram showing the percentage composition of the diet of Carcinus maenas individuals in Blouberg Lagoon

From the analysis of the gut contents (Figure 1) the most important prey items were gastropods (snails and slugs), isopods (order of crustaceans) and polychaetes (worms) . Mussels were, surprisingly, absent from the gut contents of all the crabs analyzed. Algae found in the gut were usually present in small quantities, possibly accidentally ingested.

The size-frequency distribution (Figure 2) will show the size class of the crabs. If the crabs are more larger, they could have established for a longer duration of time and adapted to their new environment well. If the size is smaller, they might not have adapted as well, or they are newly introduced. If the growth rate of the crabs in Table Bay Docks is similar to that elsewhere, then the size-frequency distribution of captured crabs suggests that the population in Table Bay Docks is well established and at least seven years old, whereas that at Blouberg Lagoon is between four and five years old. Isolated records and sizes of crabs suggest that the populations at Green Point, Sea Point and Camps Bay are between three and four years old.

Fig. 2. Size frequency distribution of C. maenas populations in (a) Table Bay Docks (February-October 1984), (b) Table Bay Docks (April-May 1989), (c) Blouberg Lagoon (August 1984), and (d) Blouberg Lagoon (March-May 1988). Arrows indicate in which size class the majority of male and female crabs are found at each site. Data for (a) obtained by trapping, for (b), (c) and (d) by diving

Efforts of Abolition

Laws/Education

In 1998, Washington State made it illegal to possess or transport European green crabs. It is a prohibited species in Oregon and California. In the same year it was formally recognized as an Aquatic Nuisance Species (ANS) by the Federal ANS Task Force.

There are numerous programs in the US to help monitor the invasive. For example, Washington Department of Fish and Wildlife started a non-profit volunteer group to raise awareness of the invasion and to monitor for its presence along the coast of the state. They hope that their efforts will be continued and expanded by increasing available funding for programs that raise public awareness and sustain increased monitoring by training and incorporating citizen scientists.

Biological Controls

A proposed biological control agent of C. maenas is their native parasite, Sacculina carcini. Although abundant in their native range, it is most likely that they did not get transported with their host in the non-native areas. Trials have shown that the introduction of the parasite to non-native areas will have a slight positive outcome. The parasite would parasitize both the European green crab as well as not target species. Researchers are looking for a species-specific parasite in hopes of targeting strictly the invasive, but more studies will need to be conducted.

Chemical Controls

There have been different attempts with this use of this control, but the easiest and cost-efficient is the use of poisoned bait. Also, it is not size-selective (as traps are) and more efficient for large scale reduction. Scientists used this method on the east coast of North America, and there was some success. The bait is soaked in a chemical poisonous to the crabs when consumed, and knowing the diet of the crabs, they are more likely than not to eat it. However, there is a chance non-target species might consume the bait, thus reducing their species as well.

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