Green Crab Lesson

Carcinus maenas: Green Invader

 The green crab (Carcinus maenas) is native to Europe, but was introduced to the East Coast of the United States in the 1800's. In 1989, it was found in San Francisco Bay, and has recently been found as far north as the state of Washington.  Green crabs can be found in a variety of habitats including rocky shores, sandflats, and tidal marshes.  They can tolerate wide salinity ranges and temperatures, allowing them to establish themselves easily in new habitats as an “invasive species.” Invasive species are also referred to as non-native, exotic, non-indigenous, or nuisance species.  They are plants and animals that have evolved elsewhere and have been purposely or accidentally relocated, and can cause great harm to natural ecosystems.  These species can spread quickly and compete with native species for essential resources such as food and space that are necessary for survival and reproduction.  The green crab is particularly adaptable to new environments, and can produce up to 200,000 eggs at a time.  Under certain conditions, it can survive up to two months out of water.

   There are many ways that the green crab may be introduced to its new environment. They are sometimes present in seaweeds that are packed in with seafood such as shellfish and lobsters. If the packing material and containers are not disposed of properly, the crabs can find their way into waterways. Recreational boaters may transport the animals through bait buckets or the crabs may be attached to boats.  Large cargo ships carry and empty ballast water in various ports, sometimes transporting larvae or young species. Well-intended people who keep the crabs as pets may also release them into the wild

   Here is one example of how the food chain might be disrupted by a crab species that is invasive and non-native: The crabs or larvae arrive through the ballast water of a ship that exchanges water when it heads into port. The conditions are right for the existence of the crab in these new waters, and it establishes itself, feeding on smaller invertebrates, including native crab species, that are usually eaten by migrating shorebirds. Food supplies for the shorebirds are now less plentiful, and native crab populations are reduced, sometimes to extinction. A chain reaction occurs, and the food chain is altered because of the single species that was introduced into waters where it didn’t naturally belong.

   The identification and management of non-native, invasive species is gaining attention world-wide, as scientists realize the harm these species can do to an ecosystem. In the U.S., public education is a critical component in helping to curb the spread of invasives, and expansive research is being conducted to find the best ways to prevent further invasions.

Student Activities

1. More on Invasive Species:  Besides the green crab, investigate what other animal and plant species are considered to be invasive and potentially destructive to native habitats in your state.  Choose one, and design a poster that would inform the public about that particular species. Include what the organism or plant looks like, why it is considered invasive, and what actions are being taken to minimize its effects on local habitats.

2. Using the National Marine Fisheries Statistics and Economics website, http://www.st.nmfs.gov/st1/commercial/landings/annual_landings.html determine the annual commercial landings of all crabs in the Atlantic from 1996-2000 in table format. Begin by typing CRABS into the box next to SPECIES. Then fill in the rest of the parameters as noted above. Why might the landings from 1996 have been so much lower than later years?

Create a line or bar graph from this data table and determine the mean and median landings. Then write a summary paragraph explaining any trends in the data.

Assessment

1. Students should complete the Student Worksheet on Crab Anatomy

2. Have each student, or small group of students, choose one species of crab to research. They should provide pictures, describe the animal, its habitat and range, its life history, and whether it is considered to be an invasive species. 

3. Ask students to explain what makes a species “invasive” and why these species are potentially harmful to native ecosystems.  Illustrate a food chain that could be disrupted by the presence of the green crab.

Additional Resources

Atlantic Coast Crabs

http://www.ifmt.nf.ca/mi-net/fishdeve/atlantic.htm

Green Crabs

http://www.cobscook.org/Links/Green_Crabs/green_crabs.htm

http://www.pac.dfo-mpo.gc.ca/ops/fm/shellfish/Green_Crab/LINKS.HTML

http://www.sei.org/crabnews.html

http://www.wsg.washington.edu/outreach/mas/aquaculture/crab.html

Blue Crabs

http://www.blue-crab.org/

http://water.dnr.state.sc.us/marine/pub/seascience/bluecrab.html

Invasive Species

http://esa.sdsc.edu/invas3.htm

http://www.wsg.washington.edu/outreach/mas/aquaculture/pathways.html