Neighborhood Bullies
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Just when the typical Nantucket Bay scallop was enjoying a day in the eelgrass, a European green crab sidles up and attacks! This invader, whose Latin name is Carcinus maenas, is a recent invasive species in North American waters and has traveled here from Europe to disrupt the ecological balance of our harbor. Common names for this small crab that quickly became a big threat include the European green crab, shore crab, and the New England nickname used by fishermen in the 1900’s of “Joe rocker.” C. maenas was first observed on the east coast of North America in Massachusetts in 1817. It was first introduced to the Pacific coast in San Francisco bay in 1989. Since its introduction along the Pacific coast, it has traveled over 500 miles in 10 years (from Monterey Bay to Vancouver Island), which is pretty darn fast for an aquatic creature. The European green crab is listed as number 5 on the top 10 list of “Animals Least Wanted” by the Nature Channel with other alarming interlopers like fire ants, European starlings, and brown tree snakes. Today, the green crab has the distinction of being one of the most prolific crabs in the New England area.
Carcinus maenas is a small shore crab (adults measure about 3'' across) whose native distribution is along the coasts of the North and Baltic Seas. Although known by the common name of green crab, color is not its distinguishing feature. The shell (carapace) color can vary widely from green to gray, yellow, or brown. Juveniles can change color to match their surroundings. Adults are generally dark greenish with yellow markings. The underside is often bright red or yellow. Green crabs do not have the last set of swimming legs or paddles seen on the rear of blue crabs, although their last pair of legs are slightly flattened and lined with little hairs or setae. For this article I found out the green crabs can be right or left handed, and for left handed crabs, and especially males, the claw (cheliped) size is larger (sexual dimorphism) although the claw size differential is not quite as pronounced as it can be for other species. They can also be identified by the five short teeth along the rim behind each eye, and three undulations between the eyes.
What makes this invasive species so prolific and dangerous? The green crab is an effective forager, adept at opening bivalve shells. Studies have shown it to be quicker and more dexterous than other crabs and capable of improving its food gathering skills over time (that’s all we need, a learning crab). It preys on a multitude of organisms, including clams, oysters, mussels, marine worms, and small crustaceans, making it a major potential competitor of our native fish and bird species. At the turn of the century, this species basically wiped out the soft-shelled clam industry of Maine and the surrounding waterways, and it has been implicated in shellfish population reductions for both bay scallops and quahogs. S.T. Tettlebach’s 1986 University of Connecticut doctoral dissertation documented that bay scallops experienced 70% mortality due to green crab predation, although they were able to fend off the green crabs more successfully once they reached a decent size toward the end of their first year of age.
C. maenas can live in all types of protected and semi-protected marine and estuarine habitats, including habitats with mud, sand, or rock substrates, submerged aquatic vegetation, and emergent marsh, although soft bottoms are preferred. C. maenas is euryhaline, meaning that it can tolerate a wide range of salinities (from 4 to 52 ‰) and it can survive in temperatures from 0°C to 30°C. The wide salinity range allows C. maenas to survive in the lower salinities found in estuaries. Apparently, it enjoys opening a new niche in the predator-prey equation in its adoptive areas as it grows larger in invaded habitat versus its natural habitat.
Green crab larvae can survive as plankton up to 80 days. Ocean currents disperse the larvae many miles up and down the coast. After a period of growth and development in the open sea, green crabs in final larval stage aggregate at night in surface waters. Tides and currents sweep them back into coastal waters where they molt and settle out as juvenile crabs in the upper intertidal zone. If the conditions in their new home are suitable, the crabs may survive and even reproduce, establishing a new population and extending the species' range farther along the coast. Some scientist believe they can even be disperse by ocean currents due to El Nino and La Nina events, which have been known to move the larvae of organisms on a global scale. To top things off, recent research in Nature by Dr. Kevin Lafferty, a USGS marine ecologist at the Western Ecological Research Center in Santa Barbara, California and his colleagues found that green crabs have much fewer parasites in their adopted homes versus their native locales. So they are learning new things and are healthier here.
There are a number of ways humans can inadvertently disperse green crab to new habitats. Scientists believe that one likely pathway of introduction is through the distribution of live seafood. Green crabs are sometimes present in seaweeds packed with lobsters and commercial oysters. If the packing material and containers are not disposed of properly, the crabs can find their way into waterways. Although heavily regulated, the aquaculture industry is also a potential source of green crab introductions. Recreational boaters transport nuisance species in bait buckets or boat wells, often without realizing it. Live green crabs are also used as bait by recreational fishers, or are present in the seaweed packed with bait. In addition, they are available for purchase from marine biological supply companies.
Scientists have also identified ballast water as a major pathway for aquatic introductions, including the larval stage of green crab. Marine vessels take on and discharge millions of tons of water for ballast each day, which may contain aquatic plants, animals and pathogens. When a vessel unloads or picks up cargo, the operator often empties the ballast tanks, thus introducing a myriad of marine life from bacteria to adult fish. In addition, barnacles, mussels, seaweeds and an abundance of other marine life attach to hulls, rudders, propellers and piping systems. Once in port, the organisms can reproduce or become dislodged and swim away.
The United Nations International Maritime Organization (IMO) instituted voluntary guidelines in 1991 calling for ships to release their ballast water on the high seas and refill the tanks with mid-ocean water, based on the assumption that species from coastal zones will not survive in the open ocean. Both Canada and the United States have established voluntary guidelines for ballast water exchange and require that all vessels entering their 200 mile Exclusive Economic Zones (EEZ) file a ballast water management report. Unfortunately, compliance, particularly in the New England area, has been spotty. Experts on the transportation of marine aquatic invasive species such as Dr. James T. Carlton (Professor of Marine Sciences at Williams College) advocate the use of ballast treatment techniques such as ultraviolet light (UV) sterilization to eradicate the miniature stowaways before they are dumped in our harbors.
Once it arrives, the green crab can thrive in many types of coastal habitats and in wide ranges of temperature and salinity. The green crab can produce up to 200,000 eggs at a time, and under certain conditions, it can survive up to two months out of water.
Although green grabs are listed as an edible species, picking out the small amount of meat is tedious and so the crab has little commercial value. Fishery ecologists and economists have determined that developing a recreational or commercial fishery for populations of introduced species like C. maenas is problematic as it is a response strategy not a control method. Creating a demand for an introduced species is more likely to encourage its presence in an area and even result in people attempting to establish populations elsewhere rather than cause its eradication. Attempts to market C. maenas caught in a trial fishery in the USA were unsuccessful, in the absence of a ready market although some success in overfishing of green crabs has occurred in Europe where a market for them does exist.
In 1995, in Edgartown, Massachusetts a bounty was paid for green crabs as part of a response to the threat to commercial shellfish. Approximately 10 metric tons of crabs were trapped in the local salt ponds, which was presumed to improve survivorship of hatchery-reared scallops and hardshell clams. This approach is also put forth in our current Nantucket and Madaket Harbors Plan, although funding for a bounty would be tricky. Fortunately, for the Chesapeake Bay area’s lucrative blue crab (Callinectes sapidus) fishery, green crabs are not found in Chesapeake Bay and it has been theorized that the blue crab could be used as a control agent for green crabs. Other research by James A Macdonald et al. in 2007, showed that when juvenile blue, green and shore crabs were put together in a tank, the green and asian shore crabs were more antagonistic than the blue and the blue’s carapace was more prone to breakage (i.e. it lost more of the fights and had to expend the most energy). Some scientists have even investigated a biological control organism, the parasitic barnacle Sacculina carcini, but initial tests show that this barnacle is not host specific and would infiltrate other crabs such as Dungeness crabs.
But there is another recent arrival to our shores that may cramp the green crab's style. The invasive Asian shore crab, Hemigrapsus sanguineus, was discovered in New Jersey in 1988. Marina floats towed from New Jersey to Massachusetts in 2000 brought with them a population of Asian shore crabs. Measuring two to three inches wide, it has spread as far north as Maine and as far south as North Carolina. Like the green crab, this Japanese import reproduces in far greater numbers than native species. The crab dines on worms, barnacles, shellfish, and algae. It has even been known to eat green crabs.
In 1998, the European green crab was formally recognized as an Aquatic Nuisance Species (ANS) by the Federal ANS Task Force (a national coordinating body). In Washington State, they have a large volunteer early detection and trapping program designed to quickly verify first sightings of the invasive crab in various harbors and estuaries. Here on Nantucket, we are not sitting idle waiting for this predator to decimate our shellfish populations. Our shellfish biologist, Jeff Mercer, places crab traps throughout the harbor and counts, then kills any green crabs found. At the field station, each summer, population estimates of crabs in Folgers’ Marsh are determined through catch, mark, and recapture experiments (see accompanying photo by UMass Boston Nantucket Field Station Coastal Ecological research student Joseph Catricala from 2007). Parts of the marsh with more blue crabs have less green crabs and vice versa. We have a large green crab in our tank right now that is missing one claw. Unfortunately, at the end of the summer, he will be having a little crab funeral.
photos credits go to: 2007 UMass Boston Nantucket Field Station (you can shorten to UMB-NFS) Coastal Ecological research student Joseph Catricala.
