BIOLOGICAL POLLUTION - Marine Pollution: What Everyone Needs to Know - page 195

estuaries, where they molt and settle out as juveniles. If condi-
tions are suitable they will survive and reproduce, establish-
ing a new population. Green crabs arrived on the Atlantic
Coast of the United States in the 1800s, probably on ship hulls,
and settled in coastal bays from New Jersey to Cape Cod. Later
they began moving north, and their arrival in Maine coin-
cided with dramatic declines in the soft clam fishery. They
are a major predator of soft-shelled clams and quahogs. They
also feed on oysters, worms, and small crustaceans. They can
crack open clams and mussels faster than other crabs, and can
out-compete native crabs for food. A second major invasion
was detected in 1989 in San Francisco Bay, where they prob-
ably arrived as larvae in ballast water or in seagrass or kelp
used in packing shipments of lobsters and bait worms to the
West Coast. Ted Grosholz and Gregory Ruiz documented their
spread and effects. Their arrival was associated with losses of
up to 50% of the Manila clams in California. As they continue
to move north there is concern for Dungeness crab, oyster, and
clam fisheries in the Pacific Northwest. They also are detri-
mental to eelgrass beds since adults uproot the eelgrass, and
juveniles graze on it.
In North America green crabs have fewer parasites and
actually grow larger than they do back home in European
waters, which may contribute to their success. On the East
Coast, snails and mussels that have been living with green
crabs for over a century have developed thicker shells as a
defense, making them harder to crush than those that have
not been exposed to green crabs. The crabs, in turn, develop
stronger claws—an example of an evolutionary “arms race.”
The Chinese mitten crab ( Erocheir sinensis ) is a burrowing
crab native to the Yellow Sea in Korea and China. It gets its
name from the dense patches of hairs on its claws. They are
believed to have been accidentally released in ballast water
in the early 1900s in Germany. In the 1920s and 1930s they
expanded into many northern European rivers and estuar-
ies. The Thames River in England has also had a population