New England Fisheries

Mean, Green Eating Machines: The European green crab is “one of the world’s worst invasives”

A scientist thinks aquaculture may be the solution to protect clams from green crabs. Photo by David Reed (dreed41) via Flickr.

This post was originally featured on New England Ocean Odyssey.

The European green crab may look small, but it has an appetite of epic proportions. These tiny 2-4 inch marine invaders can consume up to 40 small clams a day- that’s more than you’d get on your average plate of fried clams!

Why are we so concerned about these crabs? Warming ocean temperatures have allowed green crabs to persist farther and farther north along the North American coastlines. Where cold winter chills used to keep its numbers in check, populations of green crabs are now booming places like the Gulf of Maine, and they are eating their way through our precious local seafood.

The European green crab, Carcinus maenas, is not exactly new to the northeast. In fact, it first arrived in the waters off Cape Cod during the 1800s, from its native range along the European coast and Northern Africa. Green crabs have since expanded their range northward through New Brunswick, and have even made their way over to the west coast, likely hitching a ride in ships’ ballast water tanks, or with commercial shipments of live seafood.

Green crabs dwell in many types of marine habitat, from rocky tidal zones to sandy beach flats, and are extremely good competitors. In a recent study, green crabs were found to be much more successful in introduced regions- including the east and west coasts of the US- as compared to their native regions. Crabs in the non-native study areas were also found to be larger and less affected by parasites, whose numbers were greater in the native region.

The green crab is a professional clammer- able to dig up and crack open young clams and oysters with ninja-like skill. A single crab can consume nearly three-dozen small mussels per day, and will basically try to eat anything around its size or smaller. Other crabs, fish, and even young lobster are all fair game for these tiny eating machines. In fact, green crabs may be the primary culprit in shutting down commercial clam harvesting in parts of Maine. Even worse, some fishermen in Maine are certain that predation from green crabs is responsible for shrinking numbers of other commercially-important mollusk populations – namely, mussels and oysters.

Fishermen worry that once the crabs work their way through shellfish populations, their next target may be lobsters. Green crabs are known to prey upon other species of crab and some fish, and have been shown to prey upon juvenile lobsters in a laboratory setting. The Washington Department of Fish and Wildlife also reports that green crabs are capable of learning and honing their predation techniques- a scary thought for our Maine lobsters!

A “parasitic castrator” of the green crab – the barnacle Sacculina carcini. Photo by Gwylan via Flickr.

What can we do to defend our coastal seafood communities? Several management strategies have already been put into practice, including trapping and removal programs, chemical controls, and even protective netting for juvenile clams. However, the most interesting, and possibly most controversial, proposed method of control is to introduce a natural enemy. Sacculina carcini is a parasitic barnacle of European green crabs, which impairs its host’s reproductive organs, rendering them unable to reproduce. Parasites that do this are collectively known as ‘parasitic castrators.’ Some sources have suggested utilizing this species to curb crab populations, but recent studies have revealed that the parasite is capable of infecting other species of crabs in addition to the green crab, which may put native species at high risk. This, along with many unknown factors associated with introducing another non-native organism, make this type of biological control an unlikely solution to our green crab dilemma.

CBC News recently deemed green crabs to be “one of the world’s worst invasive species”, reporting problems associated with spikes in green crab populations as far north as New Brunswick. With ocean temperatures rapidly rising, the green crab is likely to continue its territory and population expansions. It is fast becoming one of the biggest threats to New England shellfish populations, and will need continued monitoring and novel control strategies in order to preserve local fisheries and prevent further destruction to our marine life.


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