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Invasive Species: Asian Shore Crab

Part 5 in the Invasive Species Series:  Asian Shore Crab is Here to Stay!

by Robert Herrmann-Keeling

Excerpted/Compiled from “The Asian shore crab invades Long Island Sound” By George P. Kraemer, Ph.D.

If you have visited the rocky intertidal zone in Long Island Sound, and have overturned any rocks, you have undoubtedly seen one or more non-native species. Perhaps the most conspicuous is the Asian shore crab, Hemigrapsus sanguineus. Researchers believe it came to our shores  in the ballast water of cargo ships coming from the western Pacific  Ocean. Since its discovery near Cape May, NJ in 1998, this crab has spread north and south. Long Island Sound has been described as the “Asian shore crab hotspot.” The numbers vary from year to year, but   this crab averages more than 100 per square yard in many places. Currently, Asian shore crabs can be found from North Carolina to Maine.

asian_shore_crab

Photo Credit: George Kraemer

The successful introduction and subsequent spread were aided by ecological characteristics common to many invaders: broad environmental tolerances, waters with temperatures between 5-30°C; an expansive diet, consuming a wide range of plants and animals, and fast growth to sexual maturity along with prolific reproduction (2 to 3 three times per year, each time releasing tens of thousands of embryos.

We now know that, as a better competitor and predator, Asian shore crabs have reduced the abundance of native crabs by as much as 99% from northern New Jersey through New York and Connecticut. Asian shore crabs have caused a sharp decline in intertidal green crabs (Carcinus maenas), ironic since the green crab is another non-native species.

Larger cover (rocks and boulders, versus pebbles and sand) provide habitat for more and larger crabs. While the predominance of sandy beaches south of the mid-Atlantic region seem to limit the range expansion by Asian shore crabs, several reports have the Asian shore crab inhabiting fiddler crab burrows, though presumably not in conjunction with the original owners.

The reaction to the appearance of invaders of any kind is often “how can we eliminate them?” This is practically impossible in a marine environment like Long Island Sound. Asian shore crabs are part of what marine ecologists call an open system, meaning that all parts – biotic and abiotic – freely interact. The millions of Asian shore crabs residing in Long Island Sound release billions of larvae that drift and disperse with the currents before settling in. Asian shore crab populations from Maine through North Carolina, numbering in the billions, is one, large population of constantly exchanging individuals. Unless the crab is eradicated everywhere simultaneously, the effort is doomed to failure.

Clearly, the natural controlling mechanisms of predation, competition, and disease have not been effective in Long Island Sound. Fish in the Sound, like blackfish (Tautoga onitis) for example, eat Asian shore crabs, yet the crabs remain numerous. Human control of Asian shore crabs would also be unsuccessful, due to the openness of marine ecosystems. One approach, in which another competitor, parasite, or predator is intentionally introduced, is sometimes proposed. This strategy, known as biocontrol, fails in many instances due to unexpected consequences; for example, the control agent may attack another, desirable organism or become an invasive species itself.

With the advent of ship travel for conquest and commerce, humans began to inadvertently move marine species from place to place. And the pace of biological homogenization has increased over the past century. A number of these arrivals have established themselves in Long Island Sound. Experience reveals that marine non-natives are likely here to stay, and that we will live with the altered biological landscape.

Thanks to Dr George Kraemer, professor of biology and the chair of the Environmental Studies program at Purchase College, State University of New York, for the article on which this is based.

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One Response to Invasive Species: Asian Shore Crab

  1. Ginny Murphy

    January 2, 2015 at 8:17 am

    Great article, timely and important. I will use it in my oceanology and AP Biology classes. Thanks