Asian Crabs Gone Wild?

Thu., May 22

The question, "Which crab is king along the Atlantic coast?" just became a little more interesting.

Michael Gilman, a master's degree candidate in biology at Southern, is wrapping up a study that challenges the presumed invincibility of the Asian shore crab, an aquatic invader from the Far East.
By casting his scientific inquiry further offshore and deeper into the ocean than other scientists, Gilman has uncovered evidence of a natural control on the small, scrappy crab that has been overtaking the region's inter-tidal crab habitats for 20 years.

"For the first time, we have been able to demonstrate there is a control for this invasive species," says Sean Patrick Grace, assistant professor of biology and Gilman's adviser. "But we are finding it in a place in which no one ever looked."

The Asian shore crab, which measures about 1.5 inches across its main shell, or carapace, is native to the western Pacific Ocean. Its vast natural habitat stretches along the coasts of Russia, the Korean peninsula, China and the islands of Japan. In the Pacific, natural predators and parasites keep the population under control.

The crab, whose colors range from green to purple to orange-brown to red, began turning up on the shores of the eastern United States in 1988 at Cape May County, N.J. Scientists believe it hitchhiked to the Atlantic coast in the ballast water discharged from Pacific trade ships.

The tolerant crab is not a fussy eater and easily adapted to its new habitat. In fact, it quickly dominated and displaced native crabs in inter-tidal habitats along the shore from Maine to North Carolina, including Long Island Sound. The inter-tidal zone refers to the shoreline mudflats, where land and sea meet at low tide.

 "Today, the Asian shore crab is pretty much the only shore crab you will see on the beach in Connecticut," Grace says. "When your kids turn over rocks on the beach and uncover crabs, 99 percent of the time, they are finding this crab."

In addition to overtaking green crabs, mud crabs and rock crabs in inter-tidal zones, the Asian shore crab can "shred" lobster, Gilman says. Its presence also affects mussels and other coastal habitats.
"Any time this crab comes in, it just takes over," he says.

To scientists and beachcombers, the Asian shore crab's sudden appearance and rapid and expansive dominance suggest it might be a textbook example of an unchecked aquatic invader gone wild. Gilman's work, however, suggests that fish living further offshore have developed a taste for Asian shore crab.

"Nobody ever looked off shore before," Gilman says. "Everybody stopped looking at the tidal line." And just 30 to 40 yards off shore, the Asian crab's reign ceases, based on Gilman's findings.

Assisted by his peers, particularly Miguel Reyes, a fellow Southern graduate student in biology, and undergraduate Kristina Clarke, Gilman designed and carried out a study to investigate the stomach contents of several species of fish inhabiting  Connecticut's sub-tidal zones - that area of the ocean that remains submerged at low tide. He also set out to survey, more generally, the diverse crab species living in sub-tidal areas.

By catching and studying a broad sample of fish and examining their stomach contents, Gilman has discovered that some local fish species are feeding on Asian shore crabs.

Gilman has looked at several species, including striped bass, black fish, cunner, oyster toad fish and black sea bass. He and Reyes have gathered hundreds of crabs and fish in water ranging from 3 to 12 feet deep at Cedar Island Marina in Clinton.

Gilman and Reyes fished and trawled from about June to December 2007. Of the fish Gilman sampled, black sea bass had the highest frequency of Asian shore crab contents in their stomachs.
For his study site, Gilman chose the same location studied by former Southern biology professor Peter Pellegrino in the late 1980s. Pellegrino's study documented the diverse species inhabiting Connecticut's sub-tidal waters, he says.

While scientists believe that the Asian shore crab has largely displaced other indigenous species in inter-tidal areas, Gilman found virtually no changes in the types and numbers of crabs inhabiting the sub-tidal zone in Clinton 20 years after the Asian shore crab's arrival. The numbers of green crabs, mud crabs, blue crabs and other local crabs he found were similar to Pellegrino's findings almost two decades ago.

"Really, the only different thing I found is the presence of the Asian shore crab in the stomachs of these fish species," Gilman says. "It is being incorporated into their diet."

Gilman's study may be replicable throughout Long Island Sound and possibly the rest of the Atlantic coast, which could lead scientists to reconsider the impact of the Asian shore crab and other invasive species.

The significance of Gilman's study is that it gives future investigators a starting point from which new hypotheses may be formed and tested, Grace says.

In April, Gilman presented his findings in Providence at the Benthic Ecology Meeting, an annual event for students and researchers, mostly along the East Coast. His study is also expected to appear in an upcoming edition of Northeast Naturalist.