Writing & Photography of Jim Murtagh

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The Writings & Photography of Jim Murtagh

Invasive Crabs - Kids study Asian shellfish

Exposed and vulnerable, half a dozen dime size crabs dart for cover. Their tan and black striped legs shuffle them sideways across the moist sand. Franticly, they search for a place to hide. Some crabs disappear into crevices among nearby rocks, while others attempt to conceal themselves by burrowing into the beach. The crab’s mottled dark brown and green carapace provides effective camouflage against the wet rocks and scattered aquatic vegetation, but is still no match for the savvy hands of the group of Westbrook Middle-Schoolers that are grabbing them and dropping them into buckets.

Another rock is over-turned, and crabs scatter like cockroaches suddenly exposed to light. The kids react quickly, and additional specimens are collected. The tide is low, and the slippery rocks along Meigs Point at Hammonasset Beach State Park make for unsure footings, but this group of sixth and seventh graders is on a mission. They need to collect 50 crabs from their designated quadrat, and record the crabs length and sex before the autumn sun sets.

Julie Ainsworth is leading this scientific field trip offered by Bauer Park and the Madison Beach and Recreation Department. Assisting Ainsworth is Westbrook teacher Cathy Lepore, and her band of volunteer students. Together, they are participating in a multi-national study to document the spread of the Asian shore crab, an invasive species rapidly besieging the East Coast of the U.S. Ainsworth says “Things that are non-native may not necessarily be bad, but the thinking with this crab is that they are more troublesome.”

David Delaney, a PhD candidate at McGill University, Quebec, organized the study in an effort to predict the spread of non-native crab species, such as the European green crab and the Asian shore crab, and to validate the effectiveness of citizen scientists to collect research data. Ainsworth became involved in the study about three years ago, after responding to an open call to educators along rocky shorelines. Delaney met with Ainsworth and her Nature Club at Bauer Park, and provided all the necessary instruction and collection materials. Now several times a year, Ainsworth calls on her crew of budding marine scientists to scour the beach and collect crabs. Their data, along with data from a thousand other volunteers, is compiled and analyzed by Delaney.

The study is still in the data collection and analysis phase regarding crab populations, but has already revealed an interesting tidbit about the effectiveness of volunteer data collectors. “We found that younger kids were actually doing a better job than the older kids, who got bored after a while. The little guys just keep going at it.” says Ainsworth.

The Asian shore crab, which is indigenous to waters from southern Russia to Hong Kong , was first recorded in the U.S. at Cape May , NJ in 1988. According to Ainsworth, this species of crab was initially identified by a young student on a science class field trip. Since then, the crab’s presence has been documented as far south as North Carolina , and as far north as Portland Maine . The crab’s range is expected to increase along Maine ’s coast.

It is theorized that the crab’s introduction onto the U.S coastline resulted from crab larvae being released during ballast water discharges of transoceanic vessels originating in crab infested waters. Ships fill their ballast tanks with seawater to gain stability before heading into the open ocean. When the ship arrives at its destination port, it dumps millions of gallons of water filled with non-native aquatic organisms into the local ecosystem. Since Asian shore crab larvae suspend in water for approximately one month before developing into juveniles, they are highly transportable.

The inch and a half long Asian shore crabs thrive in the rocky intertidal zone; the area between the high tide line and a couple feet below the low tide limit. They are voracious feeders, and their diet consists of salt marsh grass, small invertebrates, mussels, clams and the green crab. Ainsworth notes why this crab is so prolific “They can withstand huge temperature and salinity ranges, as well as low oxygen levels.” She also mentions, “Females have a much longer breeding season than other species of crab. It seems like they are reproducing so fast that they can dominate very quickly.” Female crabs produce 50,000 eggs per clutch and are capable of producing 3-4 clutches per breeding season.

In just 18 years, the Asian shore crab has become the predominant crab along the rocky CT shore, and has effectively replaced the European green crab, another non-native but larger crab species which arrived in the 1800’s. The pace that this crab is spreading is astonishing. Ainsworth mentions a recent study which showed “Blue mussels are actually evolving thicker shells in response to the Asian shore crab. They (Asian shore crabs) are definitely having a sudden impact.” The students collected and examined 200 crabs on this outing, of which, only two were green crabs, and both of them were below average size.

Sitting on the large rocks along the beach, the students take each crab from their bucket and carefully measure it with a plastic ruler. This information, along with the crab’s sex, and the location of any missing appendages, is recorded on a datasheet attached to a clipboard. After all the specimens are documented, the crabs are released back into the Sound.

One boy asks “What if we came back in the winter, would the crabs still be here?” Ainsworth is ecstatic that the students are showing a real interest in the project, and decides right then that a winter trip should be planned.

If you want to learn more about Delaney’s invasive-species study, go to: http://www.invasivetracers.com For more information about the Bauer park program, or to participate in the next crab count, call Madison Beach and Rec at: 203-245-5623 or on the web at: www.myrecdept.com