Horseshoe Crab Habits Detailed

BERLIN – A horseshoe crab habitat survey begun this year showed that spawning horseshoe crabs preferred Skimmer Island and the southwest tip of Ocean City, according to a volunteer survey conducted in May and June.

“Volunteers go out every night at the evening high tide around the new moon and full moon of May and June,” said Carol Cain, technical coordinator with the Maryland Coastal Bays Program. “They just count how many crabs they see within one meter of the tide line on one section of the beach. It’s all bayside beaches.”

About 5,800 of the 7,500 crabs counted went briefly ashore on Skimmer Island, just north of the Route 50 bridge. The sheltered island, off-limits to humans, is biologically productive, both for birds and the horseshoe crabs.

Male horseshoe crabs far outnumbered the females overall, with 76 percent of the count. Biologists see this as an encouraging indicator of genetic diversity in the next generation of horseshoe crabs.

“As they come in they often have at least one male or more attached to them,” Cain said.

The number of crabs counted went up this year compared to last, chiefly because of the addition of Skimmer Island and a few more sites. The survey of Skimmer Island and the three Assateague Island sites was made possible by help from the Maryland Department of Natural Resources.

The numbers are less important than the horseshoe crabs’ use of the bayside beaches.

“We’re more concerned about where the remaining sandy beaches are on the bayside. Behind Assateague, it’s still wild. Behind Ocean City, it’s become armored and rip-rapped,” Cain said. “It’s more of a habitat concern than population.”

Few horseshoe crabs were seen north of Skimmer Island, with less than 200 crabs counted at the five spots surveyed north of that site. 

“They seemed to really be concentrated downtown,” Cain said. “It looks like they’re coming in the Inlet and finding the first available beach.”

Horseshoe crabs do not return to the same beaches every year. The strength of the wind, wave energy, and water temperature all play a part in where and whether the crustaceans spawn, but there seemed to be little variation in these factors this year, Cain said.

Overall, the 2007 horseshoe crab survey is just the beginning of data to gathered on limulus polyphemus in the northern coastal bays. Past surveys were outreach oriented, rather than data oriented.

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