Tracking horseshoes

In the dark of the new moon, people gathered at Calf Pasture Beach. They were equipped with headlamps, waders, buckets and bug spray, and they were there to solve a mystery: What’s happening to Long Island Sound’s horseshoe crabs?

Horseshoe crabs are prehistoric creatures; they look like armored stingrays until you flip one over. Then you can see its six waving pairs of legs and pincers, which resemble boxing gloves if the crab is male and scissors if it’s female.

The crab’s unique ability to fight bacteria may help explain how they’ve managed to stick around for 450 million years. But now, their numbers are falling. Jennifer Mattei, an ecology and evolution professor at Sacred Heart has observed two problems: fewer crabs are seen mating, and fewer crabs are reaching mating age.

Mattei is the founder of Project Limulus, which has been collecting data on horseshoe crabs and tagging them to track their habits since 1997. (The project takes its name from the scientific name for a horseshoe crab). The data provides a piece in the complicated puzzle of what is happening to the animals, and the volunteers assembled at 11 p.m. Wednesday night were ready to help.

Bridget Cervero, Citizen Science Coordinator at the Maritime Aquarium, demonstrated how to measure, evaluate and tag the crabs before setting teams loose to do so on their own.

Kathy Rooney, of Wilton, whose family has been volunteering to tag horseshoe crabs for eight years in a row, took up a clipboard and began handing out tags. Finding the crabs, which were coming to shore to mate, was faster than logging their information, and she was soon surrounded by a ring of people, each holding an upside-down crab in their hands like a bowl.

Rooney said she was drawn back year after year to help out and witness the horseshoe crabs during their peak shore activity, which only occurs a few weeks out of every year.

“Coming out and doing this when you’d normally be asleep is an adventure,” agreed her husband, Kevin Rooney.

During mating season, the male crabs join onto the females, sometimes forming chains of crabs four or five long. But finding pairs of more than two were uncommon Wednesday night, and many of the crabs were by themselves.

The reason why is unclear, although one theory is pollution.

“They have to see each other, right?” Mattei pointed out.

Another possibility is that it takes a certain density for horseshoe crabs to easily find each other — while there were more horseshoe crabs than tags on Wednesday night, it was nothing like the seething masses that some volunteers remembered from their childhoods.

After the data was collected, it was time to put on the tag. Citizen scientists use awls to pierce a hole for the tag through the crab’s shell, which is made of the same material as your fingernails. Cervero had reassured volunteers that the process doesn’t hurt the crab, but it does take some elbow grease.

Stick a tag through the hole, and the job is complete. She scooped up the newly tagged crab and returned it to the sea. It took a moment to reorient itself, then glided away, merging into the darkness. The last thing visible was the white circle of the tag.