One question that we were trying to answer this summer was whether or not female horseshoe crabs return to the same beaches to spawn, or if the masses of females we see are unique individuals each day. To find this out, we needed to mark them somehow. The marking method needed to be waterproof, rub-proof enough to survive continual abrasion by sand, harmless to the crab, cheap, quick, and easy. It would need to last at least a month. Grease pencils wear away and smudge. Lobster bands around the telson break too easily. Gluing things to their carapaces is difficult because they are wet and dirty, and most glue requires dry, clean surfaces for bonding.
When we’re tagging them for long term studies, we drill a small hole in the corner of the carapace, which I suppose is akin to cutting one’s nails in terms of injury to the crab, but it’s hard to impress upon someone that drilling a hole into any part of an animal would be harmless. Then again, considering the myriad human body parts that can be pierced, this is minor in comparison. Once this hole is drilled, we pass a tag through the hole that is indistinguishable from the little plastic t-tags that hold prices on clothes in stores, except ours has a brightly colored plastic sheath that has a four-digit number, our phone number, and the words “Horseshoe Crab Study.” These tags are meant to last for at least a year, and the goal is to track the movements of tagged crabs within and between estuaries. The scheme depends on the kindness, curiosity, and/or sense of duty of others to call us if they find a tagged crab.
Our eventual shorter-term solution did not require citizen participants and was as close as a bulletin board. We used thumbtacks in five colors, each color corresponding to five days centered on the full moon. Each thumbtack had a number written on it in permanent marker. All we needed to do was walk along the shore looking for females, and whenever we found one, she’d get a tack in the corner of her shell. If she was actively spawning she’d get a gold tack as well. After the first day we looked for previously tagged crabs in addition to tagging new ones, and surprisingly we got some repeats over the days that followed. Why is this a surprise? We ended up tagging around 400 females, but there are hundreds of thousands of females estimated to be living in this bay, and we were only tagging and looking over half a kilometer of shoreline.
What was even more surprising was the reaction from the people who harvest the horseshoe crabs. Apparently they got a bit upset, wondering who was sticking pins in the crabs. Perhaps they thought it was the work of some malicious kids, but trust me, if kids are going to be malicious to crabs it doesn’t usually involve pushpins. They can survive pretty much anything anyway; see the first entry about dissection for evidence on how difficult it is to kill one. We occasionally find crabs with clam rake holes in them, chunks missing, and mangled legs and tails. There’s a reason they’ve survived for so long. The battle tank body plan, complete with spines and armored surfaces, is ideal for surviving most attacks, particularly scientists armed with thumbtacks.