Out Late With the Crab Crew: Tagging Ancient Arthropods
Tag Along! This post is the first of a series about the world of animal tagging and tracking technology.
Before midnight, they emerged from the dark water at Coffin Point.
Scuttling in on the high tide, invisible under the new moon, just a few arrived at first. Then pairs formed, their low-domed shells occasionally bowled over by incoming waves. Then whole piles appeared, hundreds of creatures crawling over one another in pursuit of a single goal: a mate.
Then we showed up to crash the party. *
Refined by 445 million years of evolution, the many-legged horseshoe crab is more closely related to scorpions than crustaceans. They play a critical role both in human health and coastal ecology, which is why S.C. Department of Natural Resources (SCDNR) biologists want to learn much more about these little-studied animals – and why you’ll find them wading into the Atlantic Ocean under every full and new moon in the spring, when the romantic interludes of mating crabs are at their peak.
DO THE CRAB SHUFFLE
The evening did not start out promising. As we drove south to Beaufort, the radio station cut in and out with high-pitched interjections from the National Weather Service: warnings about severe thunderstorms in the area, 60-mph gusts, large hail. The sky behind the car shone bright with the light of an early June evening; before us, it looked like dusk had already arrived.
But after a blinding rain in the ACE Basin, the sky grew clear. SCDNR marine educator Emily Foy and I met up with our crustacean-expert colleagues, Dr. Amy Fowler and Robin Frede, to start our mission: a nighttime survey to count and tag horseshoe crabs. Our survey site for the evening was Coffin Point, a small community overlooking St. Helena Sound.
Amy and Robin outlined the plan. Walking four abreast in the water, with team members standing from ankle-deep to thigh-high water, we would shuffle along the seafloor, feeling blindly with our feet for the smooth, hard shell of a horseshoe crab. When we bumped into one, we’d have to move quickly to scoop them up (horseshoe crabs can zoom with surprising speed over the sand), haul them out, determine their sex, and deposit them upside-down high up on the beach. After surveying our stretch of shoreline, we’d work backwards to methodically measure, tag, and release each crab we’d brought ashore.
We waited until sundown and reviewed some horseshoe crab anatomy. Then it was showtime. Headlamps on, cameras and phones stowed away in drybags, we lined up in the water and started slogging.
As our eyes adjusted to the growing darkness, the team fell into an industrious groove, steadily hauling crabs out of the water and onto the beach. The rapid-fire call and answer between team members and our note-taker kept pace with our shuffling down the shoreline:
“Single male, got it.”
“Pair – and one satellite male.”
“Pair with satellite male, confirmed.”
“Pair – and the female is tagged!”
Horseshoe crabs are “tagged” with small, plastic discs that snap into the outer shell, or carapace. And tags are little nuggets of research gold – simple yet extremely valuable markers that identify an individual crab with a unique tag number. Every time a tagged crab is recaptured and reported, biologists learn a little more about the migrations and population sizes of these mysterious animals.
THE ART OF CRAB TAGGING
At the end of Coffin Point’s beach, we trudged out of the water and set straight to tagging crabs. The tagging process is quick and straightforward, pioneered by a national tagging program started in 1999 by the U.S. Fish and Wildlife Service (USFWS):
- Grab a horseshoe crab
- Figure out and record its sex (mature females are much larger than males; males have a special set of claws that some say resemble boxing gloves)
- Measure the widest point of the horseshoe crab’s shell on a measuring board
- Disinfect and drill a small hole in the crab’s hard exterior shell
- Record tag number and pop the tag into the hole with a satisfying click
- Bring the crab back to the water’s edge and release
We had a particularly fruitful night, tagging hundreds of new-to-science crabs and recapturing dozens of already-tagged animals – many of which had been tagged within the past month and were still hanging around St. Helena Sound.
REPORT TAGGED CRABS FOR SCIENCE… AND CRAB SWAG
So what do you do if you find a horseshoe crab with a tag on it? Report that baby!
Each tag has a phone number and a URL, either of which you can use to report your crab. Make sure you take a picture or write down the tag number so that you can provide it, along with information about where you found the crab and what condition it was in (alive or dead), to the USFWS.
If you’d like, the USFWS will even send you crab swag in thanks for your contribution to science: a pewter horseshoe crab pin for the first tag you report (#stylegoals), as well as information about where else your crab was spotted before it crossed paths with you.
* No horseshoe crabs were harmed in the making of this post, and all interrupted parties were quickly returned to the waters from which they were pulled for another shot at the title.