Tracking the Atlantic's Senior Citizen
Microbial Solutions
Regina Kelder

Tracking the Atlantic's Senior Citizen

What scientists can learn by observing the movement of horseshoe crabs

When Atlantic and Pacific salmon are ready to reproduce, they swim hundreds, even thousands, of miles inland to river gravel beds, where they spawn and die. This is what is known as the annual salmon run.

Atlantic horseshoe crabs are much less peripatetic. For 450 million years, these helmet-shaped arthropods have made their homes in shallow ocean waters on soft sandy or muddy bottoms. During full moons and high tides in May and June, they come ashore to spawn. Females lay up to 88,000 eggs a season before heading back to their underwater home, and they continue to reproduce, on average, for eight years.

Females typically stake out a favorite beach to lay their eggs, returning to it multiple times within a single season. Others are less loyal to a single spot. We know this because scientists have been watching them for decades.

Observing the movements of horseshoe crabs is more than an academic exercise, however. The horseshoe crab's distinctive blue blood plays a central part in vaccine contamination control. Horseshoe crabs are also important in the food chain and provide bait for commercial American eel and conch fisheries along some parts of the Eastern seaboard, including the Mid-Atlantic and New England. Learning as much as we can about the ecology of the horseshoe crab is essential.

X Marks the Spot

So using the biological equivalent of an electronic ankle bracelet, marine researchers—aided by legions of volunteers—have been able to observe the movement patterns of horseshoe crabs and to spot symptoms of ecological stress. What scientists aren't as clear about are the climatic changes or anthropogenic disruptions in habitats that might force a horseshoe crab to explore new nesting and spawning grounds and the impact those forces might be having on biodiversity.

Tagging operations work something like this. Crabs are hand collected from spawning beaches and outfitted with some type of identifier, perhaps a cinch tag or a ridged disk, which is printed with an ID number and contact information for people to call or log into if they happen to run across the marked horseshoe crab on the beach. The information supplied by these citizen scientists gets entered into databases and analyzed for migration patterns, distribution, abundance, and mortality, which in turn helps inform state and regional management practices governing horseshoe crabs.

Tagging crabs is but one tool scientists are using to analyze the health of horseshoe crab populations. Researchers are also using different genomic tools to model the genetic diversity and genetic structure of horseshoe crabs, a topic explored in the first of this two-part series about horseshoe crab conservation. These genetic studies, coupled with the old-fashioned shoe leather tactics of tagging operations, is helping to provide a much more complete picture of horseshoe crabs populations in regional waters along the Eastern seaboard.

The South Carolina Department of Natural Resources' (SCDNR) Marine Resources Research Institute, in conjunction with the US Fish and Wildlife Service (USFWS), has tagged over 9,000 horseshoe crabs since 1997, with a tag return rate of close to 20%. At least 10% of these were biomedical bled, allowing for investigation into mortality and movement patterns of these individuals, says Assistant Marine Scientist Amy Fowler.

This tag-recapture dataset has yet to be analyzed for South Carolina horseshoe crabs, but could offer long-term trends in population dynamics, including nesting beach fidelity, impacts of biomedical bleeding, distance between recaptures, time between tagging and recapture rates, and the longevity of individuals, she said.

Toward that end, the SCDNR, with the assistance of graduate student Kristin Linesch at the College of Charleston, will be combing through the USFWS tag data from South Carolina to look for trends in horseshoe crab movement patterns. The project will employ a variety of tools, including GIS software to spatially analyze the release and recapture site data, and statistical programs to estimate population size and survival rates.

Other tagging studies

Tagging studies have also been used extensively to study horseshoe crabs in the Middle Atlantic and Northeast. For instance, a 17-year tagging operation in the Delaware Bay region that began in 1986 has tagged over 30,000 horseshoe crabs. The study, led by Benjie Swan of Limuli Laboratories in Cape May, N.J., found that crabs tagged in spawning beaches tended to stick to that region, or at the very least, the same side of the bay, while crabs tagged in other parts of the Atlantic Shelf (Staten Island, Ocean City, M.D.,) migrated into Delaware Bay. There was even a case of a horseshoe crab traveling as far as southern Virginia! The study team hypothesized that the hunt for food and mates, the abundance of the population, and the potential longevity of individuals are the "most likely factors driving their peregrinations."

Another Delaware Bay study led by David Smith of the US Geological Survey's Leetown Science Center used a bay-wide mark recapture program to measure horseshoe crab abundance and to predict egg production. Further north, a study led by Sacred Heart University in Fairfield, C.T., looked at the movement patterns and population genetics of the horseshoe crabs living in Long Island Sound.

These studies have helped to alert scientists about declines in some horseshoe crab populations (the Northeast) while showing stable populations of horseshoe crabs in other regions (the Southeast). Further data are needed, so the next time you are out strolling on the beach and happen to run across a horseshoe crab, take a look and see if it's been tagged and if it has, answer the call. You could be shaping the future of an ancient creature.

How to cite:

McEnery, Regina. Tracking the Atlantic's Senior Citizen. Eureka blog. Apr 7, 2016. Available: