- RYAN BROMWELL, SCIENCE DEPARTMENT CHAIR
It’s been happening for millions of years. Ancient denizens of the Atlantic coast – horseshoe crabs – crawl from the depths and converge to mate on the shores of Delaware Bay. It turns out that much of the coastline of Delaware and New Jersey surrounding Delaware Bay is prime breeding habitat for the crabs. And they come by the thousands.
On an evening in May, when the tides are strong, the crabs so densely pack the surf that you would have to jump eight feet to clear the pile. Females dragging males clamber up the beach and lay their eggs which are quickly fertilized. A single female can lay up to 4,000 eggs at a time – up to 88,000 in a single year. This bounty doesn’t go unnoticed. Shorebirds consume large amounts of them. Some birds like the tiny red knot layover in Delaware Bay for only a few weeks to dine on the nutritious eggs before continuing their flight from Tierra del Fuego in Southern Argentina to the Canadian Arctic. Horseshoe crab eggs are an attractive source of energy, especially if you are a little bird making a long migration.
This amazing convergence of horseshoe crabs and shorebirds recently attracted three Dons (Michael Bell '16, Wyatt Rahl '16 and Erik Van Zijl '13) and three Loyola Blakefield faculty members (Ryan Bromwell - Science, Sally Waller - English and Leandra Laird - Art).
They joined students from the National Aquarium’s "Aquarium on Wheels" program to census horseshoe crabs and catch a glimpse of the elusive red knots. To count the crabs, students donned headlamps for night walk along Pickering Beach in central Delaware.
Horseshoe crabs mate under the cover of darkness at high tide. By placing a quadrat, a one meter square PVC frame, into the surf at fifteen meter intervals, the students sampled the number of male and female crabs. The data will be forwarded to the National Horseshoe Crab Survey project coordinated by scientists at the University of Delaware. The preliminary results were promising. Students were finding three to five females in each quadrat. Male counts ranged from the teens into the twenties. By the end of the late night census, the crabs had begun to crawl back into the sea, and the students were ready to crawl into bed.
The next morning students witnessed a less idyllic result of the previous evening’s horseshoe crab jubilee. At a second beach, Port Mahon, many crabs had gotten stuck. Rip-rap, rocks piled along the coast to prevent shoreline erosion, had proven a tricky obstacle for the crabs. Horseshoe crabs driven ashore by the tides and the millennia-old urge to spawn had become trapped in the rocks. Dons to the rescue! Senior Erik Van Zijl noted, “We saved a lot of horseshoe crabs. I learned how much of an impact these animals have and what a small thought they are often given.”
Another student rescued a diamondback terrapin caught in the rocks. Usually at home in the vegetated shallows of marshes, terrapins head for higher and dryer ground each spring to lay their own eggs. A closer inspection of our rescued female confirmed that she was carrying eggs. Without our help, the rip-rap may have proven an impenetrable barrier. As a light rain began to fall, the students reluctantly left Port Mahon behind. All agreed that the challenges horseshoe crabs face are significant.
Finally, the trip culminated with a visit to the DuPoint Nature Center in Slaughter Beach. Along a narrow strip of protected barrier beach shorebirds of all types flock. Hoping to spot the brown and white wings and auburn tummy of a red knot, students scanned the beach using binoculars. Patient students were rewarded. Amid the clamoring gulls, cormorants, dunlins, willets, sandpipers, and ruddy turnstones, a few red knots skittered about.
Unfortunately, this may be the only time. As horseshoe crab numbers have declined in recent years, so too have red knot populations. “The red knots need the horseshoe crab eggs to survive their migration,” notes Rahl. Staving off the decline is a problem that not only affects birds but humans as well.
Horseshoe crabs are used as bait for fishermen who use them to attract whelks (locally called conch) and eels – two fisheries that have become increasingly lucrative as Asian markets for them grow. In addition, horseshoe crabs possess specialized amoebocyte cells in their blood that swarm around invading bacteria. Chemical extracts from these cells called Limulus amoebocyte lysate or LAL, for short, are used today to test pharmaceuticals for bacterial contamination. The test works so well that it was adopted by the Food and Drug Administration as a standard test for pharmaceutical contamination in 1983.
Many students never get to witness firsthand the biology and ecology of animals and plants that we humans rely on every day. The horseshoe crab is a remarkable example of a resilient species, but one that needs our conservation efforts too. Seeing the crabs and shorebirds, especially the red knot, at this time of the year was a great opportunity for our students and we are grateful to our partners at the National Aquarium who invited us to join them.
View more pictures from the trip here.