Spring Migration III: The Red Knots vs. The Horseshoe Crabs

Red Knot; photo courtesy of Hans Hillewaert

Red Knot; photo courtesy of Hans Hillewaert

Horseshoe Crab; female with male in tow

Horseshoe Crab; female with male in tow

The contestants could not be more different; the venue is the shoreline of Delaware Bay each May and early June; the prize is the ultimate–survival as an individual and as a species.  For those of us who live around the bay, this annual saga is well known, but for those who don’t, its a story worth hearing.

The Horseshoe Crab (Limulus polyphemus) gets no respect.  It is an unattractive, prehistoric, lumbering bottom dweller of the estuaries, more closely related to spiders than crabs.  It dates back 450+ million years, several hundred million years before dinosaurs, and therefore before birds and the Red Knots.  The fact that it hasn’t changed much over that span speaks volumes to its design however unbecoming.  The domed hinged carapace protects a creature that moves with 5 paired extremities and drags a threatening appearing tail. The tail is only used to flip it back over if the surf upsets it.  Its genus name “Limulus” means askew and “polyphemus” refers to the giant cyclops from Homer’s Oydyssey.  There are two small median eyes, hence the “cyclops” designation, but surprisingly there are two other paired eyes on the lateral carapace and multiple additional eyes on the sides and ventral surface near the mouth to track you coming and going and assist with feeding .

Even the Robins get involved

Even the Robins get involved

Each spring something stirs in the loins of the male crabs and at high tide, around the time of a full or new moon, they start patrolling the shoreline, parallel to the beach, looking for the gravid, larger female coming ashore.  If he’s lucky he finds one and attaches himself to her back for the ride with the tide up the beach.  After she digs the sand nest and lays 60 to 120 THOUSAND eggs he contributes his part of the genome.  The unlucky males are called satellites and relieve their tension by finding any unoccupied nest and doing their part.

The venue, Delaware Bay, is much younger than the contestants. We tend to think of our earth as static and go to great lengths to try to preserve it just as it is, but things constantly change. 15,000 years ago there was no Delaware Bay, but just a long Delaware River Canyon extending out to the sea at the continental shelf. As the Ice Age passed and earth warmed, the water level gradually rose over 100 feet, slowly drowning the river valley and widening the tidal estuary to its current dimensions.

View from Dupont Nature Center deck

View from Dupont Nature Center deck

The Red Knot (Calidris canutus rufa) is the beautiful cinnamon colored world traveler, spending our winter on the windswept tidal flats of Tierra del Fuego and Patagonia and migrating 9300 miles northward, usually with just two stops. This is one of the longest migratory bird routes.  The first stop is the Brazilian coast at Lagoa do Peixe, feeding on clams and snails and doubling their weight before the strenuous, 5-8 day, non-stop, 7000 mile trip to Delaware Bay.  Somehow rufa knows that the time to arrive in Delaware is when the army of horseshoe crabs are pulling themselves ashore and depositing their myriad eggs.  Wasted and starving, the birds arrive to feast on the eggs and replenish body fat for several weeks before the final 2400 mile leg to the Arctic breeding ground.

Shorebird Frenzy;  The Red Knots are grouped near the waterline

Shorebird Frenzy; The Red Knots are grouped near the waterline

The best place to witness this on the west side of Delaware Bay is at the DuPont Nature Center at Mispillion Harbor Reserve and nearby Slaughter Beach.  The center is a small observation house, education center, and deck on stilts giving you spotting scope views of the surrounding tidal flats and marsh.  Lighthouse Road leading to the center takes you along a huge grass wetland, home for abundant Seaside Sparrows and Clapper Rails. Slaughter Beach, about a mile to the south, gives you a chance for close-up views of all the action.

DuPont Nature Center

DuPont Nature Center

Seaside Sparrow

Seaside Sparrow

Willet

Willet

Timing is everything for this event; for the birds, horseshoe crabs, and for the birder.  I finally had a free day in late May and made the trek across the Delmarva Peninsula and was lucky enough to see this saga, almost.  The birds and crabs were there, but it was low tide on Slaughter Beach and the water line and birds were across a wide mudflat and only visible with scope and telephoto lens. What I didn’t anticipate was the teeming frenzy of many kinds of shorebirds and gulls; this feast is not just for the Red Knots.  Forget about counting birds, there are just too many.

Red Knots and friends at breakwater, Mispillion Harbor

Red Knots and friends at breakwater, Mispillion Harbor

Unfortunately both the Horseshoe Crabs and Red Knot populations are under stress.  The crabs have been over-harvested for eel bait, and their blue blood is drawn and used for a medical test for bacterial endotoxins.  They are returned to the bay after blood drawing, but the mortality rate may be as high as 30%.  Their population has declined 90% in the last 15 years.  The birds are stressed by the fewer crab eggs as well as the loss of habitat at both ends of their route and stopover points.  Their numbers in South America are down 50% since the mid 1980’s. There are currently some measures enacted to address all this, but only time will tell.

This saga of Delaware Bay is not really a one-on-one competition as the title of this post suggests, but rather a win-win or at least a draw. The Red Knots successfully regain their weight and strength feasting on the eggs and set out again on the final leg of their long migration to their breeding grounds in the Arctic tundra.  But what does the Horseshoe Crab get out of all of this?  For the crab, survival is a numbers game.  Its strategy is to overwhelm the sandy beaches with trillions of fertilized eggs, so even the ravished shorebirds can’t find them all before the next high tide washes the eggs to sea.  This has worked for hundreds of millions of years.

6 thoughts on “Spring Migration III: The Red Knots vs. The Horseshoe Crabs

  1. Very interesting story on the crabs. Good question on what they get out of it too. I never saw the birds feeding on them except in nature shows. Very wonderful images of yours too.

    Like

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