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Twice each year shorebirds along the eastern seaboard migrate thousands of miles between North and South America. In the fall they head south to their wintering grounds, and then return to their northern breeding grounds in the Spring. To make this trek and still survive means finding high protein foods to provide the energy they need. And there aren’t many items on a bird’s menu that packs in more protein than the eggs of other animals. This is where the horseshoe crab comes in.

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Delaware Bay is home to the largest colony of horseshoe crabs in the world. Each spring tens of thousands wait for the sun to set on the evening of the first full moon, when the gravitational effect of our satellite is strongest, and the ocean rises far up onto the beach. Males and females have spent the previous few weeks pairing off using a process known as aplexus, with the male laying claim to the much larger female by attaching to her exoskeleton using special hook-like appendages on the front pair of legs. When the tide has reached its peak the females haul themselves and their mates out of the water at the tide line. Here she digs a shallow pit and deposits tens of thousands of tiny gray eggs in the sand.

Once the cluster of eggs is laid she lumbers forward to position the male over the pit, and he promptly fertilizes them with milt deposited from seminal glands attached to the book gills. The female then pushes her way back towards the water line, covering the eggs with sand as she leaves. This is repeated several times during the night as the tide recedes, each nest positioned a bit closer to the low tide line each time. In all, the female will have laid over a quarter million eggs.

Every adult that was originally born on this beach has migrated from deeper water to converge on this shore. On some beaches as many as ten thousand individuals can be seen spawning on the same night. There won’t be another tide this high until the next full moon and the embryo's development is timed so that hatching occurs during this next tidal event, allowing the tail-less young to be washed away as they emerge from their shallow nests.

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Gray horseshoe crab eggs mixed with dark sand.

Simply by looking at the moon phase on any calendar we can predict when this spawning event will occur. And by a remarkable feat of co-evolution, so can the shorebirds. The migration north to the Canadian and Arctic breeding grounds is timed so that the flocks of birds arrive at Delaware Bay exactly on the night the horseshoe crabs come ashore.

There are six species of shorebirds that depend on the horseshoe crab embryos, and the punctuality of the adults, to successfully migrate. None are more dependent on the eggs than the red knot (Calidris canutus).

Before moving on, I should note that this bird’s name has nothing to do with rope. "Knot" is an englishfied version of "Knut". "Knut" is a Norwegianfied version "Canute". King Canute the Great was a Viking leader who’s followers claimed was so powerful he could stop the sea from advancing. To show that God’s will is stronger than any mortal’s, he had his throne placed on a beach at low tide and commanded the sea to stop. When it didn’t, it proved his point. The first part of the bird’s name does refer to its color, however.

The red knot is a species of sandpiper. It breeds high up in the north of Canada and Greenland, often inside the Arctic Circle. Its migration is one of the longest of any species of bird, covering over 9,000 miles to the tip of South America. A migration like this is brutal on the knot, and the energy needs to successfully complete it are enormous. This is especially so for the females making the trip northwards since they will also be using up lots of energy for egg production.

The migration pattern for the east coast knot population passes over Delaware Bay on the day the horseshoe crab spawning is due. As the various flocks arrive, they gather on the beaches and mud flats of the estuary. A full 95% of the entire knot population is present at the same time. When the crabs start coming ashore, the frenzy begins as the birds gorge on eggs throughout the night. When the spawning stops, and the horseshoe crabs return to the ocean, the knots rise into the air and head towards the arctic practically non-stop.

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Any degree of failure of the horseshoe crab spawning is a potential disaster for the red knot. Even if the number of horseshoe crabs is fairly high, but not full strength, the effect on the birds can be severe. In fact, to the red knot a half-showing on the crab’s part is no different than a total spawning failure. The reason is that the eggs only become accessible to the shorebirds if the crab breeding density is high enough that the digging females disturb each others nests. Packed tightly together on the beach, one female digging a nest will inadvertently excavate the contents of another. This is the only way the buried eggs can become available to the birds, since shorebirds can't dig down through the sand.

Negative impacts on horseshoe crab spawning can be anthropogenic or natural. Sever storms, especially those accompanied by rough surf, will hamper the spawning efforts. And prolonged storms may effectively cancel an entire breeding season. Embryos will perish if the female cannot bury them under sand in time.

Man-made threats are many, but the principal one is from overfishing. The crabs are used as bait in eel pots and whelk traps, and in areas with historically high populations the harvest is extensive. This animal takes up to ten years before it is mature enough to spawn for the first time, and fewer and fewer of them are reaching this milestone.

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It’s ironic that even the total failure of a year’s horseshoe crab crop has a much greater impact on the fate of the birds than on the crabs themselves. Outside of harvesting pressures, horseshoe crab populations are fairly stable. This is mainly because of their delayed sexual maturity as well as the long life they lead once they reach adulthood (thirty to forty years).

Still, over-harvesting has taken its toll on the horseshoe crab population. We no longer see the mass breeding events of tens of thousands of crabs on the beaches. More common today are multiple groups of only dozens or hundreds. And the effect on the shorebirds has been dramatic. Compared to the 1970’s, the number of red knots is down a whopping fifty percent.

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Other diaries in this series can be found here.

Originally posted to Mark H on Thu Oct 15, 2009 at 06:12 PM PDT.

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