Horseshoe Crabs and Red Knots
Each Spring, horseshoe crabs show up on our local beaches and barrier island inlets to spawn. They come ashore where the beach slopes gently and the sand is just the right mixture of soft and firm. On the extra high tides of full and new moons, the beach can be literally covered in crabs. I saw this once several years ago on the beach at Harbor Island and can still picture the hundreds of crabs covering the beach and us walking carefully through and around them.
As the water warms in the spring, the female, which is larger than the male, waits just offshore for a male to grab hold of her back. She then comes ashore to lay her eggs, dragging the male behind her.
She digs a hole, burrowing in with the front of her 'shell', lays a batch of eggs, then crawls out. As the male is dragged over the eggs, he fertilizes them. At the same time, there are other males crowding around trying to get their sperm onto the eggs as well. Its a crazy scene. Check out this video.
In this picture, there is one female - in the top left quadrant. The others are all males hoping to pass on their genes.
Two to four weeks later, the eggs will hatch into the most adorable little prehistoric creatures (originating 450 million years ago) you have ever seen. By the way, I am in search of a molt from a very young horseshoe crab if you ever find one :-)
Seemingly unrelated, but wait for it.....
The Red Knot is a robin sized bird that migrates 18,0000 miles every year! I can't fathom this. A friend of mine said "I don't even put that many miles on my car in a year!" They spend their winters in southern South America (where it is actually summer) and fly all the way up to the high arctic each spring to breed and raise young.
When they take off from South America, a red knot will weigh about 200 grams. By the time they land here on the east coast of North American (a nonstop flight!!!!) they weigh half of that. During their stay on South Carolina beaches they must replenish their bodies and build up stores for the remainder of their migration. Horseshoe crab eggs provide a rich source of concentrated food, when they need it, where they need it. Without that, red knots would not make it to their breeding grounds.
Red knots are not the only shorebird that depend on the horseshoe crab eggs. Semipalmated Sandpipers, Sanderlings, and Ruddy Turnstones have long migrations and replenish their stores on the eggs. Other birds who happen to be in the area also feast. How many different bird species can you find in the pictures below?
The morning after we went out on Harbor Island beach to see the horseshoe crabs spawn we went back to see the shorebirds feasting on the eggs. There were thousands! An incredible sight. Though many eggs stay buried and become little crabs, many of the eggs are brought to the surface of the sand by wave action or by other burrowing horseshoe crabs. These eggs will never hatch, but the birds are crazy about them.
A Fragile Dependence
Here in the Lowcountry we can see this amazing coming together of species. From Georgia up to New Jersey, the birds know when and where to land in order to feed their starved, exhausted bodies. The birds' lives depend on those horseshoe crabs spawning in those same locations year after year.
To keep these species alive, we need to be sure the beaches are protected where the horseshoe crabs spawn, that they are not harvested in unsustainable numbers, and we must allow the birds undisturbed time to replenish their bodies so that they can finish their migration and reproduce.
I am in awe of nature. The Horseshoe Crabs and Red Knots exhibit resilience of 450 million years, superhuman stamina (18,000 miles every year!), uncanny timing. We must respect and preserve this fragile dependence between species.