Posted by: underseaencounters | November 21, 2008

Ode to a Horseshoe Crab

My friends will never let me forget that I once turned down an invitation to a party to go see horseshoe crabs spawning on the Jersey Shore. With regularity, they rib me about this, and I admit, it does sound kind of weird and funny.

But…despite my attempts to extol the virtues of these marvelous creatures, no one, except the previously converted, seems to understand. So here is my attempt to explain why I like horseshoe crabs so much.

Horseshoe crabs are living fossils, prehistoric survivors whose basic body structure hasn’t changed much in over 300 million years. They were on Earth long, long before flowers, trees, fish, butterflies, birds, and even dinosaurs.

Horseshoe crabs give us a glimpse into earth’s truly ancient past. Their closest relatives are trilobites, which have been extinct for millions of years, and with whom they shared the shallow seas of the Paleozoic era. Horseshoe crabs are actually more closely related to spiders or scorpions than they are to the crabs you are familiar with.

Horseshoe crabs are true blue bloods—their blood is blue because it uses copper as an oxygen ferrier not iron as ours does. Its spiky tail, or telson, is used for steering or to right itself if it gets turned over. (Horseshoe crabs are completely harmless; they don’t sting or bite). They have ten eyes: one eye on each side of its shell, two small ones in the center, five light-receptive organs beneath its shell, and one in its tail.

Horseshoe crab females are 25% to 35% larger than males. Why? They need to be big to hold all those eggs–something like 80,000 eggs a year, 4 million in a lifetime. The males don’t fight each other to mate with a female. The victor is whoever gets to the female first, so smaller in this case is better. Also, a few males might attach themselves to one female to ensure that all her eggs are fertilized (the eggs are fertilized externally).

And what about all those eggs? Many eventually hatch into tiny perfectly-formed tail-less horseshoe crabs. The rest of the eggs are a feast for migratory birds, predatory fish, reptiles, and amphibians. On the Jersey Shore, Red Knotts, a migratory bird, depend on the eggs to replenish their fat supply on the long journey to Canadian feeding grounds. Without this needed fuel, they might not make it.

The peak in horseshoe crab spawning activity coincides with the new and full moon during evening spring tides. I imagine in the recent past it was a sight to see—hundreds, even thousands, of horseshoe crabs, descending on the beach. But we saw a couple of dozen at most. We did turn over a few upside down horseshoe crabs that couldn’t right themselves (that happens sometimes when they are on the beach).

Horseshoe crabs are used as bait for catching North American eel, and other fish. Like so many creatures in the sea, people have exploited horseshoe crabs almost to the point of no return. When I was a child, I frequently found their shells on the beach. They left behind light-brown replicas of themselves after moulting. But these days, I rarely find their shells on the beach.

In the span of one human lifetime, the numbers of these ancient survivors have been severely depleted.

Here is a website with more information about horseshoe crabs.

Horseshoe Crabs: Living Fossils

And a lovely picture book about horseshoe crabs:


Crab Moon, by Ruth Horowitz


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