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An Excerpt from Jim Conrad's
Naturalist Newsletter of December 15, 2005
issued from Mexico's Yucatan Peninsula

HORSESHOE CRABS

Horseshoe Crab, bottom view. Image by Karen Wise of MississippiEach week the beach is different from every other week. Often I find washed-up, dead organisms that I am completely at a loss to identify -- sometimes don't even know whether they are plants or animals. It's clear that just offshore there's an ecosystem as diverse and beautiful as any I've known, but it's beyond my reach. Still, I'm savoring the process of slowly discovering what I can about this mysterious, potent place.

This week's beach was different from all previous ones in that there were more shells -- extensive accumulations of them in mint condition -- washed up than ever before. Also, for some reason, strewn randomly along the beach, there were about a dozen dead, wave-deposited Horseshoe Crabs, LIMULUS POLYPHEMUS. Usually I find only one or two of them.

Horseshoe Crabs, image by Cotting WhiteCrabs feed on clams, worms and other invertebrates, but if you look at Karen's picture you're likely to wonder where the mouth is. It's on the shell's underside in the center where the legs come together. Tiny projections at the legs' bases help grind larger itmes so they can enter the mouth. More info on Horseshoe Crab anatomy and many other topics relating to them is available at the Horseshoe Crab Website at http://www.horseshoecrab.org.

In the spring adults move onto sandy beaches and mate. Females  lay over 80,000 egs each. When young hatch they go into the water and spend about a week swimming, then settle to the bottom. As the animal grows, it sheds its exoskeleton -- 16 or 17 molts over 9 to 12 years to reach sexual maturity. Then breeding adults live for 2 to 6 more years. Adults feed mainly on marine worms and shellfish. 

If you are familiar with the leggy, fast-moving, claw-flaunting crabs along most beaches you might guess that Horseshoe Crabs aren't crabs at all, but rather their own kind of thing. In fact, they're a very ancient creation more related to spiders than to crabs. They're closely related to trilobites, which often show up as fossils in older geologic strata. At one time trilobites were among the most conspicuous of Earth's life forms. During the massive extinction at the end of the Paleozoic Era (when 50% of all the Earth's animal families and 95% of all marine species disappeared for reasons still not clear) trilobites went extinct but their relatives the horseshoe crabs did not.

Anatomically, Horseshoe Crabs haven't changed much for the last 350-400 million years -- since well before the supercontinent Pangea began breaking into the continents we recognize today, even before the first amphibians arose from early reptiles. Horseshoe crabs are living fossils and when I find them along the beach I get a feeling like the one that comes in museums when I see fastidiously wrought but rather klutzy looking knight armor from the Middle Ages.


Excerpt from the November 27, 2006 Newsletter

A HORSESHOE CRAB'S UNDERSIDE

Usually the horseshoe crabs I find washed up on beaches are shattered, incomplete wrecks of the former organism. However the other day I came upon a very recently dead one in good shape, so I studied his parts closely, and photographed him. That's the picture below.

Horseshoe Crab

In the picture I'm holding onto the crab by one of its 12 appendages. The ten large ones, called pedipalps, are for walking but the two small ones at the top, called chelicerae, are used for manipulating food into the mouth. Notice that the lowest, largest pair of legs each end in brush-like structures. These are used for stirring up and pushing aside sediment when the crab burrows into the seafloor.

In the creature's center, at the base of the leg I'm holding, you can make out a hole. That's the mouth. If you look closely you can see that the base of each leg arising around the mouth is equipped with small, brown spines called gnathobases pointing downward toward the mouth. These keep food from moving in any direction other than toward the mouth. As food works toward the mouth the spiny leg bases grind it. It's all a very simple system for a simple, primitive organism.

The five pairs of wafer-like items covering the shell's bottom half are gills. You can't see them in the picture but from each gill arise about 150 leaf-like membranes called lamellae. The moving gills keep oxygen-rich water flowing over the lamellae, which absorb the oxygen. The gills also function as paddles for propelling juvenile horseshoe crabs through the water.

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