The Horseshoe Crab Diaries Part Six: In Which Paul Shows a Nice Photograph and Explains Molting

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So several things. We went to Cape Cod this last weekend and found evidence of everyone's favorite chelicerate: limulus polyphemus! I thought this photograph was actually one of my best in a while. Even after owning my camera for about six years I'm still learning things about it. I was playing with the various white-balance mechanisms and found that you can have the camera measure the light and tweak the white-balance. Seems really obvious but I just never bumped into the option before. That provided me with a couple of fairly nice shots and this one which I think is well above average.

So I hear this a lot: "I went to Cape Cod/the beach/the marsh/Duxbury/etc. etc. and I saw a dead horseshoe crab/my brother picked up a dead horseshoe crab/it was crawling with dead ones etc. etc." But 99 times out of 100 it's not a dead animal. It's usually what is called a molt. Limulus, like all members of the phylum arthropoda, must remove its exoskeleton periodically. It can be a slow, difficult process because every last inch of its body is covered with the chitinous armor including eyes and its little legs. This is why the molts are often mistaken for dead animals: it looks just like a complete limulus.

Not a dead limulus!

The reason why is that tough armor, great for keeping them safe from predators, is rigid and cannot grow. Instead of growing gradually like most vertebrates they grow in spurts. When they're ready to get a little bigger first they stop taking in water, dehydrating and shrinking the soft parts of their body within the exoskeleton. They then start a crack right along the front ridge of their "helmet" and slowly wriggle their way out of the old armor. The outside of their body is soft at this point but ready to become hard and chitinous again. The arthropod will then grow, usually a full 30% of their current body size, and then the outer layer of their body begins to harden again. The whole process can take a few weeks so arthropods that grow in this way are especially vulnerable to predation during this time. Limulus, lobsters, crabs all seek extra shelter and typically focus entirely on staying safe rather than feeding or any other behaviors during this time.

Many people have eaten soft-shell crab or lobster. These are not different species but crabs and lobsters that have been trapped just after molting. By designing a trap to look like a comfy, safe cave fishers are able to trick the arthropods into crawling right inside. Instead of safety they walk right into the waiting clutches of hungry terrestrial vertebrates.

The next time you're walking around on the mud flats keep an eye out for these signs of limulus.