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Horseshoe Crab

I've been working my way through back episodes of the PBS series "Nature." If you have never seen it, go to their website, they have tons of full episodes. It's amazing, really. Anyway. Recently I came across an episode titled "Crash: A Tale of Two species" and it drew me in in a way I had never expected, especially for a show whose major star is a weird looking invertebrate. Without doing an entire plot synopsis, it details the intricate relationship between a small, migratory sea bird known as a Red Knot, and the strange, ancient Horseshoe Crab. So today, inspired by that episode, we dive in to the world of the Horseshoe Crab!

Image from University of Delaware
First off, it isn't actually a crab at all. Though it belongs to the Arthropod phylum, along with crabs and other crustaceans, its subphylum is Chelicerata, home to spiders and scorpions. Moving further down the taxonomic chart, the four species of Horseshoe crab are the only living members of their entire Family, Order, and Class.  What that means in a nutshell is that they are quite unique genetically, and that they split from their nearest living relatives a long, long time ago. (The fact that there are only 4 species in the entire class is really strange when you consider that the three other classes in phylum Arthropoda- Insecta, Arachnida, and Crustacea- have one million, seventy thousand, and twenty thousands species respectively)

So I've already mentioned their ancient-ness. But just how ancient are they? Well, fossils of their ancestors have been found that date back 360 million years. That's 160 million years before any dinosaurs even showed up! They have survived massive extinctions when other related species (like Trilobites) have not. Now they are found primarily on the eastern coast of the United States. More Horseshoe Crabs spawn in Delaware Bay than in any other location on a planet. (Delaware has even designated them as their State Marine Animal)

Image from A Delmarva Odyssey
I could probably talk about these guys for an absurdly long amount of time, but I'll start to wrap this up with a few fast facts: Horseshoe crabs molt several times, stopping at around age ten. Females molt a few more times than males, making them larger. They also have ten eyes which are mostly light sensors! Oh, and their blood is Blue. Not only is it blue, but it has a unique trait in that detects bacteria and toxins and forms clots around them to contain and destroy. LAL testing, which uses this strange blood, is required by the FDA for all new IV and injectable drugs.

Anyway, to learn more about Horseshoe Crabs I would highly recommend watching the aforementioned "Nature" episode. Also, check out, and a Horseshoe Crab site put together by the University of Delaware. Both resources provide excellent information about the history, evolution and anatomy of this interesting invertebrates. Enjoy!


  1. What a wonderful site and a wonderful post about my favorite animal. I am definitely subscribing to your feeds. Keep up the great work! Cheers, Beach Chair Scientist


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