Horseshoe Crabs, or Turtle Spiders?

Some horseshoe crabs I saw in a touch tank at Ripley’s Aquarium in Myrtle Beach, South Carolina.

This post was originally going to be titled “Horseshoe Crabs Are Not Crabs,” but I feel like I’ve overused that format lately so I decided to be cheesy instead. That said, horseshoe crabs are not actually crabs. In fact, they aren’t even crustaceans at all.

That’s right, mulch lobsters are crustaceans, but an animal that literally has “crab” in its name is actually a turtle spider. I just… why? Why, taxonomists? Why do you do this to us?

Fortunately, I learned much more about horseshoe crabs than roly polies in Biology 202. Our friendly neighborhood turtle spiders are members of the class Merostomata, which is part of the subphylum Chelicerata that spiders and scorpions belong to under the class Arachnida. Merostomata basically means “leg mouth,” which is a lot less interesting than the old class name of Xiphosura, or “sword tail,” that they used to have.

Again, taxonomists: why? How is “leg mouth” better than “sword tail”? Who cares about being scientifically accurate, we just want names that sound cool!

In case the boring name didn’t give it away, horseshoe crabs’ mouths are surrounded by their legs. Because horseshoe crabs don’t have teeth, beaks or jaws, they have to crush their food with their legs, so the proximity of leg to mouth is actually very beneficial. Nevertheless, “sword tail” is a much better name in my opinion, even though the tail isn’t really a tail at all but rather a telson. The telson won’t pierce you like a sword, either, but rather acts like a carjack (or turtle spider jack) to flip the horseshoe crab over if it gets turned upside down.

Another thing I learned in my biology class is that while horseshoe crabs typically live in deep water, they will migrate to shallow shorelines to mate when the weather gets warm, and you just can’t deny how great that timing is: before I started attending a coastal college, the only time I ever visited the beach was once or twice a year during the summer, so I doubt I would’ve ever come across a horseshoe crab in the wild if they mated in the winter.

Despite just being cool to look at, horseshoe crabs are pretty incredible arthropods to have around thanks to their blue super-blood.

…I was going to make a joke here about a certain blue-blooded superhero who appeared in a couple movies last year, but that’s a copyright issue and no one would’ve gotten my nerdy reference anyway.

Seriously though, horseshoe crabs have blue, germ-exposing blood that’s used to ensure that medicine and medical equipment alike are properly sanitized. In fact, horseshoe crab blood has been a major player in recent efforts to create a vaccine for the coronavirus! However, the amount of crabs it takes to obtain an adequate amount of blood combined with the animals’ dropping numbers from pollution and other environmental issues make the turtle spider blood method less than ideal.

I bet you didn’t expect to read a paragraph like that today, did you?

Now that we’ve discussed horseshoe crab blood, let’s talk about their fancy shells. The wide, front part of the shell that looks like a helmet is called the prosoma or carapace, and the smaller piece between the carapace and telson is referred to as the opisthosoma – and that completes the knowledge of horseshoe crabs I gained from biology 202.

You know those eye-looking things on a horseshoe crab’s carapace? They are actually eyes. I thought they were false eyes, but nope, they’re genuine compound eyes. These arthropods also have additional eyes under their carapace – and now we’re beginning to see the resemblance to spiders, right? They even have photoreceptors around their telson, so there’s basically no sneaking up on a horseshoe crab. I mean, it won’t really do much if you approach it, but it’ll know you’re approaching nonetheless.

That said, while it may be tempting to pick up a lazy horseshoe crab you spot lying on the beach, please do so with caution – scoop it up by grabbing it on either side of its carapace rather by than telson. If it damages or loses its telson, that could spell trouble for the arthropod in more ways than one. Since horseshoe crab population numbers are on the decline, you probably shouldn’t pick up a horseshoe crab anyways unless you’re moving it back into the water.

That’s right, horseshoe crabs like the water because, like most of our water-dwelling arthropod buddies, they have gills. Called “book gills” for their resemblance to book pages stacked together, these appendages actually do a little more for the turtle spiders than just take in oxygen from the water: they’re also utilized in swimming, and can absorb water into the arthropod’s body to help remove its exoskeleton when it’s time to molt.

Spiders actually breathe through book lungs, so the pairing of both arthropods in Chelicerata is making more and more sense the farther into this post I get – I may owe the taxonomists an apology.

…For Chelicerata, specifically. I can handle turtle spiders hanging out with arachnids and whatnot, but I’m still not letting “sword tail” go.

Next time you see a horseshoe crab, give it a nod of respect. Who knows? Maybe soon you’ll be thanking it for a sanitary coronavirus vaccination.

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