Saturday, April 30, 2011

Horseshoe Crab - What I Learned Teaching with the Program Sea-to-See at FSU

This blog is about my favorite animal--an animal that almost predates the dinosaurs in origin and looks like it could easily be something from a alien exoplanet.  Below are the notes of what I teach at FSU's Sea-to-See program about this very special creature.  The Sea-to-See program travels to local elementary schools and provides an interactive learning experience for kids aimed at raising awareness of both the scientific method, local habitats in our area and the organisms that dwell therein.  The horseshoe crab very easily turned into my favorite to present--for a number of reasons--many of which you'll find out about below.  A fair amount of the material below was from my own personal curiosity being piqued and doing some personal research and the rest is from a few brilliant coworkers such as Beth Kostka and Heather Sneed. 

Try and imagine you're a kid with your chest pressed up against the side of a 100 gallon touch tank rapt in curiosity.  (The below is kind of a script.  I also could never cover all of the below at once.  I pick and choose as I see fit based on time/class ability.  I also use the masculine singular pronoun since I worked with mostly male crabs.)

This is me!

After inviting the children to gather around the touch tank, I explain that I want to borrow everyone's eyes for just a second and that I promise to give them back (to make sure they're paying attention).  I bend down to their level and then go on to say that there are two rules at the tank, "First rule: One person can talk at a time.  That way each of us can hear what another has to say.  That makes sense, right?  Second:  One animal at a time, that way we cover more.  Put up your thumbs if you think those rules are awesome!!"  The children are eager to play with the animals, so they readily comply.   Then I say that I need their help to pull the tarp cover off of the tank on the count of three.  Stretching out the count a little builds anticipation.  Once they're able to see in the tank I usually let out a seed "Wow!!!" almost like a laugh track is used to get other people to laugh.  haha.  I then explain that I want to talk about the horseshoe crab--the biggest, baddest, scariest crab in the whole tank--the star of the show!!!

Limulus polyphemus

"Does anyone know his name?"  A lot of kids will say 'stingray'.  I lovingly say that makes sense because he looks like one, but this is actually a horseshoe crab and that if you use your imagination a little bit his shell is kind of shaped like a horseshoe crab.  "He's my favorite because he's a 'living fossil'.  That means that he looks just like his great, great, great, great, great...grand parents from 200 million years ago!  That's almost before the dinosaurs first walked the Earth!!!"  I then say that I want to talk about his top side, then his underside and then that we'll have a time for touching and holding.

The Stinger 

"First, I want to talk about this tail.  What do you think it's for?  Many people when they look at this tail think that it's for stinging and some people even say that if you touch it that your hand is going to swell up and fall off!    Does anyone want to reach out and have their hand swell up and fall off."   (Some will say, "That's ridiculous!" and reach out to touch the tip of the tail.  Then the other kids will follow suit.  Some classes will become really worried and quite literally jump back from the tank.  During those times I'll have to touch the tip and explain that I was just being silly and that the rumors people tell aren't true.)

Once they know it isn't a stinger I can then ask, "What's it for then?  If it isn't stinging, why does the horseshoe crab have it?  What's its purpose?"  Kids may guess for protection, steering, to look scary, maybe to look like a stingray (mimicry), etc.  Next I ask if anyone wants to know a secret.  Kids seem to love secrets for some reason.  haha.  After that I say that I'm going to flip him over and that I want them to figure out what the secret use of that tail is.    Crab will most likely have trouble flipping over in tank due to plexiglass bottom and shallow water.  I say that he needs some cheering on.  I then start a chant of, "Horseshoe crab, horseshoe crab, horseshoe crab..."  I give him a boost to help him a long a little by lifting the tail and explain that if he wasn't in this tank he'd already be right side up.   "So, what's he use that tail for?  What would  happen if that tail broke off and he got flipped over?"  The answer, of course, is that it's essential to flip over from his back. 

The Creeper Watching Eyes

  "Reach out and touch the top of the crab.  How does it feel?  Give it a light tap to feel how strong it is.  Now, can anyone find his eyes?  How many eyes do you think he has?"  Kids will guess low numbers like 2, 3, 4, etc.  As they go up in numbers I'll say enthusiastically, "Getting warmer!!!  Getting warmer!!!  Warmer!!!"  This back and forth antiphonal response will elevate in speed and volume until I finally break in and say, "Would you believe if I said that he has two...thousand eyes!!!"  Kids often let out a gasp of disbelief.  "Take a close look!  Each one of those eyes has almost one thousand eyes packed into it."  I ask the kids if they've ever walked by a painting and felt like the painting was watching them no matter where they were standing.  Guess what?  He has eyes like that!!!"    An appearance of a black ocular dot will appear to follow the observer.  "Why? Why do we only have two eyes that see really, really well and he has 2,000 eyes that don't see as well?"  Kids will speculate that maybe it is to see in multiple directions simultaneously.  I say that maybe we can test that!

"We should first test on ourselves.  Let's test our field of vision, that's the range that we can see in."  Next I ask the kids to pick out a point across the room to look at.  I tell them to fix and lock their heads and eyes on that object.  (Kids will want to cheat and move their eyes.)  I then ask them to take their hands and put them in front of of their body like they're going to give someone a big hug.  Then to take their hands and spread them eagle until they can't see their hands any more.  That is their field of vision.  Mine is about 170 degrees.  Review how many degrees a half circle is.  Then I excitedly say that we can test the field of vision of the horseshoe crab.  I then pick a volunteer across from me.  They will then need to pick one of the large eyes on top to experiment with.  I then say that if you can see the crab's eye that he can see you.  Does that make sense?  I then say that I'm going to rotate the top of the crab and that they should let me know when they start to see the eye and when they stop seeing the eye.  Having completed that I gesture with my hand and review for everyone what the volunteer stated as when they started and when they stopped seeing the eye.  Ask for an estimation of how many degrees (reviewing that a full circle is 360, half circle is 180, etc.)  Then figure out the total of both eyes combined, explaining that there will be overlap in fields of vision between the two eyes.  The crab can see in a full circle!!!  I then use examples from the kids attire:  "He can see your cool baseball cap.  He can see your purple sweater over there!  He can see your red quicksilver shirt right here!  He can see all of us at once!!  Imagine what that would feel like to see all around you at once!!!"

A Horseshoe Crab's Calendar and Watch

Inevitably, one of kids will point out the two dots on his carapace that looks like a nose.  I then ask, "Well, if that was his nose, would it be a hole or a dot?"  Kids may flounder in answering, but eventually one will realize that it would need to be holes to be a nose since air or water must circulate through it.  I then ask for them to look very closely to see which it is.  I then explain that it's another set of eyes that specialize in UV light, an invisible light to humans, and that they use that to tell what phase of the moon it is (or time of day) to synchronize their reproduction (said euphemistically, usually).

The Underbelly of the Great Beast

"Who's feeling brave today?!?!  I mean really, really, really, super, duper brave!!!    I'm going to ask you to do something really, really scary.  Like so scary that I don't think ____ grade could do it." Pick one grade below whatever the kids are.  "Here's what I'm going to ask you to do.  I'm going to flip this crab over and he has one dozen scary claws.  How many is a dozen?    If you're feeling brave, and not everyone has to do this, I want you to stick your hand in that mess of claws!!!"    The group dynamics of this are fascinating.  If the ice isn't broken by one of the kids (preferably a girl so the boys have to defend their masculinity) then I may have to demonstrate and explain that his pinchers aren't for defense like a blue crabs/stone crabs.  They're just for picking stuff up and for walking with.  Other times one of the 'cool' kids will do it and subsequently every single kid will want to.

The Mustache of the Great Beast

"Here's what I want you to do.  I'm going to flip him over again and I want you to touch his back again and while you're touching it I want you to think about this--he can't feel that.  He may not be able to feel that, but he does have a way of feeling.  What could it be?  Can anyone help me solve this mystery?  Alright, everyone stick out your arm.  Now, take your hand and very lightly touch your arm.  I know that you've done that before, but this time I want you to think about it like a scientist would.  What method does your body use to feel?  We see by our eyes letting in light and our brain interprets that light as sight, vision.  We feel by pressure.  When we touch our arm we feel an ever so slight, teeny-weeny, microscopic dent or indentation on our arm.  Does that make sense?  We feel by our soft skin being pushed on, but the crab doesn't have soft skin.  How does it feel then?  Stick your arm out again.   This time don't touch your arm, but just barely tickle the hairs on your arm.  That's how the horseshoe crab feels!  Let's flip him over again and you point out every where that he has a bunch of hairs.  Those are places that he really wants to feel!"  (Kids point out mouth area and hind quarters.)  "So, why do you think that he wants to feel so well right around here?"  (Waves finger around mouth. Kids make guesses.)  "Why does a cat or a dog have whiskers?  Well, where's his mouth?  How might a scientist make sure that's his mouth?"  It surprises me that this can be a tough question.  One right answer is to put some food there and see if it gets eaten.

Death by Funky Chicken

"The other weird thing about the horseshoe crabs mouth is that his teeth are attached to his legs!!  Who wants to be a horseshoe crab with me for a minute?"  Kids raise hands.  "We're going to do something silly.   So, how do you eat if your teeth are attached to your legs?  You do the funky chicken!!"  Kids have fun acting this out.

Half Billion Year Old Habits Die Hard

"The ancestors of the horseshoe crabs, the eurypterids, started laying eggs on the beach almost 500 million years ago.  Why?"  Wait for people to think about it.  If I'm presenting to adults I'll see if they know what was happening at the time period and whether or not the dinosaurs had arrived on the scene.  "Because there was literally nothing on the land at that time--no plants, no animals and certainly no predators.  The land was the perfect safe haven for their offspring and now they've continued that tradition for half of a billion years.  Now, more recently, it's a very important part of the diet of many migratory birds (like the endangered Red Knot that flies from Canada down to the tip of S. America yearly--9,300 miles)."

Blue Blood

"Horseshoe crabs have blue blood.  Does anyone know why our blood is red?  Iron!  Well, they have blue blood because they use copper instead of iron to carry oxygen around in their blood!"

Limulus Amoebocyte Lysate

"Who has gotten an immunization or flue shot recently?  Did you know that those shots get tested with a compound from horseshoe crabs?!  It's true!  Because the environment the horseshoe crabs live in isn't exactly the cleanest--it can have billions of bacteria per gram of mud--they have very special adaptations to deal with the bacteria.  One of which is that their blood has a compound, Limulus amoebocyte lysate, that coagulates in the presence of gram-negative bacteria (the kind we need to worry about the most for infections).  The government requires that medical companies test their shots with this compound and if they put into the sample and it coagulates they know the batch is contaminated and they know to throw it out.  Your life could have been saved by a horseshoe crab and you didn't eve know it!!"

Male 'boxing glove' claw used to hold on to female's carapace.

Boys vs. Girls

(Hold up crab.)  "Is this a boy horseshoe crab or a girl?"  (Kids will never not answer.  Funny to think about how ingrained gender is into our psyches, even at a young age.  Flip crab over after taking vote.)  "Here's how we tell: the males crabs have a 'boxing glove' pincher that they use to hold onto the backs of of the females (right next to the base of the tail).  Females have all the same kind of claws."

Snow Shoes

"What do you think these weird flower feet are for?"  Pause for guesses.  Allow the kids to speculate.  "Pushing on soft mud.  There is also a tiny claw on that leg that is specific for cleaning the gills.  His underside is kind of a Swiss army knife of tools!"

All Booked Up

Point out book gills.   "Why do you think they're called book gills?"  Wait for guesses.  "Because the individual flaps look like the pages of a book!"

Fed Up

(This is acted out dramatically.)  "Here's how the horseshoe crab eats.  He takes his big head and then rams it into the mud.  Then he takes all those pinchers and starts ramming them into the mud to find anything he can get--worms, clams, crustaceans, etc. and then pulverizes them with his leg-teeth."

O-So Hungry

"What's the longest you've gone without eating?"  Kids may say a day or most of a day.  "Guess how long a horseshoe crab can go without  eating?  Some sources say up to a year!"


"This tiny, insignificant little flap has a very important use.  It has over 1 million chemical sensors and can be used to detect carbon dioxide so as to let the crab know when he ought to move out of stagnant water."  (Flabellum means tiny fan in Latin and in Roman Catholicism it is the name of the tiny fan used to shew flies away from the Sacraments.)

Spines - Why Science Is Still Cool

"What do you think these move-able spines are for?"  Pause for kids to process out loud.  "Maybe you're right!  I have been told that scientists aren't sure the exact function of them.  I love that fact.  Do you know why?  I love it that there is still so much to learn!  I love that maybe you can grow up, become a scientist and help to solve some of these mysteries!!"

  Pictures from here, here, hereherehere, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.

Works sighted [sic]:


  1. Seriously, that post was pure awesomeness. What you did for those kids was cool enough, and then you trumped that by composing a superbly written description. I am very impressed. Your blog has all the factors I aspire to in my own.

  2. Awesome post. Have you thought of posting on the cognitive abilities of arthropods? It's an area I feel isn't covered enough and is something I write about often on my blog.

  3. Enjoyed the information on this ancient critter

  4. Horseshoe crabs can float upside down and use their book gills to "paddle" themselves.

  5. Thanks, Jason! I definitely subscribed to your blog as well! I'm excited to hear about your school prospects and really appreciate your blog suggestion! I'm going to have to dig a little bit more into that subject! Also, feel free to friend me on Facebook if you so desire!

    LKW/Tedd, you guys are why I write about science! Thanks for being my inspiration!

  6. you sem very informed on horseshoe crabs. I have 3 at a museum that i keep in the saltwater tanks. They are losing their feet and dont look near as beautiful as they should. What is the best way to keep them. please reply to

  7. Hello, anonymous! Thanks so much for commenting! I personally have not kept the beauties, but I wonder if these folks would be able to help: They are a local aquarium and supply company and have always found them to be knowledgeable. Hope that helps a little! Good luck! Let me know if you find out anything interesting!


Please comment! You can comment anonymously! Please send ideas and topics to research and post on!!!