Crash: A Tale of Two Species
Horseshoe Crab Anatomy

horseshoe crab at sunset

The horseshoe crab has been on Earth for 350 million years. An ancient and complex anatomy hides within its domed shell. From its 10 eyes to its tube-like heart, the horseshoe crab’s unique physique may surprise you.

SHELL

When you first look at a horseshoe crab, chances are the first thing that grabs your attention is its large, hard carapace, or shell. Like all invertebrates, the horseshoe crab lacks an internal skeleton. Instead, this external shell acts as an exoskeleton, providing structure from the outside and protection to the animal against predators or other threats. Made of the cellulose-like material called chitin, the shell is so hard that only sharks or sea turtles can penetrate it. The crab will shed its shell continually throughout its lifetime, as many as 17 times, including four times while still inside the egg.

The carapace of the horseshoe crab is made up of three sections: the cephalothorax, abdomen and tail. The largest section of the animal, the cephalothorax, houses parts of the intestinal tract, nervous system and circulatory system. The size of the cephalothorax differs greatly between males and females. A female’s cephalothorax can reach almost twice the size of a male’s. Attached by a hinge to the cephalothorax, the abdomen contains the musculature for the operation of the book gills and the tail. The tail is attached to the abdomen at the terminal base. Misunderstood as a stinger, the tail is not at all poisonous. It acts as the horseshoe’s rudder, helping it steer and right itself if it gets flipped on its back by the surf.

MOUTH & LEGS

Flip the animal over (gently) and you’ll easily see its six pairs of feeding and walking appendages. Starting from the front of the crab, the first pair of appendages is called the chelicerae. These are feeding appendages used to place food into the animal’s mouth. Going down the body, the next pair of appendages is the pedipalps. These are the first walking legs and they enable the horseshoe crab to move along the rugged seafloor. Each pedipalp has a small claw at the tip except the last pair, the pusher legs. This pair of legs is used for locomotion but also has been equipped with a leaf-like structure that is used for pushing and clearing away sediment as the crab burrows into the sea floor.

NERVOUS SYSTEM

Don’t let its hard exterior fool you. The horseshoe crab is actually quite a sensitive creature. This invertebrate uses a system of specialized nerves that extend from the brain throughout the body. Several large nerves supply the crab with information about its surroundings, including two optic nerves and eight pairs of hemal nerves that are spread throughout the body.

An interesting feature of the pusher leg is the flabellum, an organ that tests the composition of the water passing to the gill chamber. There are approximately one million sensory cells in this organ alone.

upside-down horseshoe crab 

The horseshoe crabs uses its tail as a rudder, and to help it turn over when it gets flipped upside-down.

EYES

A total of 10 eyes help the horseshoe crab get around. These eyes are distributed around the body including on top of its shell, on the tail and near the mouth to help orient the animal when swimming.

Two compound eyes are easily seen on each side of the animal’s shell. The main function of this set of eyes is to find mates during the spawning season.

LUNGS AND HEART

On the horseshoe crab’s underside is a series of six page-like structures called book gills. These organs absorb oxygen from the water while keeping the water out. Each gill contains approximately 150 large flap-like membranes called lamellae that look like pages in a book.

The book gills are versatile organs used not only to breathe but also for swimming. Swimming is an alternative mode of transportation used in emergencies, mainly to escape from predators or if the animal finds itself in rough surf. The gills also function as paddles to propel juvenile horseshoe crabs through the water.

The horseshoe crab’s heart is a long tube that runs down the middle of the cephalathorax and abdomen, extending almost the entire length of its body. On average, the heart rate of the horseshoe crab is about 32 beats per minute.

  • Bill Gates

    Horseshoe crabs are amazing. More worthy than humans. If they could write code, they’d be perfect.

  • RA Hanes

    These are amazing creatures, why do we have to destroy everything around us? It is time for people to take a stand and write to NJ legislatures to keep the protective ban they put in place to protect this important link. They have survived thousands of years only to be destroyed by today’s Americans. We CAN do something to stop this. Please help.

  • Dominic Anderson

    these extraordinary creatures inspired a primal prejudice of fear and revulsion when l was five,and encounterd them for the first time off kowloon, l cannot understand the source of this reaction, l have great empathy for the unjustly reviled,lets take care of our nature on every level….LD50

  • Barb Green

    Many years ago while crabbing in Barnegat Bay in NJ I
    caught one in my crab trap. It was nothing I had ever seen and it scared me! I had no idea how extroardinary the horseshoe crab is. Thank you for enlightening me.

  • I.C. Wiener

    Horseshoe crabs are very intelligent creatures.

  • d.ameden

    there needs to be a moratorim that covers the whole east coast for the next ten yrs to allow them to get back up in numbers. and to allow the companies that are researching alternatives to make headway and when they achieve the better mousetrap make it possible for them thru tax incentives to make it affordable to those fisherman that need it and leave these ancient creatures to thier destiny certainly not extinction as that would have been manmade. wake up humans!!!

  • B.J.

    The program and this page were fascinating. I wish it had included more information on the crabs blood, it’s circulation,from where and how it is removed, how much is removed, etc. The multiple eyes, amazing number of sensory cells that you describe make me cringe when I think of how abominably the crabs are treated by the fisherman and the blood-letters …

  • A.Louisin

    I think this is amazing. I have to write a newspaper report for school. i hope you don’t mind me using this as an info guide. Anyways, I can’t imagine what the horseshoe crab is feeling and…..

  • sarara

    eww

  • daniel

    wat do they do all day

  • ♫Sarara ♫

    the picture eww but i loveeeeeeee this site (not really) but yea rock on ppl! ♫♫♫
    ☺ Sarara ♫

  • ashley

    great ;D

  • Meredith

    We live in South Carolina and actually went to see the facility where they bleed the crabs. This whole topic will probably end up being my research paper for biology.

  • Marlys-Jean Natonick

    I have vacationed in DE for over 25 years…always trying to save and turn over the horseshoe crabs with my sons…4 years ago I moved here to teach. I am amazed how little the children of DE know about the horseshoe crabs. I am on a mission and this video was phenomenal. Each year I have taken students to the beaches to see the horseshoe crabs spawn. I believe they have gained a respect for this ancient creature. Thank you! :-)

  • bbetances

    How big is that crab in the first picture? Curious because around here we get crabs that are about 1 foot or so long, and a few inches wide. This guy looks like he’s about 1 ft high and 3 ft long! I love these guys, the most under appreciated animal in the world, and I think they are so cute! and valuable to society, unlike some animals that ruin the Earth (humans!). So many people think they can hurt you, but they spend most of their time AVOIDING confrontation from predators! How big do these guys get? I would really like to know because I only find answers like “a foot or so” but he looks HUGE!

  • Tony McGuane

    These animals do look really cool. And this one was pretty interesting.

  • cathy

    We are working with Maryland DNR to raise “Horseshoe Crabs in the Classroom.” Hopefully this experience and shows like this will enlighten children and adults about these amazing animals. Instead of hurting and exploiting them, we could learn to appreciate and protect them. We need them, but they certainly wouldn’t miss us if we disappeared from the Earth. Think about it the next time you get an injection.

  • Betty

    I live near the Indian River Lagoon in Florida. In the past we saw many horseshoe crabs in the lagoon, but in recent years they seem to have declined. I don’t know about any harvesting of them here, so I wonder if water quality might be a part of the problem.
    They are incredible creatures and I woul like to understand them and their needs for survival more.

  • Joey Bag A’ Donuts

    These things are a nuisance and all over the Jersey shore, there is hardly a shortage of them. I remember as a kid my mom used to take me to a beach, don’t remember which in NJ and it was difficult to even get to the water without stepping on one of these things.

  • EdFan

    I agree that these creatures should not be used for bait, but there is nothing wrong with using their blood to test for bacteria. I doubt they feel any more pain than I do when I donate blood, maybe we should stop allowing humans to donate blood because it’s too painful. 10 to 13% die when harvested, but only 300,00 were harvested last year for their blood, that’s less than 40,00 dead, that seems like a pretty small number when you consider the benefit to mankind.

    to Joey Bag A’Donuts – apparently the population started to drop in the nineties when their use as bait increased. I also remember seeing many of them at the Jersey Shore when I was a kid in the seventies.

  • lynn

    Amazing creatures. There needs to be a ban on catching them for bait for 10 years because that is how long it takes them to reach maturity and reproduce!

  • billy younan

    this was an amazing discovery, this creature it is so vital to the human its unbelievable.
    our whole existinance will come down to discoveries like this,.
    these are the sources for cures and eliminating diseases as we know it, as matter and fact discoveriese like this can enhance our own life span.
    bottom line these creatures are more important than almost anything out there.
    we can learn so much more from creatures and animals and insects and nature. we are part of that.
    jsut because is not effecting us imediatly dose not mean it was not a part of our existance.

  • candycab

    bbetances

    If you look at the size of the grains of sand in that picture it’s easy to see that its no bigger than most Horseshoe Crabs :)

  • Teanna Byerts

    The Delaware Bay is near my home in south central PA (where the Susquehanna flows down to the other great estuary, the Chesapeake). I grew up a landlubber and discovered the sea mainly as an adult: the Horseshoe crab was one of my first encounters, and still one of my favorites. Most of the visitor’s centers (Cape Henlopen on the Delaware Bay, Chincoteague and Assateague Islands) have touch tanks with horseshoe crabs in them. Most of the naturalist programs include some on the wonderful crabs. It’s great to see kids’ expressions of “eeeewww!” change to wonder as they find out the true nature of these totally cool, ancient beings.

    Educate, educate, educate. Take a kid to the beach, pick up a crab in a touch tank (or the beach) and have an encounter.

    You only protect what you understand and care about.

  • Crystal

    Woohoo…Horseshoe Crab Success!

  • melanie

    The difference between humans donating blood to help other humans and horseshoe crabs having their blood harvested to help humans is that you are volunteering to have it done, you know what you are getting into. You are not being forcibly taken from your home and taken to a lab and having your blood drawn. There is real stress suffered by these creatures, and it’s incredibly self-centered to believe that our lives are worth so much in the grand scheme of things that it’s worth hurting other sentient beings. When people donate blood, care is taken to ease the amount of pain they endure. It’s highly likely that the same care is not being taken when harvesting blood from horseshoe crabs.
    And certainly human blood donations do not involve a 10-13% mortality rate for the donors, or we’d stop doing them.

  • DL

    I am trying to find out about something very similiar to the horseshoe crab. I am from northern arizona, it has been raining very good for the last month, elevation is 6000 ft. frogs come out when it rains good, now instead of frogs we found hundreds of very similiar horseshoe crabs in a big mud puddle. They are not like the pictures we find on the internet, very similar, but small about 1 inch to 2 inches, the tail is also different it is soft and split at the end. When we turn it around it looks disgusting just like the horseshoe crab but the color underneath is red, does anyone have pictures we can view to determine a similiarty? we live in the high desert and have never seen anything like these creatures.

  • TrishaF

    Horshoe crabs are amazing! One of my favourite animals —– Hello Bill Gates!! I’ll gladly write code for you on their behalf if you’ll donate toward this species’ support! Currently there is no regulation for horshoe crabs at all and given their rapidly depleting numbers they should actually be on the endangered species list already! P.S. — I’m completely serious about the coding offer, Bill!

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