LiteraSea Photo Blog

Horseshoe Crabs...Alien or Ancestor?

10/31/2014

They may look like they've just beamed down off of a spacecraft, but in fact, these incredible species have been on this planet for millions of years. They're also a species responsible for saving countless human lives- a hard shelled hero!

Horseshoe crabs are one of the ocean's most enduring and unique creatures. Keep reading for more facts and lesson plans that will help you beam these strange creatures straight into your next biology class!

Horseshoe diagram from Exploringnature.org.

Horseshoe Crabs are also known as Limulus Polyphemus (limulus, in latin, means sideways motion; polyphemus is the name of the cyclops of Greek myth who was blinded by Odysseus)
Horseshoe crabs are thought to have evolved between 300-520 million years ago. Horseshoe crabs have existed since the time of the dinosaurs, and were here before the evolution of land plants, insects, and humans.Check out this great article on what the Horseshoe crabs have evolved through!
Some people refer to horseshoe crabs as a “living fossil”-meaning a species that has retained the same body plan for millions of years, and represents a “sole surviving lineage from an epoch long past” (mnn.com)
Horseshoe crabs are not actually that “crabby” at all. Originally classified by Linnaeus in 1759, the Horseshoe crab is actually a distant relative of the crustaceans, and a more closely related to Arachnids, which include species like spiders, ticks, and scorpions.

Horseshoe crabs are protected from the environment by a hard exoskeleton.
The females are usually larger than the male horseshoe crabs, with a carapace (or outer body shell) of up to 60 cm. These animals can also weigh up to 5 kilograms.
Horseshoe crabs have six pairs of appendages. You can tell a female from a male horseshoe crab by looking at it's first set of appendages. Males have a pair of mating claws commonly known as "boxing gloves". Females have pincers similar to those seen in other crab species. When it's time to mate, the male uses his boxing gloves to hold himself onto the female's carapace.

Did you know that horseshoe crabs also provide valuable real estate for other marine creatures? Epiphytic plants and epizootic animals can live on the surface of the horseshoe crab's carapace.
Scuds, ghost anemones, Asteriids, snail furs, blue mussels, barnacles, sea strawberries, seal lettuce, red beard sponges, eastern oyster, northern rock barnacles, skeleton shrimps, sand builder worms, bushy bugulas, hard tube worms, flat worms, oyster drill eggs and Agardh’s red seaweed all make their home on the backs of Horseshoe Crabs!

Close-up photo of a horseshoe crab's compound eye. Photo courtesy of sciencephoto.com

Horseshoe crabs can have up to ten eyes. Most of them are found around the top of the carapace, but some even have eyes on their long, thin tail. These eyes vary in their complexity. Some are simple eyes, with only photoreceptors, while others are large and complex. The eyes found on the back of the horseshoe crab have up to 1,000 photo receptor clusters, known as ommatidia. Each ommatidia has a lens, a cornea and a cluster of photosreceptors.

Horseshoe crabs have the largest rods and cones of any known animal. The rods and cones found in the eye of a horseshoe crab are approximately 100 times the size of the ones found in the humans eye. In spite of the number of eyes, horseshoe crabs still have "poor" eyesight used only to sense light and locate mates.

Photo courtesy of Steve Greer Photography.

Horseshoe crabs have no known predators in the sea. Their only concern for survival comes from human consumption.
Humans have used Horseshoe crabs as bait for other commercially collected species, like the American Eel and the Conch (a type of Whelk snail). They also have traditionally collected Horseshoe crabs as food for other farmed species, such as pigs and poultry. Leftover horseshoe crab material was also used as a crop fertilizer, up until the 1970s.
Traditionally collected only as By-catch, horseshoe crabs are now commercially harvested, mainly for medicinal purposes.

Photo courtesy of http://www.lehsd.k12.nj.us

Horseshoe crabs have blue blood-baby blue in fact! This is because horseshoe crab blood contains copper as it's oxygen carrying molecule, unlike our blood, which uses iron.
In the 1950's, a researcher named Frederick Bang discovered something amazing about horseshoe crab blood. He looked at a special type of cell found in horseshoe crab blood called amoebocytes (more information here) and found that they contain a chemical called coagulen. Coagulen's job is to detect invading bacteria-and to stop it in it's tracks. When the coagulen detects even a part per trillion of bacteria or it's endotoxins in the horseshoe crab's blood, it releases a gooey gel that coats the bacteria completely,and kills it. This prevents the bacteria from spreading through the crab's body, and keeps it protected.
Coagulen is so good at what it does that it is now used all over the world to detect bacteria in many medicines given to patients by injection. When doctors want to administer a medicine to a patient, they first test it using the Limulus Amebocyte Lysate Test.
Bacteria can be detected by mixing the medicine with extracted coagulen. If there is bacteria is present, the coagulen will trap them in a thick gel, visible to the human eye. If no bacteria is present, the coagulen will not react.
The only way to get the Coaguluen for the LAL test is to harvest live horseshoe crab blood. Biomedical companies harvest up to 30,000 horseshoe crabs per year, and take approximately 30% of their blood at a time.
Horseshoe crabs are harvested during their mating season, when they congregate on the sandy shoreline in shallow water. Once taken, the horseshoe crabs are returned to the area where they were found.
1 quart of horseshoe crab blood is worth approximately $15,000 dollars US.
Horseshoe crab blood has also been used in cancer therapy reserach, leukemia diagnosis and to detect vitamin B12 deficiency.

Curricular Links:,

Investigation Questions:

What about the Horseshoe crab has made it so capable of survival and reproduction? Why has it lasted in the ocean as long as it has while other similar creatures might have died out?
Why do you think that Horseshoe crabs would need amoebocytes? What about where they live might make them prone to having bacterial invaders?
Do you think Coagulen would work in human blood? Why or why not?
How do you think harvesting their blood effects the Horseshoe crabs?

Lesson Plans and Activities:

For a lesson on how horseshoe crabs are connected with seabirds and intertidal ecology, check it out this lesson plan from The US Fish and Wildlife Service.
For a great high-school level biochemistry or chemistry lesson, try out this LAL lab by Gary Kreamer of the Delaware Aquatic Resources Education Center.
For lessons on the horseshoe crab's compound eyes, external anatomy and molting behavior, check out horseshoecrab.org's great lesson plans!
For your own 3D model of a horseshoe crab, check out the Deleware Sea Grant's life-sized model set!

References:

The Atlantic Online. Feb 26, 2014. "The Blood Harvest." Accessed online October 31st, 2014.

Mitchell, B. July 27, 2014. How we Harvest Horseshoe Crab blood to Save Lives. Inhabitat article.

Accessed online October 31st, 2014.

Sacred Heart University. "Horseshoe Crab History." Accessed online October 31st, 2014.
Sea Kayak Chesepeake Bay. "Horseshoe Crab-Living Fossil-Champion Survivor."Accessed online October 31st, 2014.
Marinebio.org. "Horseshoe Crabs:Limulus Polyphemus."Accessed online October 31st, 2014.