Horseshoe crabs are not actually crabs but chelicerates. While they resemble crustaceans like crabs, they have specialized appendages called chelicerae and their bodies are divided into two parts rather than three. Horseshoe crabs are also considered "living fossils" as they evolved over 450 million years ago before the dinosaurs. Their blue blood contains hemocyanin which carries oxygen throughout their bodies and turns blue when oxygenated due to its copper content. The blood detects bacteria and is used to test for contamination in medical supplies and devices.

1. 10. They’re not really crabs.
A scorpion, the horseshoe crab’s close relative
(Img Cred: Flickr, Furryscaly)

2. 10. They’re not really crabs.
Like scorpions and spiders, the American horseshoe crab (Limulus
polyphemus) is a chelicerate.
Chelicerates are a subgroup of arthropods with specialized fang-like
feeding appendages called chelicerae. The bodies of these creatures are
divided into two parts, and they lack antennae.
True crabs are crustaceans. Crustaceans are also arthropods, but unlike
chelicerates, their bodies are divided into three parts, they typically have
two pairs of antennae, and they lack chelicerae.

3. 9. They are “living fossils.”
A horseshoe crab fossil on display at the Harvard Museum of Natural History
(Img Cred: Hillary Hoffman)

4. 9. They are “living fossils.”
Horseshoe crabs evolved before dinosaurs. Like the long-extinct trilobite,
their closest relative, they evolved during the Paleozoic era, which took
place 540 to 248 million years ago.
Horseshoe crab fossils have been dated at 360 million years old, while
dinosaurs began to evolve about 200 million years ago.

5. 8. Their eggs feed migratory birds.
Horseshoe crab eggs
(Img Cred: Flickr, U.S. Fish and Wildlife Service – Northeast Region)

6. 8. Their eggs feed migratory birds.
Horseshoe crabs come ashore to breed in early spring, around the same
time that migratory birds head north to their summer breeding grounds.
Many species of birds stop to feast on the horseshoe crab eggs.
The fat and protein in the eggs provide an important source of
nourishment for the birds, giving them energy for the remainder of their
flight and for breeding.
Each shorebird can eat as many as 9,000 horseshoe crab eggs per day.

7. 7. Their populations are declining.
Horseshoe crabs swarm to shore
(Img Cred: Flickr, U.S. Fish and Wildlife Service – Northeast Region)

8. 7. Their populations are declining.
Scientists have reported declines in horseshoe crab populations up and
down the Atlantic coast. The magnitude of the declines remains uncertain
due to inconsistencies in counting methods and locations.
Many scientists attribute the horseshoe crabs’ demise to the
overharvesting of the crabs for use as eel and whelk bait.
Declines in horseshoe crab populations affect other species as well,
including the migratory birds that feed on their eggs.

9. 6. They are a good fertilizer.
A horseshoe crab washed up on Assateague Island, Maryland
(Img Cred: Hillary Hoffman)

10. 6. They are a good fertilizer.
Native Americans and farmers in the 1800s used nitrogen-rich horseshoe
crabs as fertilizer. Well into the 1950s, horseshoe crabs were the base of a
strong fertilizer industry in Delaware and New Jersey, but the decline of
horseshoe crab populations and the rise of artificial fertilizers ended this
practice.

11. 5. They help wounds heal faster.
Chemical structure of chitin, a substance found in horseshoe crab shells
(Img Cred: Wikimedia Commons, Dschanz)

12. 5. They help wounds heal faster.
Medical product manufacturers use chitin, a polymer found in the hard
shell of horseshoe crabs and other arthropods, to coat filaments for
suturing and wound dressings for burn victims.
Chitin has a strong positive charge, which allows it to bind strongly to
negatively charged surfaces like skin. Chitin–coated materials can reduce
wound healing time by 35 to 50 percent.
Although there are numerous natural sources of chitin, scientists often
prefer to use the chitin found in horseshoe crabs for research purposes
due to its high level of purity.

13. 4. They have 10 eyes.
Locations of the horseshoe crab’s eyes
(Img Cred: Flickr, cliff1066)

14. 4. They have 10 eyes.
Horseshoe crabs have two main compound eyes, which are similar to a
fly’s eyes. Each eye has about 1,000 receptors. Additionally, the crabs
have two simple eyes located next to the compound eyes.
They also have three additional eyes on the top of their shell that eyes
detect UV light and help the crabs follow the lunar cycle. Two more eyes
are located near the mouth: their function is unknown. The last “eye” is
composed of photoreceptors on the crab’s tail.

15. 3. They helped us learn about vision
Research performed on horseshoe crabs taught us about human eyes.
(Img Cred: Flickr, _StaR_DusT_)

16. 3. They helped us learn about vision
Dr. H. Keffer Hartline received the Nobel Prize in Physiology or Medicine in
1967 for his research on horseshoe crab vision.
Hartline studied electrical impulses from the optic nerve of horseshoe crab
eyes and confirmed his hypothesis that nerve impulses relay visual
information from the retina to the brain.

17. 2. They’re blue-bloods. Literally.
Copper-containing proteins give horseshoe crab blood its blue color
(Img Cred: Flickr, jkirkhart35)

18. 2. They’re blue-bloods. Literally.
In humans, the iron-containing protein hemoglobin, present in red blood
cells, carries oxygen through the bloodstream. The binding of oxygen to
iron gives our blood its characteristic red hue.
Horseshoe crabs have a protein with a similar function called hemocyanin.
Unlike hemoglobin, hemocyanin is not contained in cells, but flows freely
through the horseshoe crab’s colorless blood.
When oxygenated, the copper-containing hemocyanin turns blue.

19. 1. Their blood detects bacteria.
Chemicals in horseshoe crab blood can detect bacteria in medical supplies
(Img Cred: Flickr, publik15)

20. 1. Their blood detects bacteria.
Cells in the blood of horseshoe crabs called amoebocytes recognize the
presence of gram-negative bacteria. Researchers in the 1950s showed
that these cells act as a primitive immune system, swarming, coagulating,
and forming a viscous gel that isolates the bacteria.
Today, scientists make use of this coagulation property as a way to test
drugs and other clinical materials for the presence of gram-negative
bacteria. To do so, scientists remove blood from horseshoe crabs and
break open the blood cells. Scientists then purify and freeze-dry the
chemicals inside – called the lysate – to produce a substance called
Limulus amoebocyte lysate, or LAL.
To test a sample for the presence of gram-negative bacteria, researchers
mix the sample with LAL and water. If coagulation occurs, the sample
contains gram-negative bacteria. The FDA approved LAL as a standard test
for the presence of bacterial endotoxins in 1983. Today, all injectable and
intravenous drugs and prosthetic devices must undergo a LAL test before
being administered to patients. LAL can also be used to diagnose diseases
caused by gram-negative bacteria, such as spinal meningitis.