When Is A Crab Not A Crab?

How did the horseshoe crab get its name?

The horseshoe crab's shell is shaped like a horseshoe, and its legs look like those of a crab.  But--a horseshoe crab is not a crab.  

How are horseshoe crabs different from crabs?

• Crabs have five pairs of legs.  Horseshoe crabs have six pairs.  
• Crabs have antenna.  Horseshoe crabs do not.
• Crabs molt their shell from the back.  Horseshoe crabs molt from the front.
• Crabs have jaws.  Horseshoe crabs do not.
• Crabs belong to the class Crustacea.  Horseshoe crabs belong to the class Merastomata. 

Horseshoe crabs are related to spiders and scorpions.  Called 'living fossils', they have lived on earth for 450 million years.  That's 200 million years before the dinosaur.

As benthic animals, horseshoe crabs spend most of their life crawling along the bottom of the deep ocean.  However, during breeding season, they leave their ocean home to mate in the moonlight on the sandy beach.  The female will lay 80,000 to 100,000 eggs during a breeding season, though most of them will not survive to adulthood.  Migratory birds, sea turtles, and other marine animals need these eggs to supplement their diet. 

The horseshoe crab has blue, copper-based blood that clots quickly when in contact with bacterial toxins.  Scientists use the blood to test drugs, vaccines, and medical devices to ensure they are free of contamination.

In order to make this testing possible, many thousands of horseshoe crabs are taken from the ocean each year.  One third of their blood is removed for this important purpose before they are returned to the beach.

Like other animals with an exoskeleton, a juvenile horseshoe crab outgrows its shell, called a carapace, many times before maturity.  It molts (crawls out of) its shell 16 to 18 times over a 10 year period.  The horseshoe crab grows a new, larger shell underneath, then molts its old shell, leaving it behind.  I found the one in the photo below on the beach, not far from the surf.

Horseshoe crabs are amazing!

Research Sources:

1.  "Are Horseshoe Crabs Really Crabs?"  Web.  5 Mar. 2018.  National Oceanic and Atmospheric Administration.  <https://oceanservice.noaa.gov/facts/horseshoe-crab.html>.

2.  "Horseshoe Crabs".  Web.  6 Mar. 2018.  National Oceanic and Atmospheric Administration.  <https://coast.noaa.gov/data/estuaries/pdf/hooray-for-horseshoe-crabs-student-master-1.pdf >

Photo Credits:

1.  Beach Horseshoe Crab by Kylienne is licensed under CCO Creative Commons. 

2.  Horseshoe Crab Molt [Image by Author].