Limulus polyphemus

Atlantic Horseshoe Crab


What was that Darwin?  Are You Talking About Me?

The horseshoe crab is a very unique character!  It is a very old organism that has been around sinc e the Ordovician period which was close to 500 million years ago!  Many of the horseshoe crab's closest relatives such as the trilobites and the eurypterids have gone into extinction. So what makes this organism so special to have survived throughout millions of years without becoming extinct like the trilobites and other closely related arthropods during that time?  Horseshoe crabs have been favored because they are known as ecological
generalists, meaning that they can live in a wide variety of environments without too much inconvenience.  What makes this organism so successful is that they are not fixed to one specific habitat.  Horseshoe crabs can endure variations in temperature and salinity. 

 Burrowing is an adaptation that multiple organisms have developed.  The horseshoe crabs are one of the largest organisms that burrow in sandy environments so that it can lay its eggs in the sediment.  This provides more protection for the eggs increasing their survival.

Top: A photo of a fossilized trilobite.
Left: A picture of two horseshoe crabs burrowing.
Bottom: A group of burrowing horseshoe crabs.

The embryos and larvae stage of a horseshoe crab's life has a high tolerance to the broad range of salinity levels in the water and harsh conditions of the intertidal habitat.  During this  stage, it can handle the extreme conditions much better than when it is in its juvenile or adult stage.



Due to its flexibility with its environment, horseshoe crabs have been able to make the changes necessary to carry on through time!



Survival of the horseshoe crabs are also due to the following adaptations:


The chelicerae are the first pair of appendages that have one main job, to place the food into the horseshoe crab's mouth.


Horseshoe crabs have two compound eyes on the top part of its shell.  These are its lateral eyes.  T hey are utilized for the sol purpose of finding mates.  There are five more eyes located on the top, center top of the shell.  Two of the eyes can actually see ultraviolet light as well as visible light!  The ultraviolet helps them see during the night!  In addition to the seven eyes already mentioned, there are two more eyes located underneath the crab.  These eyes help this organism see through the water when swimming.  Last, but certainly not least, the horseshoe crab's tail has light sensors which keep the organism aware of night and day.

Above: A close up of one of the two lateral eyes.
Below: A simple breakdown of the anatomy of the horseshoe crab


A horseshoe crab, in total, have five pairs of legs!  The first eight legs are utilized for walking around, or in the case of males, they use these legs to grasp onto the female horseshoe.  The fifth pair have an extra muscle that allows it to push itself around the sand without getting stuck. 

Book Gills:

The book gills are multitasking organs.  When a horseshoe crab goes swimming, the book gills actually propel it through the water.  This is where the breathing takes place under water.  Oxygen is allowed to pass through but the book gills keeps the water out.  

If you would like to learn even more about the anatomy and adaptations of the horseshoe crab visit!


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