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Roger Weller, geology instructor

Horseshoe Crab
by Paul Foss
Historical Geology
Spring 2007

                                                          Horseshoe Crab



Credit photo to


          Limulus polyphemus or the horseshoe crabs are considered one of the oldest living fossils today. Fossil remains of horseshoe crabs have been dating as far back as 360 million years ago. Horseshoe crabs are believed to be the only living relatives to the trilobites, an invertebrate which died out hundreds of millions of years ago.


A horseshoe crab fossil. Credit photo to


           Despite the horseshoe crab being called a crab it actually belongs to the arthropod group and has more in common with insects than it does with crabs.

Credit photo to


The body of a horseshoe crab is made of three parts: a head, abdomen, and tail spine. The head is fused with thorax and is protected by a tough exoskeleton.

photo from R.Weller/Cochise College

On the top of its body is a pair of compound eyes, one on each side of its body. Also there are light receptors on the top part of the head. Its eyes allow it to see in the ultraviolet light spectrum as well as in the regular light spectrum.

photo from R.Weller/Cochise College

On the underside there is the mouth, five pairs of legs, and a pair of small pinchers. Despite being closely related to insects the horseshoe crab lacks mandibles on its mouth.  Like the head the abdomen is covered a protective shell. On the underside are the six gills that the horseshoe crab uses to breath.

photo from R.Weller/Cochise College

             Extending from the rear of its body is a rigid tail. This tail is used to help a horseshoe crab flip its self over if the crab is flipped on its back. Horseshoe crabs have the ability to regenerate lost limbs almost like starfish.


Horseshoe crab in Delaware Bay. Credit Photo to



Horseshoe crabs live in the Pacific, Atlantic, and Indian Oceans. During mating season they can seen in large numbers in Delaware Bay. After the females lay thousands of eggs in the sand the males fertilize the eggs. Then the adult horseshoe crabs return to the sea, leaving the eggs to develop and hatch. The small larvae then enter the sea and within 10 to 11 years they develop into full adults. Horseshoe crabs can live up to 25 years and can grow up to 20 inches.


Long ago humans realized that horseshoe crabs could be useful in their everyday living. Native Americans used the shells of horseshoe crabs as tools and the tails as spear tips. They also discovered that the horseshoe crabs made good fertilizer as their bodies were high in Nitrogen. This knowledge was later passed on to the European settlers and was used up into the late 1900ís.

More recently, scientists in the medical field have started to study the horseshoe crab for medical reasons. Research on the compound eyes of the horseshoe crab has lead to a greater understanding of how our own eyes work.


Horseshoe crabs being bled. Photo credit to


In the 1950ís Frederick Bang discovered that the blood of horseshoe crabs contained a clotting agent that when it comes in contact with certain types of bacteria it clots the blood and forms a gel that surrounds the bacteria. This allows for the bodies antibodies to take care of the bacteria. Bang and an associate created Limulus amoebocyte lysate or LAL which is now used to test for certain types of bacteria. Horseshoe crabs are captured and after they are inspected, blood is carefully bled. The procedure does not usually harm the horseshoe crabs and death rates and very low. One quart of LAL sells for $15,000.


Despite having been around for millions of years the horseshoe crab seems to have changed very little from its ancestor. They seem to be creatures that are well adapted to their environment and will probably be around for a long time to come.



Works Cited