Limulus polyphemus

Atlantic Horseshoe Crab


It's Not Your Normal Relationship...

Red Knot, horseshoe crab eggs, and close up of horseshoe crab.

 Horseshoe crabs are classified under secondary consumers.  They consume organisms such as bivalves, worms, and multiple kinds of clams.  The pumpkinseed sunfish tend to feed on a lot of mollusks as well!  To learn more about the pumpkinseed sunfish click here!

 Horseshoe crabs are important prey for multiple organisms.  Over 11 different species of shorebirds, especially the red knot, eat horseshoe crab eggs as their main food source when migrating.  The eggs that are found are nests that had been destroyed by waves or storms. These birds do not affect the horseshoe crab after it is out of the egg and growing.   

Shorebirds are not the only organisms that feed off of horseshoe crab’s eggs. Many species of fish such as striped bass, white perch, killifish, kingfish, silversides, all crabs, many gastropods, other invertebrates, fish, sharks, and sea turtles prey on the eggs and larvae.    

Above: shorebirds feeding on the horseshoe crab's eggs.
Photo by Sheila Eyler, USFWS
Right: Loggerhead Sea Turtle

The older the horseshoe crabs get, the slower they grow and the less they molt.  At this point in their life it becomes easier to spot relationships between them and smaller organisms.  The horseshoe crab forms multiple relationships with small organisms.  Majority of their interactions with other organisms are commensalism.  Sponges, bivalves, and many snails are often found just living on their backs and undersides.  Want to learn more about some cool sponges?  Click here for the sea sponge and click here for the marine sponge!  Periwinkles, basket, and mud snails lay their eggs on their backs.  However, there are organisms that are considered parasitic.  The limulus leeches lay their eggs around the book gills and leg joints of the crabs.  Old female crabs are generally the main target.  Often the cuticle of the gills are utilized as the place for chemical activity causing harm to the horseshoe crab. 

Multiple organisms on a horseshoe crab's back.  An example of some of the many symbiotic relationships this organisms exhibits.


Medical Uses

The use of horseshoe crabs in the medical field often goes under the radar.  Not many people know that we use the horseshoe crab's blood for many medical purposes including the safety of drugs and many medical devices.  There is a protein in the blood called Limulus Amebocyte Lysate, LAL and it is used to test medical products for endotoxins.  Endotoxins can cause fevers as well as the death of humans.







 Above: This is a bottle of LAL, the gel that came from

horseshoe crab blood


Left: This is a photo of harvested horseshoe crabs being prepared for LAL production.





Biomedical companies harvest horseshoe crabs to extract a certain amount of its blood.  Only a small amount of blood is drawn from the organism so that it can continue to live.  To learn more about some of the the research studies on Atlantic horseshoe crabs, go to


If you would like to learn some interesting facts about the horseshoe crab click here!

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