Every time I teach human physiology and reach the blood coagulation (hemostasis) unit, I find myself talking about horsecrabs, Limulus polyphemus.   Not only are horseshoe crabs vital to biomedical research and just outright cool, but they also have blue blood (hemolymph as it is known in most crustaceans).  The bluish hue of their hemolymph is caused by the presence of copper as the central element of their hemoglobin, instead of iron as we use for our hemoglobin.   This blue color is interesting in and of itself, but it is not the reason I usually talk about horseshoe crabs in a human physiology course. 

The reason I expound on horsecrabs in human physiology is because biomedical research companies have worked out that their blood can be used to test drugs and to detect endotoxins associated with gram-negative bacteria.  When a horseshoe crab’s hemolymph comes in contact with the bacteria via an injury, the hemolymph will coagulate to protect the animal against infection.  Biomedical companies have used this defense to create a preparation called Limulus Amebocyte Lysate (LAL).  LAL is used to by pharmaceutical and medical industries to ensure that their products (e.g., intravenous drugs, vaccines, and implantable medical and dental devices) have no bacterial contamination.  They take a sample of their product and introduce LAL, and if it coagulates they know their samples are contaminated with pathogenic bacteria and must be destroyed.  So if you have ever had a flu shot, or any kind of shot for that matter, a horseshoe crab made it possible for you to not get sick or even possibly die.

LAL is so important and useful that biomedical companies need a large supply of horseshoe crabs in order to extract their blood.  Luckily they don’t have to kill the horseshoe crabs.  What they do is they harvest the horseshoe crabs from the wild and then extract about 1/3 of its blood in the lab and later release the horseshoe crabs back into the wild.   It is estimated that it takes about 30 days for the horseshoe crab to replace the blood that was extracted.  Biomedical companies report that the process of bleeding causes a 10 - 30% initial mortality rate, but this could be much higher.  What is not considered in that initial mortality number is that sub-lethal effects on the horseshoe crabs occur and these effects could have very lethal consequences.  For example, a recent study in the Biological Bulletin, demonstrated that recently bled crabs move slower, decreased their activity levels, and had a disruption in their circadian rhythm.  Moreover, their reduced hemocyanin levels would likely reduce immune function and cuticle integrity.  These results taken together indicate that horseshoe crabs recently released back to the wild may be increasingly susceptible to predators and infections.  Further tracking of horsecrabs after their re-release is likely needed to ensure that these animals are surviving.  They are invaluable assets to human health and we cannot take the chance of horsecrabs becoming threatened or endangered.