horseshoe crab
Horseshoe crab is a marine arthropod of the family Limulidae and order of Xiphosurida or Xiphosura
Health and Medicine

The Importance of the Horseshoe Crab in Science

The impact of the horseshoe crab, Limulus polyphemus, in science is historically vast and vitally significant. Ranging from their use in the study of vision nearly one hundred years ago, to quantification of marine bacteria, to current and future applications of potentially life saving diagnostic tests containing extractions of their blood cells, the horseshoe crab has made an impact in a remarkable way.

Most scientific applications utilizing the horseshoe crab are based upon the ability of the Limulus amebocyte lysate— an extract of horseshoe crab blood cells – to react with a unique component of gram-negative cell membranes. One of the early applications involving horseshoe crab extract revolved around devising a way to quantify marine bacteria. Since horseshoe crab extract is able to bind to gram-negative bacteria so precisely, researchers were able to use the horseshoe crab to produce sensitive reagents that were ultimately able to replace more tedious methods of quantifying marine biomass, such as direct counting using microscopy. (Watson, 1977).

This unique binding ability was more recently examined in the study of blood clotting. Structural proteins from blood clots of three different organisms— horseshoe crab, mouse, and lobster– were employed to study how a blood clot traps the endotoxin responsible for gram-negative septicemia, an infection of the bloodstream that has a very high mortality rate. Understanding this cascade of events in a comparative study provided the building blocks for researchers in the study of how the blood clot is able to form protection from the bacteria responsible for more than 100,000 human deaths annually in the United States alone (Armstrong, Rickles, Armstrong, 2013).

The theory from which the marine bacteria quantification and blood clotting study are based has provided a powerful platform for many practical applications. Most notably, horseshoe crab extract is the key reagent in the production of a diagnostic test that has contributed in saving many lives– the LAL (Limulus amebocyte lysate) test. This test has become essential for sterility monitoring of many medical products and is, in fact, the gold standard when it comes to the detection of pyrogens, as these fever inducing substances are not eliminated by standard sterilization processes. Pyrogens, when introduced into the bloodstream, are a great risk to human health. And while some reactions may be as mild as fever, others can be as severe as septic shock and death (Fennrich, Wendel, Hartung 1999). The application of the LAL assay in the quality assurance of medical products is quite far reaching. Injectable drugs, implants, and cellular therapies have understandably benefitted greatly from this diagnostic assay (Fennrich, Wendel, Hartung 1999), and it is not difficult to surmise why the impact of this assay on products for immunocompromised patients, such as chemotherapeutic agents, could be immense (Siegel, 1976).

Not only is the horseshoe crab indispensible in the production of diagnostic assays, but also it may provide the building blocks for drug therapy. A peptide isolated from the horseshoe crab—LALF— has demonstrated the capability of forming a protective effect against meningococcal endotoxin shock in rabbits. In fact, rabbits which were administered this horseshoe crab derived treatment were more than ten times more likely to survive when exposed to the endotoxin responsible for gram negative septic shock as compared to the rabbits which did not receive treatment. Therefore, this peptide isolated from the horseshoe crab is a potential therapeutic agent for the treatment of gram-negative septic shock (Alpert, 1992).

In summary, the horseshoe crab has proven to be a key player in the production of reagents for the LAL assay and LALF, both of which have had significant impact in the world of biomedical science. Two notable investigations of practical adaptations of the LAL assay were the adaptation for private physician use in the diagnosis of gonorrhea (Spagna, 1976) and adapting to use the test’s high sensitivity for the detection of endotoxemia associated with gram-negative sepsis in neonates (Goldstein, Reller, Wang, 1976).