Why is the Blood of a Horseshoe Crab So Valuable?

Horseshoe crabs have blood that is 'blue' in color. This is because they have hemocyanin instead of hemoglobin (which humans possess), that uses copper to carry the oxygen throughout their bodies.

Horseshoe crabs are large marine arthropods but not actually crabs. They are characterized by their horseshoe-shaped dome carapace and pointed tail. Belonging to the family Limulidae, they are referred to as living fossils as they are said to have existed since 450 million years. They dwell in or near shallow ocean waters. They prefer sandy bottoms.

They have a peculiar feature about their blood. Their blood tends to clot only in the presence of endotoxins. This is a way in which their immune system protects them from an infection, triggering a clotting action when bacteria are present. Some believe that their bacteria-rich habitat is a possibility for them to acquire such a tendency.

▶ It is the presence of a compound called Limulus Amebocyte Lysate in their blood that causes the coagulation process in case of an infection. That is, the compound binds and clots around the bacteria in order to get rid of the infection.

▶ It is this feature that compelled scientists to consider a horseshoe crab's blood for endotoxin-related testing that is being carried out in the pharmaceutical and medical industries today. This testing has gained a lot of importance as the blood of a horseshoe crab enables the detection of bacterial endotoxins in intravenous drugs and medical equipment.

▶ The pharmaceutical and medical industries need to exercise great care and caution for the production of drugs and medical equipment that deliver drugs, etc. Most importantly, their products need to be sterile and non-toxic for which these are usually sterilized.

▶ Endotoxins are bacterial toxins that tend to remain even after heat sterilization. The introduction of endotoxins into the bloodstream by use of such contaminated intravenous drugs, vaccines, or other medical equipment would cause serious health problems. These serious consequences can result even when contaminated with very small amounts.

▶ In order to avoid such consequences, horseshoe crabs are being gathered from the oceans in large numbers. They are then taken to laboratories where their blood is collected. Then, ambeocytes (a type of immune system cell) is separated out and the clotting element is extracted by breaking them open.

▶ After the bleeding process, most of the horseshoe crabs are returned to the ocean. However, there are a few that die due to the overall process.

▶ The testing that is done is called Limulus Amebocyte Lysate or LAL test. The sample to be tested is mixed with the test solution and kept aside for some time. Then based on whether the clotting process has occurred or not, the presence of endotoxin can be detected.

▶ Due to this, the blood of horseshoe crabs has become very valuable and important to humans. Moreover, it is said to be the only reliable and easy technique for the detection that has successfully saved so many human lives.

▶ In spite of these drawbacks, the LAL test has been used widely in the diagnosis of health conditions like urinary tract infections and spinal meningitis.

▶ It has also been used in the assessment of food spoilage, quality of air and water, as well as to investigate whether new drugs are capable of neutralizing the toxic results of endotoxins.

▶ It is believed that there are two major drawbacks of this testing. Firstly, the difference between live and dead bacteria cannot be recognized.

▶ Secondly, the species of bacterial endotoxins cannot be differentiated.

The LAL test is an expensive process. Hence, the high cost factor is a reason for research studies being conducted to obtain a synthetic imitation that is able to substitute the use of the live horseshoe crabs.

Furthermore, other compounds in the horseshoe crab blood have also been detected, which are believed to be useful for medical purposes. These detection have enabled a new test for fungal infections.

Moreover, it is believed that an endotoxin-neutralizing protein (or ENP) that is present in the blood can be made synthetically. With this, the use of live species for the LAL compound can be completely avoided. Presence of proteins that exhibit antiviral and anticancer behavior is also one of the detection.

▶ A great decline in the population of the horseshoe crabs has been observed. Farming of horseshoe crab blood may be a reason.

▶ However, there is another cause that contributes to this decline to a larger extent―the use of horseshoe crabs by fishermen as a bait to trap eels and conch.

For this, each year, around a million horseshoe crabs are harvested, while a lesser number of species are harvested for medical testing purposes, out of which many do survive. They were also used in fertilizers wherein these species were harvested in large numbers.