Science //

The crabs you want to have

Felicity Nelson discusses the blue equivalent of liquid gold.

Illustration by Monica Renn.
Illustration by Monica Renn.
Illustration by Monica Renn.

Blood is rarely worth its weight in gold, but horseshoe crab blood is an exception. The bright blue liquid extracted from these ancient sea creatures costs around $15, 000 per litre.

More closely related to spiders than crabs, horseshoe crabs resemble large brown helmets with legs. Just as the bright red colour of human blood is due to the iron in haemoglobin, the cool blue colour of horseshoe crab blood comes from their copper-based oxygen receptor, haemocyanin.

Every year, 20,000 Atlantic horseshoe crabs are pulled from the ocean and sent to five licensed companies that extract their blood into glass bottles. Those bled are returned to where they were found within forty-eight hours, and most fully recover from the blood loss.

From these samples, a unique clotting agent, Limulus amebocyte lysate (LAL) is extracted and used to test for the presence of dangerous fever-inducing endotoxins that persists in medical equipment, vaccines or blood donations after sterilisation. Without a highly sensitive test, it would be almost impossible to perform a lot of surgery safely as even a microscopic trace of these chemicals can lead to grim immune responses or even death.

Before the mysterious properties of horseshoe crab blood were unravelled in the 1960s (and I honestly don’t know how someone came up with the idea of using horseshoe crab blood), scientists and medical professionals resorted to Rabbit Pyrogen assay (rabbit-lovers cover your eyes!) A small amount of the solution of interest would be injected into a rabbit for the monitoring of its body temperature. A fever in the rabbit indicated the presence of an endotoxin, however, unlike LAL assays, this could not predict the concentration of the toxin.

Today, rabbits are still used to test for toxins in hepatitis B vaccines, but horseshoe crab blood is generally the preferred method. The irony is that the horseshoe crab, like all invertebrates, lacks a “proper” immune system – it can’t create specialised antibodies to fight specific infections and instead relies on a set of primitive immune cells. Luckily for us, one of these bacteria-fighting agents works perfectly for our drug-testing purposes.

In case you aren’t convinced that these armoured dinner plates are the most amazing things ever; the chitin skeleton of horseshoe crabs is used to make wound dressings for burn victims, reducing healing times by 50 per cent. What’s more, the study of their optic nerve has resulted in major breakthroughs in the understanding of human vision.

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