A True Blue Blood

Let’s have a look at a real oddity of nature: the Horseshoe Crab.

The horseshoe crab isn’t really a crab; its closest relatives are the spiders.

Now, normally, I am allergic to spiders, seriously allergic…the sight of a hairy arachnid can have me fleeing from a room, while screaming at the top of my lungs. Yes, I know that I shouldn’t be scared, but my brain hasn’t any say in the matter; in fact, all of Lynne’s higher brain functions shut down in the presence of a huntsman spider. I love crabs, on the other hand, usually steamed with tartar sauce. But I would never, ever try to harm a horseshoe crab. I certainly wouldn’t treat it like one of its terrestrial cousins, such as hitting it with a broom and seven litres of insect spray.

The horseshoe crabs, Class Merostomata, are represented by just five species. One species, Limulus polyphemus, is a native to the Americas, while the other four species inhabit Asian waters from Japan and Korea south to Indonesia and India.

These arthropods have survived relatively unaltered since the Silurian era, part of the Palaeozoic era; their larva even mimics the form of a miniature trilobite, but this is only a superficial resemblance. The Silurian was time of remarkable stability in our environmental history, following on from a period of unstable climatic conditions; this the period when the coral reefs first formed, and there was a major diversification in all types of fish, and vascular plants first evolved. It is when animals first began to colonise the land, including the horseshoe crab’s relative, the spiders.

In recent times, horseshoe crabs were considered of little economic importance, except as an Asian delicacy or as fertilizer. This changed when science discovered the special properties of the blood of the Limulus species.

Horseshoe crabs have blue blood. Their blood is oxygenated by haemocyanin, rather than haemoglobin as is the case in our own blood. The haemocyanin contains copper, instead of iron, and hence the bright blue colour. But this isn’t the only peculiarity of the Limulus blood and the horseshoe crab’s circulatory system.

Human beings have a ‘closed’ circulatory system, where our blood flows enclosed in arteries and veins. The horseshoe crab contains its blood in large sinuses that allow the liquid direct contact with the animal’s internal organs. This means that a crack in the shell has potentially fatal consequences; invading bacteria can enter such a crack and then have instant access to a large volume of the crab’s interior. The horseshoe crab is cold-blooded; it can not raise its body temperature to fight off an infection, so it can never ‘run a fever’ to fight colonising bacteria. Instead, it has special, sensitive blood cells, called amoebocytes. These cells are activated when they detect a bacterial toxin; they release a clotting agent that traps the bacteria. The clotting agent also acts to prevent further invasion of the crab’s interior by sealing the crack in the shell. This process was discovered by Dr Fred Bang, while researching the crab’s circulatory system.

Now, how does this make the horseshoe crab so important to science?

Glad you asked.

Medicine has found numerous uses for the unique properties of the blue crab blood. The blood’s sensitivity to toxins, not just bacterial toxins, makes it the fastest and cheapest method of detecting endotoxins and contaminants, and the one of the most accurate. All you have to do is add your sample to the blood, and if there is a problem, a clot will form within forty-five minutes.

The old method involved injecting rodents, usually rabbits, and waiting for the rabbit to get a fever or die. This could take days…

The blue-blooded method can be used to detect contaminants in our blood, and to confirm the sterility of medical equipment. There is a huge industry producing injectable drugs, surgical tubing and threads, irrigation fluids and the rest, and they all have to be free of bacteria and their associated endotoxins.

Endotoxins have been studied for years, as they are fatal to most mammals. They cause a range of symptoms: fever, a rise in the number of white blood cells, blood coagulation, hypotension, shock and death. Even a small dose of bacterial endotoxins can be lethal to mammals. So, a cheap and quick method of detecting any sort of bacteria, not just dangerous bacteria, is big business.

And so, the horseshoe crabs have become big business. This has been a blessing for the arthropods. They are now a protected species, no longer harvested for fertiliser; you need a special licence to collect them. They are not killed to extract their blood, as a dead horseshoe crab can not produce more blood further down the track, nor – more importantly – breed more animals. So, captured individuals have a small, carefully regulated amount of blood removed, and then are released back into the wild.

It’s one of those rare win/win situations. The horseshoe crabs are thriving, and the medical industry benefits from a quick & cheap method for detecting endotoxins. There is something to learn from this: not even an ugly or boring animal should be allowed to go extinct, as you can never predict how they may benefit the environment or mankind. I could harp on about the Platypus Frog at this point…but as I’ve previously lectured on that topic, I won’t. Still, even I might just try to chase the next hairy spider outside, rather than kill it.

Just in case.