Meet the humble horseshoe crab.

It’s not an especially good-looking arthropod. It’s not especially good eating, either. Indeed, apart from its rubbery, barely-edible roe, this hangover from the Paleozoic era, which is often described as a living fossil, is more likely to give you food poisoning than not. No wonder it has historically been used as bait and fertiliser.

A waste of aquatic space? Think again. You owe this ugly, inedible specimen far more than you know.

The horseshoe crab has been protecting you your entire life. Whenever a person has gotten an injection, the horseshoe crab has been there. Whenever a drug has been certified for human use, the horseshoe crab has been there.

Or at least its blood has.

That’s because the cells of this remarkable liquid – which is a vibrant baby blue in colour, as though its medicinal properties weren’t otherworldly enough – contain a chemical that can detect the merest traces of bacteria and trap them in inescapable clots. Pharmaceutical companies use the chemical – which is known as coagulogen – to detect contamination in any solution that might come into contact with blood.

If there are dangerous bacterial endotoxins in the solution in question – even at concentrations as small as one part per trillion – the coagulogen kicks into gear and turns the solution into a gel.

The Limulus amebocyte lysate (LAL) test – named after the crab’s scientific moniker – is not only simple, but almost instantaneous, and has replaced the rather macabre method that preceded it: getting together a bunch of rabbits and injecting them with potentially contaminated substances.

Of course, it also means that scientists need a lot of horseshoe crab blood. Enter the horseshoe crab milkman.

This stage of the process is a little rough, at least for the horseshoe crab. The tissue around its heart is pierced and up to 30 percent of its blood drained away. The LAL is extracted and goes for about A$19,750 per litre.

There is an upside for the crabs: most of them live to tell the tale. While between 10 and 30 per cent of them die, according to various estimates, the rest are returned to the ocean a long way from where they were initially picked up, reducing the risk of them undergoing the process, which takes between 24 and 72 hours, again.

The upside has a downside, though, with scientists finding that the bleeding process makes the animals more lethargic and less likely to follow the tides, with the result that females, in particular, are less likely to breed. This is obviously not only a bit of a raw deal for the crabs, but also for the future of, you know, medicine.

Luckily, science is on the case. Biologist Ding Jeak Ling from the National University of Singapore has already succeeded in producing the key bacterial detection enzyme, known as Factor C, in yeast. German company Hyglos has been working on synthetic endotoxin detector, too.

Should these or other projects be successful, the horseshoe crab will be out of a gig, at least as far as medicine is concerned. It will return to its position in the scheme of things as bait, fertisliser and a source of third-rate caviar.

We should never forget what it’s done for us, though, or how much we owe it. It really is a prince among shellfish.

The blue blood makes sense after all.