Horse-shoe crabs aren't actually crabs, and are more closely related to scorpions and spiders. They are sometimes referred to as 'living fossils', because their species has remained virtually unchanged for 200 million years. Though they look dangerous, they don't actually bite or sting; their tail is only used to keep them upright. They also have blood the color of a cloudy blue sky. This blood is vital for many human medical applications.
In the 19th century, doctors would give patients injections using sterilized needles, but the patients occasionally reacted badly to the toxins left behind by bacteria. Sometimes the patients even died from it.
By the 1950s, doctors had found a method to check the sterilized needles for any toxins. The standard test was to inject a live rabbit, then check it's temperature every half hour. If the rabbit didn't catch a fever, the needle was almost certainly toxin-free. Under a microscope, a rabbit's blood cells could also be seen clumping around the toxin, if it was present.
A pathologist discovered that horse-shoe crab blood clumps in a similar way. Scientists learned how to create a Limulus amebocyte lysate (LAL) test using this blood. The LAL tests are absolutely essential to humans; we use them for every vaccination, insulin injection, medical IVs, and implanted devices (like hearts!).
The Process, and the Problem
The horse-shoe crabs are scooped in nets and shipped to labs. From there, they are hooked up to machines, and scientists collect some of their blood. The creatures are then returned to the ocean.
Unfortunately a significant number of them don't survive. Sometimes they die because of the bleeding, but most often the cause is injury from the process of collection. They also suffer because of their time out of water.
Humans Helping Horse-Shoe Crabs
Using biotechnology, scientists have found a way to perform these life-saving tests without harming any more horse-shoe crabs. The scientists spliced the gene responsible for the creature's amazing blood properties into insect gut cells, and it worked: they now had an alternative to the LAL tests that didn't harm the creatures. The alternative is referred to as rFC (recombinant Factor C).
For various reasons, rFC didn't take off right away. First, it had many regulatory hurdles to clear. Afterwards, there was only one source for rFC, and people were reluctant to risk their entire business on a product that only had one supplier. People were also reluctant to leave known technologies for ones that were still in their infancy.
Diminishing horse-shoe crab populations forced researchers to take another look at rFC. It's even been approved for use in the European Union. While it hasn't yet reached the popularity of LAL tests, it's use has expanded dramatically. Good news for us, great news for horse-shoe crabs!