Photo credit: PBS. Horseshoe crab blood being drained for use in checking the sterile state of medical equipment.
During World War Two, soldiers learned to fear treatment as much as enemy bullets. Unsanitary conditions and equipment in field hospitals made open wounds a breeding ground for bacteria that killed thousands, particularly the fast-acting and barely detectable gram-negative strains that caused toxic shock syndrome, meningitis, and typhoid.
Today, vast improvements in medical hygiene have greatly reduced the odds of patients being poisoned on the treatment table. And our safety is protected by an unlikely source — the bright blue blood of the horseshoe crab.
The helmet-shaped creature has developed a unique defense to compensate for its vulnerability to infection in shallow waters. When faced with toxins produced by bacteria, amebocyte cells in the blood — colored blue by their copper-based molecules — identify and congeal around the invading matter, trapping the threat inside a gel-like seal that prevents it from spreading.
You don’t survive for 450 million years without learning a trick or two. In the case of horseshoe crabs, one of those is developing blood with remarkable antibacterial properties. Humans haven’t been around nearly as long, but we learn fast, including how to harness those properties in ways that save our lives, but aren’t so great for the crabs.
Do you give much thought to horseshoe crabs? No, me neither. But it turns out that without them, we could be in a very precarious position. Horseshoe crabs – or to be more precise, their incredible, baby blue blood – are used to test for bacterial contamination, thus saving countless lives each year during medical procedures. The only trouble is, we have to catch a quarter of a million horseshoe crabs each year to do this, and then we have to drain their blood.
The most obviously unusual aspect of crab blood is that it is bright blue, a consequence of using copper-based hemocyanin to transport oxygen where vertebrates use iron in hemoglobin. Instead of white blood cells to fight infection, many invertebrates have amebocytes, and Atlantic horseshoe crabs (Limulus polyphemus) have evolved these to such a peak of refinement that they are of enormous medical value.
Horseshoe crab amebocytes coagulate around as little as one part in a trillion of bacterial contamination. Even better, the reaction takes 45 minutes, not two days as with mammalian equivalents. Coagulan, the chemical that makes this possible, is used for testing medical equipment and vaccines prior to use, without which many more people would die from infections. Unfortunately, coagulan synthesis is in its infancy so a quarter of a million crabs are harvested each year for their blood, as shown in this video:
Unfortunately, overharvesting in North America has led to a decline in crab populations, with worrying implications if this continues. In order to sustain the species, the harvesters take 30% of the blood from each crab, after which they are returned to the ocean. While this portion of blood is meant to be an amount the crabs can survive, it is thought that 10-30% don’t. Moreover, among the females that do recover they often breed less after being bled. However, at $15,000/l plenty of people still think the crab blood is worth bottling.