I’m not very familiar with horseshoe crabs. It turns out there’s a lot to learn – because any species that has managed to survive for 450 million years likely has at least a couple of evolutionary tricks up their sleeves.
If crabs had sleeves, I mean.
First up: horseshoe crab blood is bright blue, due to copper-based hemocyanin it uses to transport oxygen (instead of the hemoglobin that makes our blood red). Also, instead of using white blood cells to fight infection, they use amebocytes – and the Atlantic horseshoe crab has evolved to the point where their amebocytes of great value to the medical community.
These amebocytes coagulate around extremely small amounts of bacterial contamination, and the reaction takes only 45 minutes as opposed to the 2 days it takes most mammals’ immune systems to respond. Medical laboratories use it to test equipment and vaccines in a much more efficient manner, which prevents people from dying of infections.
The value of horseshoe crab blood is unfortunately leading to overharvesting – a quarter of a million crabs are harvested for their blood every year – and the population is in a steep decline that may be impossible to recover from, unless extreme measures are taken.
The crabs aren’t killed for their blood, though; 30% of their blood is harvested, then they’re returned to the ocean. That said, around 10-30% of the crabs don’t survive the process, and females who are bled often breed less afterward.
But the blood goes for $15k a liter, so I doubt people are going to stop anytime soon.