HORSESHOE CRABS DONATING BLOOD. Horseshoe crab blood is harvested in much the same way that human blood is donated. There is a good picture in Caren Chesler’s article (in the section captioned “The Catch”) of a row of horseshoes crabs giving blood (the blood is an unusual and beautiful blue.) Only about one third of a crab’s blood is taken. Then it is returned to the sea. But how do crabs do after giving blood? Much of Chesler’s article consists of current speculation about whether horseshoe crabs are harmed by giving blood. At the moment 500,000 crabs are bled each year. Global demand is growing at about 8% a year. While not an endangered species, horseshoe crabs are listed in the next category down—“vulnerable”.

From an evolutionary point of view, it seems remarkable that horseshoe blood is so useful in modern medicine, given that the crabs go back 450 million years.

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