Why do horseshoe crabs have blue blood?
Horseshoe crabs are twice as old as the dinosaurs, and despite their name are not crabs at all, but are more closely related to spiders and scorpions and the largest, scariest insect that ever lived, the prehistoric eurypterids.
Considered living fossils themselves as they are known from as far back as the Cambrian period (500 million years ago), there are four species of horseshoes known today and they can be found living in waters around the USA and Asia.
Their bodies are full of ancient features. They have 10 eyes - located across the shell and even on their tail - as well as book gills. These flap-like structure are held externally, not internally like most gills and lungs. They also have bizarre blood.
Vertebrate blood uses haemoglobin, an iron-based chemical that is effective at binding with oxygen, the fuel used to power muscles and organs. This explains why our blood is red, it's the oxygen reacting with the iron contained with the haemoglobin. Horseshoe crabs use another substance called hemocyanin. Instead of iron this contains copper, and when it carries oxygen to the muscles and organs of the invertebrate, it changes from a colourless substance to blue.
Importantly, the blood also uses amebocytes against pathogens in the same way our blood uses white blood cells. In medicine this can be harvested and used to create Limulus amebocyte lysate, which can be used to detect things like E. coli on many medical instruments and injectable drugs like insulin. This makes the blood valuable ($14,000 per quart) and an industry has grown where the creatures are collected as they approach the shoreline to spawn (an estimated 500,000 a year), bled of up to half their blood and then returned to the sea supposedly unharmed.
In reality, it's believed 3 to 30 per cent of the animals die from this process. A synthetic form has been developed recently and this will hopefully end the pressure on this ancient, blue-blood living fossil.
Response: Phil Hore
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