Virtually unchanged, the horseshoe crab has existed for at least 450 million years. Having had plenty of time to perfect its immune system, horseshoe crabs developed feisty and highly specialized immune cells, called amebocytes. Similar to our white blood cells, amebocytes roam the blood of horseshoe crabs and fight invasive pathogens. The clotting agent in amebocytes is the only existing source of a very special medical product called Limulus amebocyte lysate or LAL. One litre of the highly-priced substance is worth USD $16,000. Since its approval in 1970, LAL has been the most widely used method of identifying bacterial contamination in vaccines and prosthetics, such as artificial heart valves. The clotting agent in the LAL reacts to a group of substances in the cell wall of bacteria called endotoxins. The LAL binds the endotoxins and forms a jelly-like cocoon around the bacteria, in this manner the LAL signals the contamination of the vaccine or prosthetic. Before the approval of LAL, lab workers had to test every new vaccine badge by injecting a sample into a cohort of rabbits and waiting for signs of infection to occur.
The pharmaceutical industry currently catches more than 430,000 horseshoe crabs every year to satisfy the market’s need for their blue blood. It is common practise to drain 30 % of the horseshoe crabs’ precious blood and soon after to release the animals back into the wild. A recent study on survival rates estimated that about 5 to 20 % of crabs don’t survive the bleeding procedure. Those lucky survivors face a difficult first two weeks back in the ocean. Many crabs are disorientated for several days and females have difficulties breeding. Although the pharmaceutical industry has been working on an artificial replacement for amebocytes for some time, experts predict that it will still take many years until drug authorities find a potential replacement fit for approval.
There are four species of horseshoe crabs currently alive. One inhabits the shallow waters along the east coast of North and Central America. The other three occupy the Indo-Pacific region. Over the last 450 million years horseshoe crabs have not only survived the dinosaurs but an additional three mass extinctions in earth’s history. In 2016, the International Union for Conservation of Nature (IUCN) added the blue bloods to the list of vulnerable species. Delaware Bay, situated between Phaldelphia and Washington DC, once boasted the densest population of horsehoe crabs in the USA. Now the numbers have declined by 90 %. This is only in part due to harvesting for blood. Once a year, from May to June, the horseshoe crabs need to go ashore for spawning. In a large breeding frenzy, several males compete for a female and try to fertilize her eggs. The females leave their eggs buried in the soft sand of the beach. The continuous increase in coastal development leaves less beaches suitable for breeding.
Hence, a large number of migratory shore birds that depend on the eggs as a valuable food source struggle to fill up their energy stores that are depleted from their long journey. Fishermen catching horseshoe crabs for bait causes yet another threat. Although local authorities in the US recently restricted such practices, scientists still estimate that more horseshoe crabs are caught for bait by fishermen than for blood harvesting by the pharmaceutical industry. Another threat to the species is the decrease of coastal water quality. In recent years, fertiliser run-off has caused re-occurring algae blooms that killed large numbers of marine life including the horseshoe crab.
One general assumption about the horseshoe crabs was wrong all along. They are not actually crabs. In fact, they are much closer related to spiders. Although scientists suspected this for quite a while, a recent genetic study led by Jesús Ballesteros from the University of Wisconsin-Madison finally concluded that horseshoe crabs belong to the same class that also includes spiders, mites, ticks and scorpions. They are arachnids after all.
Blue blood fun fact:
It is not the precious amebocytes that are responsible for the seemingly unusual blue colour. It is the blood’s haemoglobin equivalent. Like many other marine invertebrates such as octopus, horseshoe crabs do not utilize iron-based haemoglobin for oxygen transport, like mammals do. Instead they use haemocyanin which is a copper-based metalloprotein and has a blueish colour. Scientists believe that copper fulfils the oxygen requirements of marine invertebrates much more efficiently than iron. As invertebrates can’t regulate their body temperature, like mammals do, they take on the temperature of their surroundings. In low temperature conditions, copper-based haemocyanin transports oxygen through the blood more reliably than haemoglobin.
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