A horseshoe crab lies on sand.

Horseshoe crabs live around North America and Asia. They are threatened by fishing, the medical industry and habitat loss. Image © Jarous/Shutterstock.com.

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Horseshoe crab blood: the miracle vaccine ingredient that's saved millions of lives

During 2020, COVID-19 vaccines were created in record time.

Nature plays a huge part in the medicines we rely on every day. And when it comes to vaccines, we have horseshoe crabs and their blood to thank for keeping us safe.

Horseshoe crabs are older than the dinosaurs. They've been around for 450 million years, which means they watched the rise and fall of millions of other species, and survived ice ages.

As well as being incredible 'living fossils', they have also helped to keep most of us alive. If you have ever had a vaccine, chances are that it was tested for safety using horseshoe crab blood. And they're about to save even more lives, because they're playing their part in the creation of a Covid-19 injection.

Horseshoe crabs strapped into a lab, being 'milked' for their blue blood.

Horseshoe crab blood is a bright shade of blue, as well as having remarkable antibacterial properties that have proved invaluable to the medical industry. Image via Business Insider.

What is horseshoe crab blood used for?

Horseshoe crab blood is bright blue. It contains important immune cells that are exceptionally sensitive to toxic bacteria. When those cells meet invading bacteria, they clot around it and protect the rest of the horseshoe crab's body from toxins.

Scientists used these clever blood cells to develop a test called Limulus Amebocyte Lysate, or LAL, which checks new vaccines for contamination. This technique has been used all over the world since the 1970s to stop medical professionals giving out jabs full of bad bacteria that could make humans very sick.  

It's great for humans, because vaccines save us from all sorts of unwanted diseases, including measles and mumps. It's not so great for the horseshoe crabs: thousands of them are rounded up and bled every year.

A group of horseshoe crabs gather together to spawn.

A group of horseshoe crabs gather together to spawn. Image © Justin Warner via Flickr, licensed under CC BY-SA 2.0.

Horseshoe crabs and Covid

The world is rushing to find a safe vaccine to fight Covid-19, the viral lung disease which has swept the planet.

More than 100 different vaccines are being tested in the hope that one will work. The successful jabs will have to be carefully checked before they are rolled out.

In many parts of the world, researchers will be relying on horseshoe crab blood in those important tests. And since we'll want to vaccinate millions of people in a short space of time, horseshoe crabs could play a big part.

Why we should care

There are four species of horseshoe crab. Three of them live in Asia, around the coasts of India, Vietnam, China, Borneo and southern Japan. These are the tri-spine horseshoe crab (Tachypleus tridentatus), the coastal horseshoe crab (Tachypleus gigas) and the mangrove horseshoe crab (Carcinoscorpius rotundicauda).

The fourth is the American horseshoe crab (Limulus polyphemus) which lives along the east coast of North America, with a particularly large number going to the beaches of the Delaware Bay to mate each year.

These crabs may look prehistoric, but they do an important job of supporting other animals around them: their eggs are a nourishing snack for migrating birds. They are also good for fishermen because they help keep the sediment around coastlines healthy.

The International Union for the Conservation of Nature (IUCN) says, 'The horseshoe crab is a critical link to coastal biodiversity. One of their ecological functions is to lay millions of eggs on beaches to feed shorebirds, fish and other wildlife. Its large hard shell serves as microhabitat for many other species such as sponges, mud crabs, mussels and snails.'

But living alongside humans can be difficult for horseshoe crabs. They are often used as fishing bait. In Asia, they are also harmed by pollution, rising sea levels and building work.

Some die after being bled for medical testing, although we don't know how many. Plus, lots die because they strand upside down on beaches after coming to land to mate.

Horseshoe crabs on a barbecue.

The medical industry isn't the only threat to horseshoe crabs. They are also used for bait, eaten and pushed out of their natural habitat. Here, they are barbecued at a street food market. Image: Gunnerchu/Shutterstock.com

Another way

For years, no one managed to find any alternative ingredients to use in a test that were as sensitive as crabs' blood.

But there is hope: in the late 1990s biologists at the University of Singapore realised that a synthetic alternative could be created in a lab by cloning a molecule in the crab blood. This genetically engineered protein is called Recombinant Factor C, or rFC.

Some governments, including the Japanese and Chinese, have approved the rFc test for use. It is likely that a new Covid test manufactured in the UK will use synthetic ingredients, which are also approved by the European Union. Pfizer, a pharmaceutical company developing one of the biggest potential Covid vaccines, said they won't be using crab blood in their jab.

There's a catch in this story. Synthetic ingredients and alternative tests are not yet widely used in some countries. For instance, America still bleeds many crabs every year. A small percentage of them die after being bled, although medicine producers are becoming ever more careful about keeping population numbers healthy.

It's also arguable medicine manufacturers aren't the biggest problem facing horseshoe crabs: in America many more are killed for fishing bait and lots are struggling in Asia because their habitat is disappearing.

A scientists looks at a crab in a lab.

Synthetic alternatives to horseshoe crab blood have been developed. Image © Rattiya Thongdumhyu/Shutterstock.com 

Will horseshoe crabs go extinct?

The American horseshoe crab is not considered endangered (although it is classified as vulnerable on the IUCN Red List of Threatened Species) and the number of crabs caught is monitored.

American regulators and manufacturers also created guidance on how to reduce the number of crabs killed during the bleeding process, which is helping. Some crab populations are now increasing, although others are still struggling.

Some medicine companies have started sustainability programmes that rescue eggs from crabs that have been caught for bait. The eggs are fertilised, raised in a hatchery and released back into the ocean to try to keep population numbers stable.

The story in Asia is less promising. Crab mating grounds are being destroyed more quickly by rising sea levels and building work. The tri-spine horseshoe crab is classified as endangered. It is locally extinct in Taiwan, and may soon disappear from Hong Kong. The other two Asian horseshoe crab species are not thriving either.

In 2019 the IUCN and other conservation groups around the world called for stronger rules to protect horseshoe crabs, more scientific research, and better protections for their coastal habitat.

There is hope yet - and it perhaps won't be long until we can phase out the use of crab blood for good.

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