Horseshoe crab: Blue-blooded living fossil of the sea (Guest writers)

By Sim Yan Ling (Millennia Institute)

Horseshoe crab

Many of us who travel often to the seaside in Malaysia or Thailand may be familiar with the horseshoe crab. In this blog post, I would like to share what I have found out about this blue-blooded marine creature.

A horseshoe crab reminds me of the lucky charm, though you’d have to imagine the charm in the shape of a crab with a horseshoe-like imprint on its shell, as if it was trodden upon by a horse.

The horseshoe crab is considered to be a prehistoric creature as it has not evolved for 300-400 million years. It is an important creature to ecologists, as it predates most species on the planet – its evolution extends back far before humans, before the dinosaurs, and can be traced all the way back to the era in our planet’s history when visible life first appeared.

Cooked horseshoe crab


There are currently four species of horseshoe crabs around the world with three of them found in Asia. In parts of Asia, its roe is considered to be a powerful aphrodisiac and commands a hefty price. This has led to over hunting, poaching, and even incidents of theft of the horseshoe crab.

Despite its rumoured powers, it is said by some that the taste of the roe is not really pleasant to the palate and over consumption can even cause poisoning.

Horseshoe crab and its lifesaving blood

The horseshoe crab’s blood is blue because it does not possess haemoglobin, an iron rich chemical which carries oxygen and gives blood a red appearance. Instead, the blood possesses hemocyanin, a copper rich blood compound that transports oxygen to the vital parts of their bodies. It is the copper present in hemocyanin that gives their blood its well-known bluish appearance.

One little-known fact is that the horseshoe crab plays a significant role in medicinal science today, so much so that it might not be an exaggeration to say that it protects public health.

Anyone who has received an injection of medicine would likely to have benefited already from the horseshoe crabs’ contribution. Their blue blood has super healing and bacteria fighting powers, and scientists have been using this ability in many of our medicines today. Pharmaceutical laboratories use an extract of the horseshoe crab’s blood to ensure that their products, such as intravenous drugs and vaccines, are free of bacterial contamination. It is said that no other test works as easily or reliably for this purpose.

Testing with horseshoe crab blood is less cruel too compared to the infamous bunny-testing. In the past, the only way to test for the presence of bacteria is to inject a bunny (the poor thing!) with the vaccine and see if it gets sick.

Thankfully for horseshoe crabs, the blood extraction process is akin to blood donation for us human beings. After harvesting about 1/3 of their blood, the horseshoe crabs are returned to the wild. Laboratories also noted that some of the same horseshoe crabs were recaptured every year, meaning that their survival rates are quite good after the release.

Role of horseshoe crab in ecology

While its many uses made them very valuable to us, horseshoe crabs also play an important ecological role in the food web. A decline in the number of horseshoe crabs will impact other species, particularly shorebirds and sea turtles.

Horseshoe crabs usually thrive in the shallow banks of oceans which are the first to become contaminated from human activities such as littering or destroyed due to land reclamation activities. It is sad that these ancient creatures, which have survived for so long, are becoming endangered now. Humans have become their largest predators with the consumption of their roe.

Seeing the horseshoe crab was an eye-opener for me. It is quite a remarkable thing that nature has bestowed on us and I count myself lucky to be able to see these ancient creatures in real life. I hope that it will survive the threat of human’s greed, so future generations can see a live horseshoe crab like me.

Yan Ling is a student from Millennia Institute who writes for MI Wired, an online platform for students to showcase their writing. She is an advocate of social and environmental causes.


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