The horseshoe crab is not actually a crab

(Photo by Kev McLoughlin)

The horseshoe crab is an ancient creature that was around way before the dinosaurs 445 million years ago and has not really changed: they are known as “living fossils”. This marine animal is an arthropod, closely related to the well-known scorpions and spiders on land and the Pycnogonida, a small sea spider. They belong to the Limulidae family, containing only four species, which differ in their habitats.

They are good dancers with eyes everywhere!

These funky animals swim upside down with their five pairs of legs! Take care not to step on them; they have a hard carapace, which changes over their lifespan. And there’s no need to worry about your feet – their favourite meals are worms, molluscs and crustaceans.


The horseshoe crab’s eyesight is better at night – they have nine eyes, some of which can even detect UV light! Their body is separated into two distinct parts, with a tail at the end called the telson, which is not a sting; they use it to flip their body over if they’ve turned upside down on the beach.

Mating: they stick together but not forever

(Photo by Kev McLoughlin)

When mating season comes around, horseshoe crabs meet on the beach to find the best partner. Surprisingly, the male chooses the female, even though females can be 30% bigger than them … brave guys! A female can produce between 60,000 and 120,000 eggs over a year and, with the male clinging to their back, will dig a hole to lay a cluster of around 4000 eggs in the sand.

After checking the tide (just as we do before we go diving), she will return to lay more eggs until the end of the season. This will increase the chance of offspring survival, as birds are sneakily waiting for a fresh lunch …

Horseshoe crabs’ blue blood can save millions of lives!

Unlike us humans, horseshoe crabs have light-blue blood. Our blood is red because iron molecules react with oxygen in haemoglobin. Instead of haemoglobin, horseshoe crabs have haemocyanin, which contains copper and this becomes blue when it reacts with oxygen.

The amebocytes in the blue blood have the ability to detect bacteria, and this is being used in the medical field to save humans lives. For this reason, it is very valuable and the price can be up to USD15,000 per litre!

The process of harvesting the blood from the horseshoe crab
(Photo: National Geographic/Getty Images)
A lot of them died as heroes

Sadly, not every horseshoe crab can survive the harvesting process that drains one-third of their blood. In fact, scientists estimate that 10 to 30% cannot handle the blood loss, not to mention the stress experienced during handling and transportation. Studies also show that, after being bled, females travel less frequently to spawning areas and this, therefore, reduces the reproduction rate. Breeding horseshoe crabs in captivity is difficult, and with the high market price for their blood, they are still being harvested from the wild.

Horseshoe crabs could be endangered soon

Besides being over-harvested, they are also in jeopardy due to shoreline development. The world’s population of horseshoe crabs has reduced greatly. The North American species, Limulus Polyphemus, is under the “Vulnerable” category in the IUCN Red List, and the status of the other three species is unknown due to lack of data. With growing concern over declining populations, regulations on the harvesting of horseshoe crabs have recently been established in some countries.

(Photo by Kev McLoughlin)

We can all help by putting more effort into conserving and restoring their natural habitat and hope that this cool creature that has survived all five of the Earth’s major extinction events can escape human threat!

Article by: Héléna Bouyer & Manman Mok


1. “Oldest horseshoe crab fossil discovered” on
2. “Limule” on
3. “The horseshoe crab” on
4. “How Horseshoe Crab Blood Saves Millions Of Lives” on
5. “How We Harvest Horseshoe Crab Blood to Save Human Lives” on
6. “Crash: A Tale of Two Species. The Benefits of Blue Blood” on
7. “As Horseshoe Crab Populations Steadily Decrease, Their Indispensable Medical Use Is Threatened” on