Horseshoe Crab

Eyes on the tail and brain around the mouth 

Is that a swimming helmet in the water? No, it is a horseshoe crab, a very peculiar animal with a rounded shell and a long pointed tail. The tail may look like it is used for stabbing, but in reality it is used to allow the horseshoe crab to turn over if it happens to end up upside down.  

This strange animal has about 10 eyes, scattered across its body. Two eyes are slightly larger and sit on top of the shell, but the rest are smaller, light-sensitive receptors. Several of the little eyes are on the tail! The brain of the horseshoe crab is positioned in a thin, round ring around the mouth. The horseshoe crab has 6 pairs of legs, but only 5 pairs are used for walking. The main pair is used for feeding, and since the species lacks both jaws and teeth, it needs to tear the food apart before it can be swallowed. 

Blue blood used for vaccines 

The horseshoe crab has blue blood. This is because they have a substance, a copper-based haemocyanin, that oxygenates their blood. Humans and many other animals have another substance, haemoglobin, which turns their blood red. When a foreign bacterium enters a wound on the horseshoe crab, it is encapsulated with a jelly-like material. This property has been very useful in medical research. The blood of the horseshoe crab can be used to ensure that no bacteria are present in medicines and vaccines – something that was discovered in the 1970s. This has become very important in order to ensure that fewer people die from infections. Most likely, the blood of the horseshoe crab has been used to test just about every vaccine you have received! 

However, the increasing need for horseshoe crab blood could become a problem for the species. The animals are captured and some of their blood is drained while they are alive – and then they are usually released back into the sea again. However, sometimes they are injured during handling, and some companies sell them on as bait to the fishing industry instead of releasing them. Nowadays, there is an artificial substance that resembles the blood of the horseshoe crab – which can be used in the same way. But blood from the horseshoe crab is still used on a very large scale, especially in North America. 

Older than the dinosaurs 

The horseshoe crab is actually not a crab at all. Instead, it is more closely related to today’s arachnids, such as spiders, scorpions, and mites. In the family to which the horseshoe crab belongs, there are only four living species left. The appearance and way of life of horseshoe crabs have remained largely the same for the past 400 million years – which is why they are sometimes referred to as living fossils. Horseshoe crabs are actually older than dinosaurs! 

In early summer, the horseshoe crab mates. Adult horseshoe crabs make their way from deep ocean floors up to the shores, where they gather in large groups. The females lay eggs in pits in the sand, and the males fertilise the eggs. A female can lay up to 4000 eggs in one session – and lay eggs up to 20 times in a year! Mating takes place at night, often during a full moon or new moon and high tide.  

The horseshoe crab larvae hatch after a couple of weeks, and are washed out into the water and find their way to the bottom. As they grow, they shed their shells and seek deeper and deeper into the sea. A horseshoe crab does not reach sexual maturity until around the age of 10 – and that is when it makes its way back to the beach again. The eggs and larvae of the horseshoe crab are a very important part of the diet of many birds and other animals.