Horseshoe Crab | Characterized by its big protective shell, the horseshoe crab is a great example

One of the oldest animals on earth, horseshoe crabs have been roaming the seas for hundreds of millions of years. Indeed, they have had to adapt to continents forming and reforming and have survived Ice Ages and whatever it was that made the dinosaurs and so many other species go extinct. When humans arrived they discovered that horseshoe crabs could be used as food and as bait to catch other kinds of food, and now they are used for medical research.
Some fascinating facts about the horseshoe crab include:
These animal are arthropods, which means it’s related to spiders.
They’re not crabs. They’re not even crustaceans.
They evolved about 255 million years ago, and some consider them living fossils.
The blood of Carcinoscorpius rotundicauda contains tetrodotoxin, a deadly poison that attacks the nervous system.
Speaking of blood, the blood of these animals is sapphire blue when exposed to oxygen.
The four horseshoe crab are Carcinoscorpius rotundicauda, which is the mangrove horseshoe crab. This crab is found in southern Asia. Limulus polyphemus is the Atlantic horseshoe crab and it lives along the east coast of the United States and Mexico. Tachypleus gigas is the Indo-Pacific horseshoe crab, and it is also found in southern and southeastern Asia. Tachypleus tridentatus is the tri-spine horseshoe crab, and it lives in southeast Asia and East Asia.

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