The reason why these 310-million-year-old crabs have blue blood

Horseshoe crabs are known as “living fossils” meaning they have existed nearly unchanged for at least 445 million years.

The creatures, which have been around since before the dinosaurs, have long baffled people with the colour of their blood.

Horseshoe crabs use hemocyanin to carry oxygen through their blood.

It’s the copper presence in hemocyanin that makes the blood change colour.

The animals also have a clotting agent in the presence of certain groups of bacteria.

Their blood is sometimes used in medicine to ensure that anything that gets injected or implanted into the human body is free of potential bacterial contamination.