Horseshoe crabs are marine invertebrates that live in and around shallow water with soft or muddy sands. They have not changed much since 450 million years ago and so are commonly referred to as ‘living fossils’.
Special gills found behind the back legs (called ‘book gills’) exchange respiratory gases and can also be used for swimming. When these crabs swim, they actually do so upside down - though only at 0.3 miles per hour.
Horseshoe crabs do not have haemoglobin in their blood, but instead use haemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is bright blue. This blood is used in the medical industry for a variety of purposes and is often harvested from wild crabs; the mortality rate of this process for the crabs ranges between 5-30% depending on the methods used.