Why do some creatures have blue-coloured blood?

We often think of blood as the red-coloured fluid made up of plasma, red blood cells, white blood cells, and platelets that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. While the majority of us know blood as a fluid of red colour, it also has different colours like blue, green, etc.

What creatures have blue-coloured blood running in their bodies? There are creatures like horseshoe crabs, octopuses, squids and certain insects that have blue-coloured blood running in their bodies.

What's the reason behind this blue colour of blood? It is because of the protein "hemocyanin" found in the blood of these animals. Just like haemoglobin, hemocyanin is also an oxygen carrier that transports oxygen throughout the bodies of these animals. But unlike haemoglobin which has an iron core, hemocyanin has copper in its heme part. Upon oxygen binding, Cu(I) oxidizes to Cu(II) and therefore gives the blood a blue colour.