Why are squids blood blue?

To understand why octopus blood is blue, we need to first understand why our blood is red. The blood of most vertebrates (animals containing a backbone..ie: Humans, dogs, birds, reptiles, etc...) is red because our blood contains a protein called hemoglobin. Hemoglobin is the molecule responsible for carrying oxygen. Hemoglobin is located in red blood cells. When red blood cells enter the lungs, hemoglobin binds oxygen molecules, and as the red blood cells flow throughout our bodies, hemoglobin releases the oxygen to our cells, nourishing them. The way that this is done is through the use of Iron. Iron (specifically Iron III) is an ideal molecule for binding oxygen. When iron binds oxygen it turns RED. This is why our blood is red; because the hemoglobin within our red blood cells contains iron, which turns red when bound to oxygen. When hemoglobin is not bound to oxygen, our blood is blue as well. Go ahead, look at one of your veins. Veins carry oxygen-depleted blood back to the lungs and heart. Until this blood contacts more oxygen, it remains blue. Now to answer the question: why is octopus blood blue? In short, Octopus blood lacks hemoglobin, and hence lacks iron. Oxygen transfer is accomplished by another (less efficient) protein known as hemocyanin. This protein, as well as other molecules in the octopus' blood give it the characteristic blue color.