Not all blood is red.
Crab blood is blue, because it contains copper instead of iron, which makes blood red. The blood of earthworms and leeches is green, thanks to an iron substance called chlorocruorin. Many invertebrates, such as starfish, have clear or yellowish blood.
Whether it’s red, blue, green, or yellow, all blood carries oxygen to body cells. The metals iron and copper attract oxygen.
An animal’s oxygen needs affect what kind of blood system it has. Smaller creatures, like bugs, don’t need much. Their “blood” (called hemolymph) just oozes through their body parts. Earthworms have one main blood vessel. It carries blood from one end to the other, moved along by five pairs of “hearts” (really just bulges in the vessels). Larger animals, like fish, birds, and mammals, need more oxygen. That’s why they (and we) have a bigger pump and many tubes to move the blood around.
