because they have copper in their blood instead of iron.
Most molluscs and some arthropods have blue coloured blood.
All mammals have bluish blood until it is oxygenated. That's one reason why veins appear blue.
Red bro... But some are blue
Some animals (like horseshoe crabs) have blue blood because they use hemocyanin carry oxygen to their tissues instead hemoglobin like we do. The copper in the hemocyanin makes their blood appear blue.
Some will say that the blood is blue in the veins because your veins are blue looking. This is incorrect. The blood is in fact red inside the veins, the veins just look blue due to the way the light passes through the fat and skin cells above.
Blood can be a bright red color or a dark red color. The reason it is two different colors is because of the oxygen it contains. Blood in the arteries is bright because it is oxygen-rich, but the blood in the veins contains wastes like carbon dioxide instead so it is a darker color. My anatomy teacher emphasizes that blood is not blue (except in some animals like crabs for example) and it only looks blue through our skin. :)
Hemocyanin is a copper-containing protein found in the circulatory system of many mollusks and arthropods. It functions in oxygen transport, similar to the iron-containing hemoglobin in vertebrates. Hemocyanin turns blue when oxygenated, giving these animals their characteristic blue blood.
Actually, the sky looks blue. But to some animals it might look gray. So how does anybody really know what color everything is? They don't. They're not sure.
Most mammals are believed to see blue color, including dogs and cats. Additionally, some birds, such as owls and hawks, are thought to have limited color vision and may perceive blue.
No, penguins do not have blue blood. Penguins, like other birds, have red blood due to the presence of hemoglobin, which carries oxygen in their bloodstream. Blue blood is found in animals like horseshoe crabs and some mollusks due to a different type of oxygen-carrying molecule called hemocyanin.
Blood color in animals varies primarily due to the type of respiratory pigment present. Most vertebrates, including humans, have red blood due to hemoglobin, which contains iron. Insects and some crustaceans have blue blood because their hemolymph contains hemocyanin, which uses copper for oxygen transport. Other variations include green blood in some worms and leeches due to chlorocruorin, and purple blood in certain species of octopus, which also rely on hemocyanin.
The red blood cells carry oxygen to the tissues, absorb CO2, then carry it away to the lungs to get rid of it. Haemoglobin is the name of this magic chemical. Some animals use a copper-based equivalent which is blue!