Halloween is just around the corner, which means decorations featuring ample amounts of fake deep red blood are everywhere. But if your haunted house features dead things that aren’t human, you might have to change the color of your fake blood accordingly.

If you’ve ever had a blood test done, you know human blood is deep red. That’s because of hemoglobin, a respiratory pigment in red blood cells. It’s made of four connected protein molecules, or globulin chains, and each globulin chain has a heme molecule at its center. An iron atom is embedded in each heme molecule, and it’s the iron that allows the red blood cells to carry oxygen from the lungs to the tissues and organs and return carbon dioxide back to the lungs for exhalation.

That iron also makes our blood red. Hemes are molecules that absorb light wavelengths in the visible portion of the spectrum, meaning we see them as colored. And the iron content of the hemes affects the absorption such that we see them as red. The shade of red varies slightly; blood will be more crimson when exposed to oxygen, like with a dripping cut on your arm, and a little more pink when it’s not, like when it’s in your veins.

While human blood can be varying shades of red, other species bleed the rainbow.

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Crustaceans, spiders, squid, octopuses, and some types of molluscs, for example, bleed blue. They have a respiratory pigment called hemocyanin. Hemocyanin also transports oxygen through the body via the bloodstream, but it has copper embedded in it rather than iron. This changes the color. In the absence of oxygen, these creatures’ blood is colorless. When oxygen is present, the blood looks distinctly blueish.

Workers bleeding horsehoe crabs, whose blood contains the best test for detecting pathogens in intravenous injections. Every safe shot you’ve ever had you owe to a crab.

Worms and leeches have yet another blood structure. These slimy creatures’ blood uses a respiratory pigment called chlorocruorin as the oxygen carrier, and it’s a protein that gives the blood a light greenish color, which gets darker in the presence of oxygen.

Now that’s a Halloween hue.

But there are other types of worms, specifically some species of marine worms, that bleed purple. Again, this hue is due to a different protein as the oxygen-ferrying agent, a respiratory pigment called hemorythrin. It’s similar to hemoglobin in that it does contain iron, but rather a reddish hue, this respiratory pigment means colorless blood when its deoxygenated and bright violet-pink blood in the presence of oxygen.

The purple-blooded peanut worm.

So as you get your haunted house set up before Friday night, consider adding some crustacean, sea worm, and leech blood to the skeletons.

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IMAGES: Compound Chem; Thirteen/WNET/PBS; Nereid worm pieces by Douglas Mason; Sipuncula by Vmenkov