Why do invertabrates have such wierd looking blood?

[Poetgrrl]

I was quite surprised to discover in my youth that not every living thing has red blood- some insects appear to have green blood, and mollusks and shellfish have some sort of goo which I *guess * is their blood.

Why is this? Don’t all these creatures need hemoglobin too, in order to carry oxygen? And isn’t oxygenated blood red? So if these critters breathe oxygen, why isn’t their blood red?

[Darwin_s_Finch]

Such critters have copper-based blood oxygen carriers, rather than iron-based ones. The iron-based carriers are, of course, hemoglobin. The copper-based ones are hemocyanin. When a hemocyanin molecule binds with oxygen, presto! - blue blood, instead of red. Arthropods and molluscs both have hemocyanin instead of hemoglobin.

See also here

Crustacean blood differs from vertebrate blood in one other characteristic. It is not red as it lacks hemoglobin. Hemoglobin in vertebrate blood carries dissolved oxygen to the tissues and carbon dioxide to the respiratory surfaces. In most small crustaceans, these gasses are simply dissolved in the blood and the relatively larger volume of blood ensures that sufficient gas exchange occurs. In larger crustaceans, a chemical called hemocyanin is found dissolved in the blood. Hemocyanin is a material made of protein subunits complexed with copper ions. It will bind to oxygen in areas of high oxygen concentration and release it in areas of lower oxygen concentration, so it is a respiratory pigment. It is not a particularly good respiratory pigment when compared to hemoglobin, but it does appear to assist in oxygen transport. Like hemoglobin, its color changes depending upon whether or not it is carrying oxygen. Lacking oxygen it is colorless, but while carrying oxygen, it is a beautiful pale blue or cyan; which, of course, gives the pigment its name.

[brachyrhynchos]

In addition to hemocyanin and hemoglobin, there are two other respiratory pigments: hemerythrin and chlorocruorin. Both of these are used by various invertebrate groups and both also use iron to bind. But differences in structure (such as side groups) result in differently-colored blood. Chlorocruorin blood is green when oxygen is bound.