Hemocyanin

... creatures like the Horseshoe crab have blue blood, this is because they do not use hemoglobin to attract oxygen (iron based) but instead use hemocyanin (copper based) it must be a more efficent way of delivering oxygen to the cells as they have survived all the great extinctions

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... blood is colorless, it is based - gemovanady containing vanadium ions. In octopus, spiders, scorpions, crabs, respiratory pigment of the blood is hemocyanin, in which iron is present instead of copper (Cu2 +). Combines with oxygen of air is blue hemocyanin, and by giving oxygen to tissues - a ...

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It has to do with the fact that hemocyanin changes its colour right after it contacts the air, whilst hemoglobin, being intracellular, doesn't react instantly unlike someone wrote in a reply in this thread, where they claimed that venous ...

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... pigments need to be locked inside cells because it is too small and it would clog the excretion systems(be they nephridia, nephrons or whatever). Hemocyanin and other pigments are large enough to float around in the plasma/hemolymph with no ill effects on the excretory system.

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The horseshoe crab has blue blood! These odd arthropods have copper-containing protein hemocyanin instead of hemoglobin, and it is indeed blue - although when oxygenated. Although this has nothing to do with human blood, it proves one point: someone earlier in this ...

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