respiratory pigment

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various colored conjugated proteins, such as hemoglobin, that occur in living organisms and function in oxygen transfer in cellular respiration.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biochemistry any of several coloured metal-containing protein pigments which combine reversibly with oxygen and which are found in the tissues of invertebrates and whose role is to transport oxygen