The respiratory proteins hemoglobin and hemocyanin share the function of oxygen transport, but the proteins belong to separate gene families, and their active sites and the metal ions that bind the oxygen differ. Either hemoglobin or hemocyanin, but not both, is expressed in the hemolymph of many arthropod crustaceans. Hemoglobin is present in Branchiopoda, Ostracoda, Copepoda, rhizocephalan Cirripedia and one suborder of amphipodan Malacostraca, while hemocyanin has been described in Malacostraca. Recent work by several laboratories have provided new information on the gene structure, exon-intron patterns, site of synthesis and expression of hemoglobins in the branchiopods Artemia and Daphnia. These studies suggest the branchiopods are excellent model organisms for studies of oxygen sensors and hypoxia inducible transcription factors during developmental and adult stages. The focus in our laboratory on the ontogeny of hemocyanin in the Dungeness crab, Cancer magister, has demonstrated that both structure and function of hemocyanin change from megalopa to adult crab. The hemocyanin of an oceanic megalopa contains four subunits. Another subunit appears about the time of metamorphosis to first juvenile instar, and expression of a sixth subunit begins four or five molts later. The timing of onset of adult hemocyanin can be altered experimentally by food levels and temperature. Gene expression and functional properties of both red and blue oxygen transport proteins of crustaceans change during ontogeny to insure oxygen delivery appropriate for each developmental stage.