I have heard that some sea creatures such as horseshoe crabs have blue, copper-based blood. The oxygen-carrying capacity of the various pigments varies with oxygen concentration, temperature, p H and carbon dioxide concentration. It depends on the nature of the protein part of the pigment as well as the metallic component, and this differs from species to species. The occurrence of these pigments does not appear to be strongly related to organisms' evolutionary relationships.
Some organisms have no oxygen-carrying respiratory pigments, some have one type, others more.
It is difficult to compare the efficiency of the respiratory pigments of different species. For example, the diagram shows how much oxygen binds to the pigments of different species at different concentrations, or partial pressures, of oxygen. For simplicity, only one haemocyanin curve is shown, but its saturation curves are as variable as those of haemoglobin. Similarly, neither the haemocyanin of the horseshoe crab nor the haemoglobin of the seal appear to be able to unload oxygen at the partial pressures found in the bird. The effect of p H provides another example of the difficulty of comparison.