The learned Lalith Vipulananthan directs me to the last word on the color of the blood of horseshoe crabs (and, implicitly, of Mr. Spock):
Posted by DeLong at August 23, 2002 11:59 AM | Trackback
I have heard that some sea creatures such as horseshoe crabs have blue, copper-based blood. Why is this, and what advantage does this kind of blood give them over creatures with the more common red, iron-based blood, like ourselves? Do any creatures have blood that is based on metals other than iron or copper?
Kiyotaka Tanaka , London Answers
Blood gets its colour from oxygen-carrying respiratory pigments, and there are a number of different types. Their job is to bind oxygen in areas of higher concentration (usually gas exchange surfaces such as lungs or gills) and release it in areas of lower concentration (usually tissues).
The oxygen-carrying capacity of the various pigments varies with oxygen concentration, temperature, pH 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 iron-containing pigments found in blood include haemoglobins (red), myoglobins (red), chlorocruorins (green) and haemerythrins (violet). Haemocyanin (blue), which is found in horseshoe crabs and other organisms, contains copper not iron. 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. These oxygen saturation curves reveal how respiratory pigments bind plenty of oxygen when the partial pressure of oxygen is relatively high (as in lungs or gills) and release it when the partial pressure is low (as in muscles). For simplicity, only one haemocyanin curve is shown, but its saturation curves are as variable as those of haemoglobin.
Neither haemocyanin nor any of the other haemoglobins shown would work in the low-oxygen environment for which the haemoglobin of the marine tube-dwelling worm is adapted. 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. However, the seal haemoglobin and the crab haemocyanin have similar saturation curves and (were such a thing possible) might be interchangeable.
The effect of pH provides another example of the difficulty of comparison. In most cases, a decrease in pH shifts the oxygen saturation curve to the left. So as the amount of CO2 in the tissues increases, the pH decreases and more oxygen is unloaded from the respiratory pigment.
Decreased pH at the oxygen uptake surface (lung) of an air breather is seldom of significance. However, decreased pH at the oxygen uptake surface (gill or skin) of an aquatic organism is not uncommon. Water often becomes acidic, and as its pH decreases, oxygen uptake decreases. Eventually, animals may die because their respiratory pigment is no longer able to carry enough oxygen to support their metabolism.
To counteract this, organisms that live in environments in which the pH varies usually have a respiratory pigment that is less sensitive to pH change than the pigments found in animals which live in more stable environments. Thus a haemocyanin that is very sensitive to pH would be detrimental to an organism that lives in a pH-labile environment, even if in other respects it is a "better" oxygen carrier. Conversely, a pH-stable haemoglobin might be a "better" oxygen carrier, even though its saturation curve is less efficient than a particular haemocyanin at a certain pH.
However, this does not explain why crabs have haemocyanin rather than haemoglobin. In some respects, the original question asking which respiratory pigment is better is moot. Living organisms do not have the ability to swap one respiratory pigment for another. If an organism has haemoglobin, it is stuck with it.
Even if it were possible to change pigments, the required physiological adjustments would probably be so far-reaching that the organism would no longer qualify as the same species. In which case, you could find yourself asking the same question again.Peter Morgenroth , Retired lecturer in zoology Royal Melbourne Institute of Technology