August 23, 2002
More Blue Blood (and Green, and Violet, and Red)

The learned Lalith Vipulananthan directs me to the last word on the color of the blood of horseshoe crabs (and, implicitly, of Mr. Spock):

New Scientist: The Last Word Science Questions and Answers:

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

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

Posted by DeLong at August 23, 2002 11:59 AM | Trackback

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We have walked along the edge of New York City beaches when baby horseshoe crabs were hatching and watched as all the moist sand seemed to be moving. How many baby crabs could there have been. Egrets were everywhere. Gulls were everywhere. Enough of the baby crabs would survive in deep water to breed in turn. What a wonderful sight.

Blue bloods indeed.

Posted by: on August 23, 2002 12:22 PM

The really interesting one is the icefish in Antarctica, which apparently has no hemoglobin or anything else. My efforts to find out about this fish have all dead-ended.

The symbolism of hemaglobin is interesting. It makes blood red, and rust makes iron red, and the planet Mars is red because of iron oxide (?not sure of that), and in tradition blood, iron, and the planet Mars were all associated symbolically with war. Not completely wrongly, since iron oxide was common to the three (if I'm right about Mars).

Meanwhile the green of green plants comes from chlorophyll, which is functionally a sort of anti-hemoglobin (picking up CO2 and exhaling O2), and not only that, structurally chlorophyll is like amirror-image of hemoglobin. As I've been told.

So if I'm right, the old poetic color-symbolism really did have some meaningful content.

Posted by: zizka on August 23, 2002 01:47 PM


Please tell us more about the icefish....

Posted by: on August 24, 2002 12:57 PM

Unfortunately, I've told you everything I know. I thinbk that there's more than one fish named icefish, some of which may have hemoglobin. I did send a query to the journal quoted here and will get back to you guys if I find anything out. I believe that actually it's an area of serious research, with lots of unanswered questions.

Posted by: on August 24, 2002 03:15 PM

OK, it was easier than I though. Google icefish, hemoglobin.

Icefish have no hemoglobin and some species no myoglobin. They are also able to survive in supercooled water below the normal freezing temperature of water.


Posted by: zizka on August 25, 2002 07:27 AM

Wonderful posts. Imagine a translucent icefish....

Posted by: on August 26, 2002 09:13 AM

I need to kno what the adaptive advantage of haemoglobin is?

Posted by: luky on November 21, 2002 07:53 PM

I need to kno what the adaptive advantage of haemoglobin is?

Posted by: luky on November 21, 2002 07:53 PM

Could anyone please tell me why the absorption spectra of the different respiratory pigments of different animals differ?

Posted by: sarah mason on November 30, 2002 01:00 PM

Could anyone please tell me why the absorption spectra of the different respiratory pigments of different animals differ?

These differences are genetic adaptations developed due to changing environments. In the early days of the world, There was much more CO2 in the air, and therefore Plants had a decided survivability advantage. So did creatures with a high tolerance to CO2. Traces of this adaptation can be found in all varieties of it's decendants, to varying degrees.

Posted by: Eric Heyworth on March 22, 2003 04:59 PM
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