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XIII. Oxygen Minimum Zone

Life in Low Oxygen

I want to finish up by say a few things about the oxygen minimum zone. The oxygen minimum zone is a layer in which the descending food material that's passing through the water column has been degraded by bacterial activity. As the food is degraded, a lot of oxygen is used by the bacteria. The oxygen left in the water column is minimal. None the less, certain organisms seem to thrive in the oxygen minimum layer, and the idiot fish is one of those. I want to tell you very quickly what some of the mechanisms are that allow fishes and invertebrates to work well in these areas of very low oxygen. As you would expect, they have low rates of oxygen consumption, and they can maintain very stable rates of respiration as oxygen decreases. They have blood binding pigments that are very effective at pulling from the water any available oxygen that's there.

Here is the idiot fish sitting on the sea floor in the oxygen minimum zone. These are very sedentary species that move about infrequently and slowly. Nonetheless, they do manage to thrive under conditions of low oxygen that would preclude the survival of most fishes.

How does one examine the properties of the idiot fish or other animals from the oxygen minimum zone to discern precisely what adaptive changes allow their success in this habitat? In doing comparative studies, one must be careful to make "apples to apples" comparisons, that is, one must use similar species that differ in their habitat preferences. Dr. Tzung-Horng Yang examined the idiot fish and Scorpaena guttata, a shallow-living member of the same family (Family Scorpaenidae), and found a number of physiological differences that reflected the different concentrations of oxygen found in the two species' habitats.

Consider the respiration rates of the two species. The idiot fish respires more slowly, a trait that would be to its advantage in the oxygen minimum zone. This species also is able to sustain a stable rate of oxygen consumption in the face of decreasing oxygen concentration; even at oxygen minimum zone concentrations, as shown by the dark segment of the X-axis of this graph, the idiot fish maintains a relatively stable rate of oxygen consumption. In contrast, S. guttata shows a large drop in rate of oxygen consumption. The ability of these two species to maintain stable rates of oxygen consumptions as the oxygen content of the water of the respirometer falls is due to the higher rate of opercular ventilation that occurs as oxygen level decreases. However, we again see differences between the two species. For the idiot fish, ventilation frequency rises steadily as oxygen concentration falls, whereas for S.guttata, ventilation rate rises but then falls rapidly as oxygen concentrations characteristic of the oxygen minimum zone are reached.

There are also differences between the species in the ability of the blood to extract oxygen from the medium. The curves shown on this diagram are "saturation curves" that show the extent to which the blood is saturated with oxygen as the oxygen level of the medium is changed. The curve for the idiot fish is shifted to the left of that for S. guttata, a sign that the blood of the idiot fish is more fully saturated at low oxygen concentrations. At the levels of oxygen found in the oxygen minimum zone, the blood of S.alascanus is almost fully saturated - all oxygen-binding sites are filed with oxygen whereas the blood of S. guttata is only about 30% saturated. Thus, blood of the idiot fish is much more effective in extracting oxygen from an oxygen-poor environment.


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