X. Capacity Adaptations

a. life vs. depth

I want to shift over now to what I call capacity adaptations. These are adaptations that concern the questions of how do rates of life vary with depth and why do we find these depth related patterns? When I teach my physiology class, I like to tell the students that when we're asking questions about cause-effect relationships, there are two different types of causes that we're concerned with. One is what Ernst Mayr would call ultimate causes. These are the ultimate reasons why a particular trait is present in an organism. The proximate causes are the underlying mechanisms that bring this trait into existence. I'll approach the question of capacity adaptations, by looking both at the ultimate causes of these trends and their physiological and biochemical causes.

What do we see as we measure life as a function of depth through the marine water column? What we find is that for a variety of different examples of life there is a steep drop off. Particularly as we descend from the surface through the aphotic zone, the first couple hundred meters, we find a very steep drop off, for example, in the rate at which organisms move and the rates at which they respire. Thus, a very large difference is found between shallow-living species and deep-living species in many vital rate processes.

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