X. Capacity Adaptations
c. darkness
This slide is basically the answer to that question. It's not a faulty slide
although I think I put it in upside down. It's the view of the marine water
column out of the window Alvin, for example. Now, everything that Bruce
and George showed you earlier was kind of a lie because they were shining
lights on things. If you're going to see these beautiful animals, you have
to light them up. If you leave the lights off and you go down in the submersible,
you may see a few flecks like this. The point is when you leave the aphotic
zone, you leave behind a lot of stresses. You leave behind a lot of potential
sorts of activity.
Darkness
appears to be one, if not the, major ultimate cause of the decreased life
with depth. I'll explain how we've come to that conclusion. Perhaps the
most obvious reason why reduction in light would lead to reduction in life
concerns photosynthetic activity. Below about 200 meters, and this number
depends on the clarity of the water, there is no longer any photosynthetic
activity. So you might conclude that below 200 meters and things should
slow down because there is no primary production unless, in fact, you're
at the hydrothermal vents.
It turns out, photosynthetic productivity really doesn't have a lot to do
with the drop-off in respiration of fishes with depth. The deep-water column
isn't so much food limited as it is limited in terms of what allows the
organisms to find this food. When you're in darkness, two things are going
to pertain in terms of predator to prey interactions or finding of food.
One is that it's going to be difficult to find food; unless the MBARI submersible
is down there lighting up the environment you're not going to see much of
what's out there. You may bump into it. The other side of the coin is that
you are also hard to locate by predators. So darkness may make it harder
for you to find food but if you turn out to be food for somebody else, darkness
works to your advantage.
When you think about what this might mean for organisms and you think about
what the effects of darkness might be on respiration rate, you can come
up with a hypothesis that is based on estimates of cost of locomotory activity.
Swimming activity, particularly in fish, is usually the major cost of living
for an animal. Those bursts of activity that a fish goes through when preying
on another organism or when it's trying to avoid being prey for another
organism are very expensive energetically. A huge fraction of the ATP an
organism turns over every day is spent powering the locomotory apparatus.
If you're in the dark, the costs of locomotion might be greatly reduced.
How can we test this hypothesis that darkness leads to reduction of locomotory
activity. Again, I'll go back to the point that it's very difficult to bring
deep sea organisms up to the surface and examine their locomotory activity.
Now, with the development of ROV's and advanced crewed submersibles, we're
beginning to see what goes on in situ but again, there are a lot
of organisms that we haven't been able to see; a lot of organisms that are
very difficult to study in a living state.
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