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A fish lives at the depth of 1000-2000m and another at the depth of 200-700m. Which of them would have a more developed gill system ?

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Can you expand on what you consider "better"? As long as both can live fine at their respective depths, then I would call both sets of gills "good enough", which is really all that matters. –  kmm Sep 1 '13 at 15:29
    
Moles living in dark don't have well developed eyes. Similarly,which of these fishes would have more developed gills ? –  biogirl Sep 1 '13 at 15:33
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up vote 3 down vote accepted

200m is still pretty deep...

However, I'd give my money for the surfacers. Oxygen concentration decreases with the depth, and so abyssal fishes have very low metabolism, mainly focused on anaerobic muscles. This is also true for fishes that live in the bottom (benthic), which probably also applies for those living in a few hundred meters. If not, they will probably be active swimmers and then they will probably need bigger gills in order to maintain aerobic movement. Even if we're talking about abyssal and non-abyssal benthic species, I think the abyssal ones would probably be even more anaerobic.

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Does Oxygen concentration decrease with depth? The opposite is true for the first few meters. –  terdon Sep 1 '13 at 17:52
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Yes, but that is because there's a biotic oxygen consumption (and production) that is higher in the most illuminated area. Once you have descend a bit beyond, the oxygen source is both photsynthesis and diffusion from air, both of wich may be hundreds of meters above. In this dark area, oxygen diffusion is limited, and carbon dioxyde accumulates too. –  Miguel Ángel Naranjo Ortiz Sep 1 '13 at 20:43
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You make a good point but the solubility of Oxygen increases with depth since it depends on the pressure. I have no idea if this increase will continue or if it reaches a plateau after a few meters. I agree that at great depths, the lack of photosynthesis should mean that the oxygen content is lower, but I wonder how much that is balanced out by the greater solubility. I haven't been able to find a reliable reference citing oxygen levels at greater depths. I would guess the concentration goes up for the first ~10-15 meters and then falls from there. –  terdon Sep 1 '13 at 22:32
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Solubility may increase due to pressure, but it will remain pretty low no matter what. Without an internal production, and with so many organisms consuming it (below a certain point there is no photosynthesis, but there is still sediments to metabolize and there are animals and bacteria feeding by the leftovers that falls from the surface), the oxygen levels in the bottom of the ocean is minimal. –  Miguel Ángel Naranjo Ortiz Sep 2 '13 at 9:11
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