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Chapter 28: Problem 2

When a salmon or other teleost fish migrates from seawater into freshwater, what are all the changes that take place or are likely to take place in its patterns of water-salt physiology?

Short Answer

Expert verified
When a salmon migrates from seawater to freshwater, it undergoes a physiological adaptation to prevent loss of essential salts and to maintain its internal osmotic concentration. Principally, three changes occur - increased salt uptake through gills, increased excretion of dilute urine by the kidneys to get rid of excess water, and behavioural changes like increased food intake and seeking areas of slightly higher salinity.

Step by step solution

01

Changes in Osmotic Concentration

Teleost fishes like salmon are osmoregulators - they control their body fluid concentrations. When they migrate from seawater (high salt concentration) to freshwater (low salt concentration), they face a hypotonic environment. Thus, their bodies tend to lose salts and absorb more water. Consequently, the fish needs to counteract this by actively absorbing salts through its gills and excreting the excess water.
02

Changes in Gill Function

The fish's gills are the primary site for salt absorption and water excretion. Chloride cells in the gills help in actively absorbing sodium and chloride from the freshwater. This is done through the sodium-potassium pump mechanism. The low concentration of ions in freshwater prompts the fish to keep replacing lost salts, thus gills play an active role in this process.
03

Changes in Kidney Function

The kidneys of the fish also adjust to the new freshwater environment. They produce large amounts of dilute urine to excrete the excess water from the body. This also helps maintain the fish's internal osmotic concentration.
04

Behavioural Changes

The fish may indulge in behaviours like eating more, which would help in gaining salts and amino acids. They might also show avoidance behaviour, i.e., actively avoiding low ionic concentration areas and seeking areas of slightly higher salinity.

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Most popular questions from this chapter

We discussed the fact in Chapter 23 (see Figure 23.11) that the efficiency of \(\mathrm{O}_{2}\) exchange across the gills of teleost fish is enhanced by countercurrent exchange between the water pumped over the gills and the blood flowing through the secondary lamellae of the gills. A recent review article makes the point that the countercurrent arrangement of blood flow and water flow-which has positive consequences for \(\mathrm{O}_{2}\) exchange-has disadvantageous side effects because it enhances osmotic water uptake by freshwater fish, osmotic water loss by marine teleosts, loss of \(\mathrm{NaCl}\) by diffusion in freshwater fish, and \(\mathrm{NaCl}\) gain by diffusion in seawater teleosts. Do you agree or disagree? Explain.

Theory predicts that when both terrestrial and freshwater animals evolve higher metabolic rates, they can be expected to evolve greater challenges to maintaining water balance. Explain why this is so for both terrestrial and freshwater animals.

Walter Cannon, who coined the term homeostasis, argued that lizards and amphibians are less highly evolved than mammals and birds because they are not as fully homeostatic as are mammals and birds. Considering water-salt relations and any other aspects of physiology you find to be pertinent, explain in detail why you agree or disagree.

Related species of terrestrial animals typically display allometric relations between body-water dynamism and body size. For example, the weight-specific rate of evaporative water loss tends to decrease allometrically as body size increases. What are the mechanistic reasons for these relationships?

Animals often face trade-offs, and one of the goals of modern physiology is to understand mechanistically why improvement of performance in one way may degrade performance in other ways. In freshwater fish, explain why \(\mathrm{O}_{2}\) uptake and \(\mathrm{Ca}^{2+}\) uptake probably cannot be simultaneously maximized. In other words, why is there a trade-off between the ability to take up \(\mathrm{O}_{2}\) and the ability to take up \(\mathrm{Ca}^{2+}\) ?
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