3.6 Interrelationship between Osmoregulation and Digestion
Researchers studying osmoregulation and researchers studying digestion have rarely considered each other's data. Marine fishes drink significant amounts of seawater, a relatively-well buffered solution having a pH of about 8.5, while gastric digestion requires a pH of 4 or lower in most fish. The amount of HCl required just to acidify the seawater would be substantial, that is, if the entire stomach gets flooded with seawater. There are several likely alternatives, however. In fish with Y-shaped stomachs, the seawater could travel directly from the oesophagus to the pylorus, and traverse only a small fraction of the stomach surface. If, at the same time, digestion functioned primarily as contact digestion, then it could be largely separated from osmoregulation. On the other hand, marine salmon stomachs have been found to be filled with a liquid slurry which would prevent such separation. In such cases, alternation of digestion and seawater drinking might be possible, although fish whose stomachs seemed continuously filled, and therefore would have no time for drinking, have also been observed.
3.6 Interrelationship between Osmoregulation and Digestion
Researchers studying osmoregulation and researchers studying digestion have rarely considered each other's data. Marine fishes drink significant amounts of seawater, a relatively-well buffered solution having a pH of about 8.5, while gastric digestion requires a pH of 4 or lower in most fish. The amount of HCl required just to acidify the seawater would be substantial, that is, if the entire stomach gets flooded with seawater. There are several likely alternatives, however. In fish with Y-shaped stomachs, the seawater could travel directly from the oesophagus to the pylorus, and traverse only a small fraction of the stomach surface. If, at the same time, digestion functioned primarily as contact digestion, then it could be largely separated from osmoregulation. On the other hand, marine salmon stomachs have been found to be filled with a liquid slurry which would prevent such separation. In such cases, alternation of digestion and seawater drinking might be possible, although fish whose stomachs seemed continuously filled, and therefore would have no time for drinking, have also been observed.
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