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.
The pH of seawater should cause little or no problem with intestinal digestion. Too high a salt content in the intestine might exceed the operational range of some enzymes and thus reduce the rate of digestion. However, one of the functions of the stomach (and in eels, the oesophagus) in osmoregulation is to dilute the incoming seawater until it is approximately equal to the osmolarity of blood, thus protecting the intestine.
The final osmoregulatory product of the gut is a rectal fluid composed of magnesium and other divalent ions having about the same total concentration as blood. Preliminary data from scale loss studies indicated that death occurred from toxic levels of magnesium in the blood. A possible cause of the high magnesium is that gut peristalsis stopped, leaving the rectal fluid to accumulate and the magnesium ions to be reabsorbed instead of being excreted.
Thus, digestion and osmoregulation are so inter-related that problems in one system could disrupt the functions of the other. Exactly how fish normally avoid such problems is largely unknown.