biology zoology blog fish water animals

Drinking like a Fish

Drinking like a Fish – Really?

When I watched my wife making pickles, the way she used all kinds of fruits and vegetables, embedding them in layers of salt, I couldn’t help but think of the fishes in the sea! The poor devils are actually swimming in a salt solution, in constant danger of drying out, of shrivelling up and turning into dehydrated, lifeless corpses. Sounds paradoxical? Let’s examine then.

The salt concentration of the body fluids (i.e., blood, lymph and interstitial fluid) of bony fishes is at the most only one half of the salt concentration of the water surrounding them. The higher salt concentration outside the fish has the property of drawing body fluids from the fish’s low salt concentration into the surrounding sea water with its high salt concentration. The fish’s scales (if the fish has any) reduce the water loss a little, but cannot prevent it altogether from taking place. If the fish would not be actively drinking water constantly, it would indeed be in great danger of being pickled just like my wife’s mangos, limes, and cucumbers. But drinking every day the equivalent amount of approximately 5% of its body weight in seawater, does that not aggravate the situation? After all, we are always told that a shipwrecked sailor is not supposed to ever drink some seawater, even when thirsty, as that would make him even more thirsty. That is physiology.

It is true that even the kidney of an oceanic fish cannot handle the salt load of the water ingested by the fish, but the fish’s gut, it can. Here sodium, potassium and chloride ions are actively taken up by the blood pulling or drawing with them water molecules. Magnesium and sulphate ions are rejected and leave the fish with its faeces. The gills of the fish receive the sodium, potassium and chloride ions with the blood, and although the gills’ job is to serve the fish as a respiratory organ, it is not the gills’ only role. At least in saltwater bony fish species, the gills are also the principal organ through which the fish can get rid of the surplus sodium, potassium and chloride, taken up from the gut. Water is retained and does not leave the fish’s body in significant amounts from the gills. That is how the saltwater fish avoids being pickled alive. Incidentally, sharks use a different method and elevate their body salt concentration to match that of seawater. They do this by retaining urea in their tissues and that’s also the reason why they smell so bad when cooked and why they taste so awful.

But now the story of the freshwater species. Well, also living in water, but this time of much lower than their own body salt concentration, their body tissues passively gain water from the environment and these fishes are in constant danger of being flooded, suffering from hydropsy (= developing oedemas). Consequently, freshwater fishes have highly efficient kidneys and never have to drink a drop of water in their lives. However, those species like the salmon, for instance, which live in the sea as adults and have to swallow a lot water in order to survive in that salty habitat, must change their behaviour and stop drinking water as soon as they invade the freshwater rivers on their way to their breeding grounds (where most of them die after having laid their eggs). For eel larvae arriving from the sea and then known as elvers, the same holds true, but when mature they, in contrast to the salmon, move back into the ocean and need to reverse their ways to deal with water a second time. Knowing all this about fish and their drinking habits adds quite a twist to that popular saying of “someone’s drinking like a fish”, doesn’t it?

© Dr V.B. Meyer-Rochow and, 2018.
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