Fish drink water, but the process differs significantly between saltwater and freshwater fish due to the varying salt concentrations in their environments.
Here's a breakdown:
-
Saltwater Fish: Saltwater fish live in a hypertonic environment, meaning the water surrounding them has a higher salt concentration than their internal fluids. This causes them to constantly lose water through osmosis. To compensate for this water loss, saltwater fish actively drink water. They then excrete excess salt through their gills and in their urine. According to McGill University, saltwater fish also purposefully drink water to have some go through their digestive tract.
-
Freshwater Fish: Freshwater fish live in a hypotonic environment, meaning the water surrounding them has a lower salt concentration than their internal fluids. Consequently, water constantly flows into their bodies through osmosis, primarily through their gills and skin. Freshwater fish don't need to drink water because they are already absorbing it from their environment. Instead, they expel excess water through copious amounts of dilute urine. While water may end up in their mouths, instead of swallowing it, the water is filtered out through the gills.
| Feature | Saltwater Fish | Freshwater Fish |
|---|---|---|
| Environment | Hypertonic (more salt than their internal fluids) | Hypotonic (less salt than their internal fluids) |
| Water Loss/Gain | Constantly lose water through osmosis | Constantly gain water through osmosis |
| Drinking Water | Actively drink water | Do not drink water (water passively enters) |
| Salt Excretion | Excrete excess salt through gills and urine | Rarely excrete salt (actively absorb salt through gills) |
| Urine Production | Small amount of concentrated urine | Large amount of dilute urine |
| Osmoregulation | Constant battle against dehydration and salt buildup | Constant battle against water gain and salt loss |
In summary, saltwater fish drink water to combat dehydration caused by osmosis, while freshwater fish do not need to drink water as they are constantly absorbing it from their environment.
