3 Main Reasons Why Saltwater Fish Can’t Live in Freshwater

Fish in saltwater aquarium

Saltwater fish can’t live in freshwater as their bodies have a high salt concentration. Hence, when saltwater fish are kept in freshwater, they bloat due to excessive water intake, which can be fatal. However, euryhaline fish can live in freshwater as well as saltwater.

Let’s understand in more detail why saltwater fish can’t live in freshwater.

3 Reasons Why Saltwater Fish Can’t Live in Freshwater

1. Physiological Structure

The primary reason saltwater fish can’t live in freshwater is that they have adapted themselves to live in a saline environment.

Saltwater fish have physiological mechanisms that help them excrete excess salt as they drink marine water continuously.

Their gills contain enzyme-producing chloride cells. This enzyme pumps out the excess salt through their gills.

Similarly, their kidneys also aid in filtering out the salinity-regulating ions.

So these fish can’t survive in freshwater as their body isn’t acclimated to freshwater.

2. Salinity Levels

There is a difference in the salinity levels of freshwater and saltwater.

Freshwater contains very low salt content, specifically sodium chloride. Freshwater bodies have less than 1% salt content.

On the other hand, saltwater marine bodies have an average salt content of more than 3.5% of the seawater’s weight.

A saltwater fish that is acclimatized to live in saline waters can’t survive in freshwater because of the difference in salinity between both environments.

3. Reliance on Osmosis for Survival

Saltwater fish rely heavily on the osmosis process for survival. Osmosis determines how much water a cell will absorb or release.

Hypertonic cells within a marine fish absorb water while retaining salt within.

The freshwater environment doesn’t allow this as it lacks salt content.

In these conditions, the cells of saltwater fish go from hypertonic to hypotonic, which doesn’t match their natural body structure.

The freshwater becomes hypotonic to the saltwater fish as water molecules continuously flow inside the fish’s body, thus diluting the salt molecules.

The saltwater fish find it difficult to replenish the lost salt and urea deposits.

As this keeps happening, the saltwater fish bloat due to excessive water intake and later explode as their internal organs burst.

What Happens when Saltwater Fish Are Kept in Freshwater?

Fish swimming in saltwater aquarium

The osmosis process helps the saltwater fish survive in their respective environments by keeping the salt concentration in their cells to a comfortable level.

A saltwater fish, if placed in freshwater, can have dire consequences.

We need to first understand the atomic theory and diffusion process to get an idea of what actually happens to saltwater fish when they’re placed in freshwater.

When diffusion occurs in water, the different particles look to spread out evenly.

As freshwater contains less salt, the salt particles drift far away from each other during the diffusion process.

When placed in freshwater, saltwater fish find it hard to locate the traces of salt needed to regulate their body cells.

As a result, the salt stored within the body of saltwater fish begins to leach out into the freshwater, while the fish start taking more water than required.

The surrounding freshwater rapidly flows into the saltwater fish’s cells.

As this continues for a longer duration, the saltwater fish begin to swell due to excessive water intake, resulting in the rupturing of their organs.

Fish have a permeable membrane that helps them regulate the amount of water entering and exiting their bodies.

Too much water inside the body causes swelling, which can rupture the internal organs.

Saltwater fish can’t survive in freshwater as the natural diffusion process acts against them, while their bodies have adapted to osmosis and saltwater living.

How Long Can Saltwater Fish Survive in Freshwater?

Saltwater fish, like swallowtail angelfish, are used to living in saline water bodies.

They can’t live in freshwater for too long as their gills and kidneys aren’t meant to handle freshwater.

A dip in freshwater effectively gets rid of Ich from saltwater fish.

However, keeping them in freshwater for a long time can be fatal.

The freshwater will get into the fish’s cells and try to balance the salt amount in the cells to the amount in the water outside.

The water entering the cells dilutes the salt concentration inside their bodies. This leads to the bursting of the cells.

Saltwater fish cannot regulate the metabolic process when placed in a freshwater environment, which can be fatal.

What Types of Fish Can Live in Saltwater and Freshwater?

Euryhaline fish can live in both saltwater and freshwater.

The tolerance of these fish to salinity makes them suitable for both environments.

True euryhaline fish species are the ones that can survive in both environments as well as reproduce in both types of waterbodies.

Largetooth Sawfish Pristis and Atlantic stingrays are the only marine euryhaline fish species that reproduce in freshwater and saltwater environments.

Euryhaline fish species are divided into the following two groups:

  • Anadromous fish: These fish are born in freshwater but spend the majority of their life in the ocean. They only return to freshwater during the spawning process. Some of these fish are sturgeon, salmon, striped bass, and smelt.
  • Catadromous fish: These fish usually live in freshwater and migrate to the sea through rivers for spawning. European Eel is a catadromous fish.

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