Why Fish Can Drink Salt Water And We Can’t

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Water is crucial for life on Earth. It allowed life to evolve and thrive.

Salt, too, is abundant and vital.

Together, salt and water form most of the water on our planet and were key to the beginning of life as we know it.

Osmosis: Balancing Salt and Water

Organisms need salt to survive. Salt draws water out of cells through a process called osmosis, which balances salinity levels.

Nature strives for a balanced level of salt everywhere.

If you drink saltwater, you can die of dehydration because it disrupts this balance, pulling water out of your cells.

Fish Adapting to Different Water Types

As bony fish moved from salty seas to freshwater, they had to eat salts to maintain balance.

They still do this today, urinating excess salt—sometimes up to a third of their body weight daily. Humans crave salt for similar reasons.

But how do saltwater fish avoid dehydration from drinking saltwater? There are two answers:

  • Gill Adaptations: Some saltwater species, like the turbot, have gills with more of an enzyme called “gill Na+/K+ ATPase.” This enzyme helps them expel excess salt back into the ocean.
  • Special Adaptations: Some fish, like eels, salmon, bass, and flounder, can move between fresh and saltwater. These euryhaline species adapt their gills or kidneys to survive in different salinities.

Examples of Adaptation

  • Atlantic Salmon: These anadromous fish are born in freshwater and live there before moving to saltwater. An enzyme called type 2 deiodinase, produced in response to longer daylight hours in spring, alters their gills to handle salty water. This process reverses when they return to freshwater to breed.
  • Tilapia: A 2013 study found that when tilapia are gradually placed into saltwater, their gills produce a specialized protein called NDRG1, allowing them to survive in saline environments.

Evolution and Osmoregulation

Over 400 million years, fish have spread into almost every water-based environment on Earth.

After a mass extinction event 250 million years ago, freshwater fish had to re-evolve ways to handle salty water and repopulate the oceans.

Some of these fish eventually made it to land, evolving into amphibians and tetrapods, like humans.

Without salt and the ability to regulate it, life as we know it wouldn’t exist. Evolution, through various adaptations, has allowed life to thrive in diverse environments.

Conclusion

Salt and water are fundamental to life on Earth.

Through osmosis and various evolutionary adaptations, animals have learned to survive and thrive in both fresh and saltwater environments.

This process of convergent evolution highlights the incredible adaptability of life.


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