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My teacher taught me that bony fishes are ammonotelic while the cartilaginous fishes are ureotelic, but why does that happen? Why do cartilaginous fishes not excrete ammonia?

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This is an interesting question. Most fish are ammonotelic, meaning the excrete ammonia (which is toxic to vertebrates) through their gills, which is a very energy efficient way (reference 1 and 2).

However some fish cannot go this route. If fish live in a very alkalic environment, excretion of ammonium ions is not possible. These fish have to convert ammonia to urea which is subsequently excreted (reference 3).

Most fish who are ureotelic use this as a strategy to control their osmotic balance. This means they keep their blood slightly hyperosmolaric compared to the surrounding sea water to prevent them from dehydration in the water and to regulate their body fluids.

This is achieved by a mechanism called "urea retention" and is an almost complete reabsorption of urea in the kidney of these fishes. Urea is then used in the plasma to maintain the high osmolarity there (reference 4 and 5).

This process also helps maintaining water for the animals but comes at a cost: The amount of energy used for osmotic regulation varies between species, but it is estimated to be around 10-15% of their standard metabolism, which is also a reason why a lot of fish directly excrete ammonia (reference 6).

Excess urea is ultimately excreted through the gills of the fish.

References

  1. The multifunctional gut of fish
  2. Ammonia Production, Excretion, Toxicity, and Defense in Fish: A Review
  3. Urea excretion as a strategy for survival in a fish living in a very alkaline environment
  4. Morphological and functional characteristics of the kidney of cartilaginous fishes: with special reference to urea reabsorption
  5. Living with high concentrations of urea: They can!
  6. The Energetics of Osmotic Regulation in Ureotelic and Hypoosmotic Fishes
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    $\begingroup$ But why cartilaginous fish? Are they more likely to live in an alkalic environment? $\endgroup$ Feb 9, 2023 at 10:55

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