How ammonotelism affects osmoregulation in fish?

Ammonotelism plays a key role in osmoregulation (water and salt balance) in fish, especially in freshwater species. It connects the removal of ammonia—a highly toxic but water-soluble waste—with the control of internal water and salt levels.

Effect in Freshwater Fish

Freshwater fish are ammonotelic, meaning they excrete ammonia directly. They live in a hypotonic environment, where the salt concentration inside their bodies is higher than in the surrounding water. This causes two main problems:

• Water influx: Water keeps entering their bodies through gills and skin by osmosis.
• Salt loss: Important salts tend to leak out into the water.

How ammonotelism helps:

• Easy ammonia removal: Since excess water constantly enters their bodies, freshwater fish can easily flush out toxic ammonia through their gills and in large volumes of dilute urine.
• Energy saving: Excreting ammonia directly uses very little energy. This allows fish to use saved energy for other tasks like absorbing salts from water through special gill cells.

Effect in Marine Fish

Most marine fish are ureotelic, meaning they excrete urea instead of ammonia. They live in a hypertonic environment, where seawater has more salt than their body fluids. This leads to opposite challenges:

• Water loss: They lose water to the salty environment through osmosis.
• Salt gain: They take in extra salts when drinking seawater or through osmosis.

For the few marine fish that are ammonotelic, the process is more complex:

• Specialized excretion: They actively push ammonia (as ammonium ions) out through their gills, even when the surrounding water already has ammonia.
• Preventing toxicity: Some fish reduce ammonia absorption by removing ammonia transporters from their gills when external ammonia levels are high.