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Metabolism
Nitrogenous Waste Excretion
How fish dispose of protein-metabolism waste - most excrete ammonia directly via the gills; sharks convert it to urea.
System: Excretory Level: Intermediate Discipline: Physiology
📖 Overview
Protein metabolism produces nitrogen waste that must be excreted. Most bony fish are ammonotelic - they release toxic ammonia (NH3) directly via the gills into the surrounding water. This is efficient in a large-volume aquatic medium. Sharks and rays are ureotelic - they convert ammonia to urea and retain much of it in their blood, using it for osmoregulation.
🔑 Key Concepts
Ammonotely, ureotely, uricotely, ammonia toxicity, urea cycle.
🎯 Applications
Aquaculture: ammonia buildup in closed systems is the single greatest health threat to farmed fish. Aquariums: the nitrogen cycle (ammonia → nitrite → nitrate) driven by nitrifying bacteria is the biological foundation of every aquarium.
✨ Fun Fact
Sharks have blood that is roughly as salty as seawater - not from actual salt, but from retained urea and TMAO. This is why shark meat smells so strongly of ammonia after death.