* Osmosis: Marine fish are adapted to a high-salt environment (saltwater). Their bodies are designed to maintain a balance of fluids and salts. When placed in freshwater, the water outside their bodies has a lower salt concentration. This causes water to rush into the fish's body through osmosis, in an attempt to equalize the salt concentration. The influx of water can lead to swelling, cell damage, and eventually death.
* Ion Regulation: Marine fish have specialized cells that help them regulate the amount of salts in their bodies. In freshwater, these mechanisms would be overwhelmed, as they are designed to remove excess salt, not to acquire it. This would disrupt their internal balance.
* Physiological Adaptations: Marine fish have evolved specific adaptations to thrive in saltwater, including:
* Gills: Their gills are structured to efficiently extract oxygen from the water and regulate salt levels.
* Kidneys: Their kidneys are designed to excrete excess salt.
* Skin: Their skin helps protect them from the harsh salinity of seawater.
In summary: Marine fish and freshwater fish have evolved different mechanisms for survival in their respective environments. Putting a marine fish in freshwater is like putting a human in a boiling pot – it is completely incompatible with their biology.