Here's why:
* Saltwater regulation: Sharks have a special adaptation to regulate their body's salt levels, which is necessary for survival in the ocean. Their bodies are hypertonic, meaning they have a higher concentration of salt than the surrounding water. This allows them to absorb water through their gills and excrete excess salt.
* Osmotic pressure: Freshwater has a lower salt concentration than sharks' bodies. If a shark were to live in freshwater, water would constantly enter its body through osmosis, leading to an imbalance and potentially fatal swelling.
While some sharks can tolerate freshwater for short periods, they cannot thrive in it for extended periods.