Saltwater fish drink salt water to compensate for water loss due to osmosis. They then actively excrete the excess salt.
Here's a more detailed explanation:
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Osmosis and Water Loss: Saltwater fish live in a hypertonic environment, meaning the surrounding water has a higher salt concentration than their body fluids. This causes water to constantly move out of their bodies through osmosis, primarily through their gills and skin.
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Drinking to Rehydrate: To counteract this constant water loss, saltwater fish actively drink seawater.
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Dealing with Excess Salt: Drinking seawater introduces a significant amount of excess salt into their bodies. They eliminate this excess salt through a few key mechanisms:
- Gills: Specialized cells in their gills, called chloride cells (or mitochondria-rich cells), actively transport salt out of the blood and into the surrounding water. This is a key process in maintaining the fish's internal salt balance.
- Kidneys: While saltwater fish kidneys produce very little urine (to conserve water), the urine they do produce contains some salt.
- Feces: Some salt is also excreted in their feces.
In short, saltwater fish drink salt water to stay hydrated and then use specialized cells in their gills to actively pump the excess salt back into the ocean. This complex process allows them to survive in a high-salinity environment.
