Fresh water bony fishes maintain water balance by

Options

A. Excreting hypotonic urine
B. Excreting wastes as uric acid
C. Drinking small amount of water
D. Excreting salt across their gills

Answer

### Step-by-Step Solution:

1. **Understanding Freshwater Bony Fishes**: Freshwater bony fishes live in an environment where the concentration of salts (ions) in the water is lower than that in their body fluids. This creates a situation where water tends to enter their bodies through osmosis.

2. **Water Absorption**: These fishes absorb water not only through their gills but also through their skin. This is due to the osmotic gradient that exists between their body fluids and the surrounding freshwater.

3. **Ion Regulation**: To maintain osmotic balance, freshwater bony fishes need to actively take up ions (such as sodium and chloride) from the water. This is primarily done through their gills, where specialized cells help in the uptake of these essential ions.

4. **Excretion of Urine**: Since these fishes are constantly gaining water, they need to excrete the excess water to prevent dilution of their bodily fluids. They do this by producing a large volume of dilute urine, which is also referred to as hypotonic urine. This urine contains a lower concentration of solutes compared to their body fluids.

5. **Maintaining Salt Balance**: While they excrete a lot of water through urine, they also need to compensate for the loss of salts. The ions that are absorbed through the gills help in maintaining the necessary salt balance in their bodies.

6. **Conclusion**: The primary method by which freshwater bony fishes maintain their water balance is by excreting hypotonic urine. This allows them to get rid of excess water while retaining essential salts.

### Final Answer:
Freshwater bony fishes maintain water balance by **excreting hypotonic urine**.

---