When a fresh water protozoan is placed in marine water

Options

A. The contractile vacuoles becomes bigger in size.
B. The number of contractile vacuoles increases
C. The contractile vacuoles disapper
D. The contractile vacuoles remain unchanged.

Answer

### Step-by-Step Solution:

1. **Understanding Freshwater Protozoans**: Freshwater protozoans are organisms that live in low salinity environments. They constantly take in water due to osmosis because their internal environment is more concentrated than the surrounding water.

2. **Placement in Marine Water**: When a freshwater protozoan is placed in marine water, which has a higher concentration of salts (higher salinity), the osmotic pressure changes. The surrounding marine water is more concentrated than the internal environment of the protozoan.

3. **Effect on Contractile Vacuoles**: Freshwater protozoans possess contractile vacuoles that help expel excess water to maintain osmotic balance. In marine water, the protozoan needs to conserve water instead of expelling it.

4. **Disappearance of Contractile Vacuoles**: To adapt to the marine environment, the contractile vacuoles of the freshwater protozoan will disappear. This adaptation prevents the loss of water, which would lead to dehydration of the cell.

5. **Consequences of Not Adapting**: If the contractile vacuoles do not disappear, the protozoan would continue to lose water to the surrounding marine environment, leading to dehydration and potentially death of the cell.

6. **Conclusion**: Therefore, when a freshwater protozoan is placed in marine water, the correct response is that the contractile vacuoles disappear to prevent dehydration.

### Final Answer:
When a freshwater protozoan is placed in marine water, its contractile vacuoles disappear.