Which is not a feature of dinoflagellates ?

Options

A. They cause red tides
B. Their cell wall has stiff cellulose plates on the outer surface
C. They release toxins
D. These are mostly fresh water and non-photosynthetic

Answer

To determine which option is not a feature of dinoflagellates, we can analyze each statement provided in the question:

1. **They cause the red tides.**
- Dinoflagellates are indeed known to cause red tides, which are algal blooms that can discolor seawater. This is a characteristic feature of dinoflagellates.

2. **Their cell wall has stiff cellulose plates on the outer surface.**
- Dinoflagellates possess a unique structure known as the theca, which consists of cellulose plates. This statement is true.

3. **They release toxins.**
- Many dinoflagellates are known to produce toxins that can be harmful to marine life and humans. This statement is also true.

4. **They are mostly freshwater and known photosynthetic.**
- While some dinoflagellates can be found in freshwater, they are predominantly marine organisms. Additionally, many dinoflagellates are photosynthetic, but not all of them. Therefore, this statement is partially true but can be misleading as it suggests that they are primarily freshwater organisms.

Based on this analysis, the statement that is not a feature of dinoflagellates is **"They are mostly freshwater."**

### Final Answer:
**They are mostly freshwater.**