Which one of the following is called as 'indicator species' for polluted waters:

Options

A. Tubifex
B. Phytoplankton
C. Zooplankton
D. Prawn.

Answer

To solve the question about which organism is referred to as an 'indicator species' for polluted waters, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Term 'Indicator Species':**
- An indicator species is a type of organism whose presence, absence, or abundance reflects a specific environmental condition, such as pollution levels in water bodies.

2. **Review the Options Provided:**
- The options given are:
- A) Tubifex
- B) Phytoplankton
- C) Zooplankton
- D) Prawns

3. **Identify Characteristics of Each Option:**
- **Tubifex:** This is a type of aquatic oligochaete worm known for its ability to survive in polluted waters, particularly in environments with low oxygen levels and high concentrations of heavy metals.
- **Phytoplankton:** These are microscopic plants that form the base of aquatic food webs but are not specifically known as indicators of pollution.
- **Zooplankton:** These are small animals that feed on phytoplankton and are also not specific indicators of water pollution.
- **Prawns:** While they are aquatic organisms, they are not typically used as indicators for water quality.

4. **Determine the Correct Answer:**
- Based on the characteristics, Tubifex is known for its tolerance to polluted waters, particularly in urban and sewage-affected areas. Therefore, it is recognized as an indicator species for polluted waters.

5. **Final Answer:**
- The organism that is called an 'indicator species' for polluted waters is **Tubifex**.