Which is an anadromous fish?

Options

A. Hippocampus
B. Petromyzon
C. Exocoetus
D. Anabas

Answer

### Step-by-Step Solution:

1. **Understanding Anadromous Fish**:
- Anadromous fish are species that migrate from saltwater to freshwater to spawn. This means they live most of their lives in the ocean but return to freshwater rivers or lakes to lay their eggs.

2. **Identifying Characteristics**:
- These fish typically face challenges with osmoregulation (the process of maintaining salt and water balance) when they move to freshwater, which can be detrimental to their survival.

3. **Examples of Anadromous Fish**:
- Common examples of anadromous fish include species like salmon and certain types of lampreys. In the context of the question, lampreys (specifically the genus Petromyzon) are highlighted as an example of an anadromous fish.

4. **Differentiating from Catadromous Fish**:
- It’s important to note the difference between anadromous and catadromous fish. Catadromous fish, such as eels, migrate from freshwater to the ocean to spawn. This is the opposite behavior of anadromous fish.

5. **Conclusion**:
- Based on the information provided, the correct answer to the question "Which is an anadromous fish?" is **lamprey (Petromyzon)**.