Why cartilaginous fishes have to swim constantly?

Answer

### Step-by-Step Solution:

1. **Identify the Class of Fish**:
Cartilaginous fishes belong to the class Chondrichthyes. This class includes sharks, rays, and skates.

2. **Understand the Structure**:
The defining characteristic of cartilaginous fishes is that their endoskeleton is made of cartilage rather than bone.

3. **Absence of Air Bladder**:
Unlike bony fishes, cartilaginous fishes do not possess an air bladder (also known as a swim bladder). The air bladder in bony fishes helps them maintain buoyancy in water.

4. **Function of Air Bladder**:
The air bladder allows bony fishes to control their buoyancy, enabling them to remain suspended at various depths without expending energy.

5. **Consequences of Absence of Air Bladder**:
Since cartilaginous fishes lack this structure, they cannot maintain buoyancy naturally. This means they will sink if they stop swimming.

6. **Need for Constant Swimming**:
To counteract their tendency to sink and to maintain balance in the water, cartilaginous fishes must swim continuously. This constant movement helps them stay afloat and navigate through their aquatic environment.

7. **Conclusion**:
Therefore, the necessity for cartilaginous fishes to swim constantly is primarily due to the absence of an air bladder, which is essential for maintaining buoyancy.