Assertion : Some marine animals find it difficult to live in fresh water and vice versa. Reason : Some animals can tolerate a narrow salinity range, while others can tolerate a wide salinity range.

Step-by-step solution

For aquatic organism salt concentration (measured as salinity in parts per thousand) is a major factor for their survival. Salinity of different aquatic habitats varies greatly. It is less than 5 per thousand parts, in inland waters, 30-35 per thousand parts in the sea and more than 100 per thousand parts in some hypersaline lagoons. Many fresh water animals cannot live for long in sea water and vice versa because of the osmotic problems they would face due to the change in relative tonicity of the surrounding wate