Fresh water animals cannot live for long in sea water and vice versa as they face:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Osmotic problems
Option (1) Temperature variations — Incorrect. Fresh water and sea water can be at much the same temperature; the difference is in salt.
Option (2) Tidal waves — Incorrect. Waves are not what kills a freshwater fish in the sea.
Option (3) Predators — Incorrect. Both fresh water and the sea have predators, so they cannot explain why the animal dies when moved. The real problem is salt: NCERT says such animals cannot live long in the other water because of osmotic problems.
Option (4) Osmotic problems — Correct. A freshwater animal in sea water loses water to the salty surroundings; a marine animal in fresh water takes in too much water.
Note: This question is from the topic “Major Abiotic Factors” in Chapter 13, Organisms and Populations, of the pre-rationalised NCERT book. This portion is not in the present NCERT book, where the chapter is now Chapter 11, Organisms and Populations. The extract below is from the pre-rationalised edition.
NCERT Extract (Pre-rationalised edition, Chapter 13, Organisms and Populations):
Major Abiotic Factors – “… The salt concentration (measured as salinity in parts per thousand), is less than 5 in inland waters, 30-35 in the sea and > 100 in some hypersaline lagoons. Some organisms are tolerant of a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline). Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face.”