Many freshwater animals cannot survive for long in sea water, and many marine animals cannot live in fresh water, primarily because of which kind of physiological problem they would face?

Introduction / Context:
Aquatic animals live in environments with different salt concentrations, such as fresh water and sea water. Their bodies are adapted to these conditions, especially in how they regulate water and salt balance. When animals are moved from one type of environment to the other, they may not survive. This question asks you to identify the main physiological problem that prevents many freshwater animals from living in sea water and vice versa.

Given Data / Assumptions:

• The animals considered are freshwater animals and marine animals.
• We are comparing their ability to survive in environments with different salinity.
• The options include osmotic, eurythermal, stenothermal, and hydrothermal problems.
• We assume normal temperature ranges and are focusing primarily on salinity differences.

Concept / Approach:
Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. Freshwater animals are adapted to low salt environments and often face the challenge of water entering their bodies, while marine animals are adapted to high salt conditions and must prevent water loss. If a freshwater animal is placed in sea water, it can lose water and dehydrate; if a marine animal is placed in fresh water, it may gain too much water and face swelling. These are osmotic problems arising from mismatched salt and water regulation systems. The terms eurythermal and stenothermal refer to temperature tolerance ranges, and hydrothermal relates to hot spring conditions, not directly to salinity. Therefore, osmotic problem is the correct answer.

Step-by-Step Solution:
Step 1: Identify the key difference between fresh water and sea water: fresh water has very low salt content, while sea water has high salt concentration. Step 2: Recall that freshwater animals are adapted to avoid excessive water intake and to conserve salts in a low salinity environment. Step 3: Recognise that marine animals are adapted to prevent dehydration and excessive salt gain in high salinity conditions. Step 4: Understand that moving animals from one environment to the other disrupts this balance and causes water to move in or out of their bodies by osmosis. Step 5: Conclude that the major difficulty they face is osmotic imbalance, which can lead to swelling, dehydration, or death, making osmotic problem the correct option.

Verification / Alternative check:
To verify, think about how textbooks explain osmoregulation. Freshwater fishes continuously excrete dilute urine to remove excess water and actively absorb salts, while marine fishes drink sea water and excrete excess salt. These strategies are adapted to specific salinity levels. Temperature tolerance (eurythermal or stenothermal) may also affect survival, but if you kept temperature constant and only changed salinity, osmotic stress would still be the primary issue. Hydrothermal refers to animals in hot vents or springs, not to typical freshwater or marine environments. This analysis confirms that osmotic problem is the central issue.

Why Other Options Are Wrong:
Eurythermal problem is wrong because it refers to species that can tolerate a wide range of temperatures; it is not about salt water versus fresh water. Stenothermal problem is incorrect because it relates to narrow temperature tolerance, again focusing on heat and cold rather than salinity. Hydrothermal problem is also wrong, as it would involve extreme temperature conditions such as hot springs or deep sea vents, which are not mentioned in the question. None of these alternatives addresses the primary salt and water balance issues that arise when animals are moved between fresh and salt water.

Common Pitfalls:
A common pitfall is to misinterpret the terms eurythermal and stenothermal and assume that any survival difficulty must be temperature related. Another mistake is to overlook the key words fresh water and sea water and not connect them to salinity differences. To avoid such errors, always link fresh water versus sea water questions to osmosis and osmoregulation. The moment you see contrasting aquatic habitats with different salt levels, you should think of osmotic problems as the core physiological challenge.

Final Answer:
The correct option is Osmotic problem due to difference in salt concentration, because mismatched salt and water regulation is the main reason many freshwater and marine animals cannot survive in each other's environments.