Question

Assertion (A): Most marine bony fishes drink sea water continuously.
Reason (R): They lose water from their body to the surrounding sea water by osmosis.

• A. Both (A) and (R) are true. (R) is correct explanation for (A)
• B. Both (A) and (R) are true. (R) is not correct explanation for (A)
• C. (A) is true. But (R) is false
• D. (A) is false. But (R) is true

Hint:

Hypotonic organisms in a hypertonic medium always lose water by osmosis.
Therefore, marine bony fishes must drink water to survive, whereas freshwater fishes do not drink water because they absorb it continuously.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question evaluates the osmoregulatory mechanisms used by marine bony fishes to survive in highly saline marine environments.

Step 2: Key Concept and Approach:
Marine bony fishes have body fluids that are hypotonic compared to the surrounding seawater.
Because of this concentration gradient, water naturally tends to leave their body through their semi-permeable membranes.
We must determine whether the assertion about drinking seawater and the reason about osmotic water loss are correct and logically linked.

Step 3: Detailed Explanation:

• Water Loss via Osmosis: Since seawater is hypertonic to the internal fluids of bony fishes, they continuously lose water by exosmosis.
This loss occurs primarily through their permeable gill membranes and body surface.
Hence, Reason (R) is true.

• Drinking Behavior: To compensate for the continuous loss of water, these fishes must take in large amounts of water.
They do this by drinking seawater continuously, making Assertion (A) true.

• Causal Link: The primary biological reason they drink seawater is to replace the water lost through osmosis.
Therefore, Reason (R) is the direct and correct explanation for Assertion (A).

• Additional Mechanisms: Along with drinking seawater, they excrete highly concentrated urine and actively transport excess salts out of their body using specialized chloride cells in their gills.


Step 4: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation for (A).