Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Marine elasmobranchs are slightly hyperosmotic to their environment.
Reason (R): Marine elasmobranchs have a considerable amount of urea and trimethylamine in their plasma.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Urea + TMAO retention is the classic physiological adaptation of sharks to remain hyperosmotic to seawater.
Without TMAO, high urea would denature their proteins.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Osmoregulation is the physiological process by which marine animals maintain the water and salt balance in their body fluids relative to the surrounding water.

Step 2: Detailed Explanation:
Let us analyze the physiological mechanism of osmoregulation in sharks and rays (marine elasmobranchs):
Assertion (A) states that marine elasmobranchs are slightly hyperosmotic to their environment.
This is correct. Their internal body fluids have a total osmotic pressure slightly higher than that of the surrounding seawater.
As a result, water tends to diffuse slowly into their bodies across the gills, preventing dehydration without requiring them to drink seawater continuously.
Reason (R) explains how they achieve this hyperosmotic state.
Instead of matching seawater's high sodium and chloride levels, elasmobranchs retain high concentrations of organic osmolytes in their blood plasma.
These osmolytes are primarily urea and trimethylamine oxide (TMAO).
Urea increases the osmotic pressure of the blood.
TMAO is a vital stabilizer that protects cellular proteins from the toxic, denaturing effects of high urea concentrations.
Therefore, the retention of urea and trimethylamine (TMAO) directly causes their plasma to be hyperosmotic to seawater.
Both statements are correct, and (R) is the direct physiological explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct and (R) is the correct explanation of (A).