Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Rectal gland, a specialised anatomical structure, is present in marine elasmobranchs.
Reason (R) : Rectal gland secretes vasopressin which helps in osmoregulation.
In light of the above statements, choose the correct answer from the options given below.

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

Hint:

The rectal gland is solely a salt-excreting organ (secreting concentrated \(\text{NaCl}\) solution), not an endocrine gland that produces hormones like vasopressin.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Marine elasmobranchs (sharks and rays) live in a hyperosmotic environment relative to their internal fluids, despite retaining high concentrations of urea and trimethylamine oxide (TMAO).
They require specialized osmoregulatory organs to excrete excess monovalent ions like sodium and chloride that constantly enter their bodies.

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Assertion (A): The rectal gland is a specialized, finger-like salt-excreting gland located in the posterior region of the intestine (rectum) of marine elasmobranchs. This statement is scientifically true. It is a key evolutionary adaptation for survival in marine environments.
- Reason (R): The primary function of the rectal gland is to secrete a highly concentrated solution of sodium chloride (\(\text{NaCl}\)) into the digestive tract to be expelled from the body. It does not secrete vasopressin. Vasopressin is a mammalian antidiuretic hormone. In fishes, the homologous hormone is arginine vasotocin (AVT), which is secreted by the neurohypophysis of the pituitary gland, not the rectal gland. Thus, Reason (R) is false.
Since the Assertion is true but the Reason is false, the correct option is (C).

Step 4: Final Answer:
Therefore, (A) is true but (R) is false.