Question

Given below are two statements:
Statement I: Freshwater teleosts operate hyperosmotically to their surrounding medium
Statement II: Freshwater teleosts produce a large quantity of dilute urine to remove excess water
In the light of the above statements, choose the most appropriate answer from the options given below

• A. Both Statement I and Statement II are correct
• B. Both Statement I and Statement II are incorrect
• C. Statement I is correct but Statement II is incorrect
• D. Statement I is incorrect but Statement II is correct

Hint:

Teleost Osmoregulation:
Freshwater Teleosts: Hyperosmotic $\rightarrow$ Copious Dilute Urine + Do NOT drink water + Active gill ion uptake.
Marine Teleosts: Hypoosmotic $\rightarrow$ Scanty Concentrated Urine + Drink seawater + Active gill salt excretion.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Freshwater osmoregulation in teleosts: maintaining internal body fluid osmolarity ($pprox 300 ext{ mOsm/L}$) hyperosmotic to hypoosmotic freshwater ($pprox 5-10 ext{ mOsm/L}$) requires copius dilute urination and active branchial ion uptake.
Key Formula or Approach:
\[ \text{Freshwater Teleost (Hyperosmotic): Osmotic Water Influx (Gills)} \implies \text{Excretes Copious Dilute Urine } + \text{Active Branchial } \text{Na}^+/\text{Cl}^- \text{ Uptake} \]

Step 2: Detailed Explanation:
1. Statement I: In freshwater teleost fishes (e.g., carps, catfishes), internal blood plasma osmolarity is $\approx 280 - 320\text{ mOsm/kg}$, which is vastly higher than the surrounding ambient freshwater environment ($< 10\text{ mOsm/kg}$). Thus, they operate hyperosmotically to their surrounding medium. Hence, Statement I is correct.
2. Statement II: Due to osmotic gradients, water continuously invades the fish's body across the permeable gill epithelia. To counter hydration, the well-developed glomerular kidneys excrete large volumes of highly dilute, hypotonic urine while ionocytes (chloride cells) actively pump $\text{Na}^+$ and $\text{Cl}^-$ from water into blood. Hence, Statement II is correct.

Step 3: Final Answer:
Therefore, Both Statement I and Statement II are correct, matching option (A).