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Zoology Question 57 – AP-EAMCET 2025

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Study the following statements regarding osmoregulatory adaptations in marine and fresh water fish and identify the correct statements. I. Marine fishes actively excrete excess salt through their gills. II. Fresh water fishes have glomerular kidney. III. Marine fishes facing the problem of exosmosis IV. Fresh water fishes have salt excreting chloride cells

Understand the fundamental challenges faced by fish in marine (hypertonic) and freshwater (hypotonic) environments concerning water and salt balance.

Step 1: Analyze Osmoregulatory Challenges✦ Active

First, identify the osmotic challenges faced by fish in different environments. Marine fish live in a hypertonic environment, meaning the surrounding water is saltier than their body fluids. This causes them to constantly lose water to the environment (exosmosis) and gain salts. Freshwater fish live in a hypotonic environment, meaning the surrounding water is less salty than their body fluids. This causes them to constantly gain water and lose salts.

💡 Teacher's Secret Hint

Remember that water moves from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration).

Step 2: Evaluate Statements for Marine Fish○ Expand

Statement I: 'Marine fishes actively excrete excess salt through their gills.' This is correct. Marine fish drink seawater and use specialized chloride cells in their gills to actively pump out excess Na+ and Cl− ions to prevent salt buildup.

Statement III: 'Marine fishes facing the problem of exosmosis.' This is correct. Due to the hypertonic marine environment, water tends to move out of the fish's body into the seawater, leading to dehydration. This is the problem of exosmosis.

💡 Teacher's Secret Hint

Marine fish adaptations are geared towards conserving water and eliminating excess salt.

Step 3: Evaluate Statements for Freshwater Fish○ Expand

Statement II: 'Fresh water fishes have glomerular kidney.' This is correct. Freshwater fish constantly gain water, so they need to excrete large amounts of dilute urine. A well-developed, large glomerulus in their kidneys allows for a high filtration rate to remove this excess water efficiently.

Statement IV: 'Fresh water fishes have salt excreting chloride cells.' This is incorrect. Freshwater fish tend to lose salts to their environment. They have specialized chloride cells in their gills that actively *absorb* salts from the surrounding water, not excrete them. Salt-excreting chloride cells are characteristic of marine fish.

💡 Teacher's Secret Hint

Freshwater fish adaptations focus on eliminating excess water and retaining/absorbing salts.

Step 4: Identify the Correct Combination○ Expand

Based on the evaluation:

Statement I: Correct

Statement II: Correct

Statement III: Correct

Statement IV: Incorrect

Therefore, the correct statements are I, II, and III. This corresponds to option 1.

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