Which one of the following reasons explains why aquatic animals, such as fish, rely on water for respiration?

Correct Option

The correct option is

Their respiratory organs can only extract oxygen dissolved in water

Explanation

Respiration in most aquatic animals, particularly fish, is mediated through specialized organs called gills. These organs are structurally adapted to facilitate gas exchange in a liquid medium, utilizing the oxygen that is physically dissolved in water.

Option Analysis

• Their respiratory organs can only extract oxygen dissolved in water: Correct. Gills consist of thin filaments and lamellae that increase surface area for gas exchange. These structures are supported by the buoyancy of water. In an aquatic environment, water flows over the gills, allowing oxygen dissolved in the water to diffuse into the bloodstream. In the air, gill filaments collapse due to surface tension and lack of buoyant support, drastically reducing the surface area and preventing effective gas exchange. Therefore, these organs are functionally restricted to extracting dissolved oxygen.
• Their lungs are not developed enough to breathe atmospheric air: Incorrect. The reliance on water is not due to "undeveloped lungs" but rather the presence of specialized aquatic respiratory organs (gills). Most fish do not possess lungs at all; they have evolved a different respiratory system entirely suited for their environment.
• Their skin requires constant hydration for gas exchange: Incorrect. While moist skin is essential for cutaneous respiration in amphibians (like frogs) and some specific aquatic creatures, it is not the primary reason fish rely on water. Fish rely on water primarily for gill function.
• They chemically break down water molecules to obtain oxygen: Incorrect. This is a scientifically invalid statement. Respiration involves the intake of free molecular oxygen ($O_2$) dissolved in water. Animals do not possess the metabolic machinery to break the strong chemical bonds of water molecules ($H_2O$) to extract oxygen. The splitting of water molecules to release oxygen occurs in plants during photosynthesis (photolysis of water), not in animal respiration.

Key Takeaway: Aquatic animals breathe oxygen dissolved in water, not the oxygen bound in water molecules ($H_2O$). Their respiratory organs (gills) are specialized for liquid-medium gas exchange and cannot function in the atmosphere.