ANATOMY

Detailed explanation-1: -Respiration in fish takes place with the help of gills. Most fish possess gills on either side of their head. Gills are tissues made up of feathery structures called gill filaments providing a large surface area for exchange of gases.

Detailed explanation-2: -The surface area of the gill filaments is an important factor in gas exchange. Water contains much less oxygen compared to air; therefore fish must have an organ (gills) with a large surface area to get enough oxygen from the water to survive.

Detailed explanation-3: -The primary site, the gills, is designed for gas exchange between blood and water. There are many accessory respiratory surfaces, which in general are associated with the movement of fish from a hypoxic aquatic to an aerial environment and are designed for gas exchange between blood and air.