GENERAL HISTOLOGY

Detailed explanation-1: -Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills.

Detailed explanation-2: -Ammonia is formed from the metabolism of protein and is the major waste product of fish. The majority of ammonia from fish is excreted through the gills, with relatively little being lost through urine and feces. Ammonia is also formed as uneaten feed or other organic matter in an aquarium decomposes.

Detailed explanation-3: -The gills are the primary site of ammonia excretion in fish (Wilkie, 1997, 2002; Weihrauch et al., 2009), because they have large surface area, perfusion by 100% of cardiac output, large ventilation rates, small diffusion distances, and contact with a voluminous mucosal medium (Evans et al., 2005).

Detailed explanation-4: -Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic acids. While aquatic animals can easily excrete ammonia into their watery surroundings, terrestrial animals have evolved special mechanisms to eliminate the toxic ammonia from their systems.

Detailed explanation-5: -Most fish are ammonotelic, that is, they excrete waste nitrogen as molecular ammonia or ammonium ion (NH4+).