Question

water bony fishes maintain water balance by
(a)Excreting hypotonic urine
(b)Excreting wastes as uric acid
(c)Drinking a small amount of water
(d)Excreting salts across their gills

Answer 1

Hint: Freshwater bony fishes live in an environment where their blood and tissue fluids are hypertonic to the surrounding medium i.e they have more salt concentration than the medium(freshwater) surrounding them.

Complete answer:
Bony fishes or Osteichthyes are the aquatic vertebrates who possess bony endoskeleton in the adult form. The endoskeleton is cartilaginous in the embryonic stage. They are ectothermic i.e. cold-blooded. There are about 25,000 species of bony fishes. Bony fishes occur in all sorts of waters – freshwater, marine, brackish. In freshwater bony fishes like Labeo, Catla, the blood and tissue fluids are hypertonic to the surrounding medium and thus water is constantly flushed inside by osmosis. To balance this, kidneys excrete hypertonic urine i.e excess of water is secreted out from the body into the environment.

Additional Information: For marine bony fishes like Exocoetus, Hippocampus the situation is the opposite. Their blood and tissue fluids are hypotonic to the surrounding medium. This poses a threat of water loss from the body of fishes into the environment by osmosis. In such circumstances, the marine bony fishes are adapted to take large amounts of water into the gut and reduce the production of urine. Also, excess of salt which has entered the body is excreted from the gills and kidneys.
So, the correct answer is 'excreting hypotonic urine'.

Note: Bony fishes are ammonotelic i.e they secrete ammonia as the main excretory product. Nitrogenous waste substances such as urea, ammonia, and uric acid are produced during protein or nucleic acid metabolism. Ammonia is the most toxic of all which is followed by urea and uric acid. Ammonia required a large amount of water to be excreted out from the body and thus is the main excretory product of most aquatic animals.