Question

Freshwater bony fishes maintain water balance by
a. Excreting hypotonic urine
b. Excreting salt across their gills
c. Drinking a small amount of water
d. Excreting waste in the form of uric acid

Answer 1

Hint: The excretion of large amounts of nitrogenous wastes and water from the body to the outside.

Complete step-by-step solution:
The freshwater fishes are surrounded by the watery environment which is hypertonic as salts are present inside their body. To maintain this salt concentration, the freshwater bony fishes need to excrete hypertonic urine which is dilute having more water and fewer salts.

Additional information:
1. In a hypotonic solution, the concentration of salt is low while in a hypertonic solution the salt concentration is high.
2. The freshwater fishes are in danger of becoming waterlogged since their habitat is hypertonic.
3. To maintain their water balance the kidneys of these are adapted to excrete large amounts of dilute urine.
4. Their kidneys have large glomeruli and capillaries to filter the blood and produce urine.
5. The movement of water and salt in these organisms is done through skin, gills, and mouth.
6. The water enters constantly through the gills and the mouth.
7. They lose salts through gills by the process of diffusion.
8. To maintain the osmoregulation, special gill cells have evolved that transport salts from the water into the body.

So, the correct answer is ‘Excreting hypotonic urine’.

Note: The main nitrogenous waste in the case of freshwater bony fishes is ammonia and about only 10% of waste is excreted in the form of urea. Ammonia is needed to be excreted quickly as it becomes highly toxic. The excretion of nitrogenous waste is mainly performed by gills.