Questions & Answers

Marine water fishes drink more water than freshwater. Do you agree? Justify.

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Hint:Osmoregulators are the animals that can regulate internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators.

Complete answer:
The osmolarity of seawater usually has an osmolarity of about 1000mOsm $L^{-1}$. The osmoregulatory problem in seawater is different from freshwater. In freshwater osmolarity is less than 50mOsm $L^{-1}$ while the freshwater fishes have blood osmolality in the range of 200 to 300 mOsm $L^{-1}$. The body fluids of freshwater animals are generally hypertonic to their surrounding environment.
Marine bony fishes have body fluids that are hypotonic to seawater, and thereby, they tend to lose water from the body though permeable in marine situations are opposite to those in freshwater environments. Marine bony fishes have the body fluids hypotonic to seawater, and thereby they tend to lose water from the body through permeable surfaces(gill membranes, oral and anal membranes). To compensate for the water loss, marine bony fishes drink seawater.
For maintaining regularity marine water drinks more water than freshwater fishes.
However, the drinking of seawater results in a gain of excess salts. The ionocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated through fecal matter.

Additional Information:
Hypertonic: Hypertonic solution has more amount of solute than other solution. For example, freshwater fishes have more concentration of salt than freshwater.
Hypotonic: the hypotonic solution has a lower amount of solute than other solutions. For example, marine fishes have a lower concentration of ions than in seawater, so we can say marine fish body fluids are hypotonic to seawater.

Note: In general, the body fluids of marine invertebrates, ascidians, and the hagfish are isosmotic to seawater. The osmolarity of the body is raised by accumulating certain organic substances of osmolytes in body fluids to reduce the osmoregulatory challenges. The best-known examples of such organic osmolytes are urea and trimethylamine oxide(TMAO).