Freshwater bony fish secrete large volumes of very dilute urine. Many marine bony fish secrete small volumes of concentrated urine. Which of the following best explains the differences in nitrogenous-waste excretion between freshwater and marine bony fish?

Question

BlackTom AI · Accepted Answer

Freshwater bony fish are hyperosmotic to their environment, so they gain water by osmosis and lose salts; marine bony fish are hypoosmotic, so they lose water and gain salts. These osmotic gradients drive different adaptive solutions for nitrogenous-waste and water balance.

Relevant knowledge points:
- Osmoregulation: organisms must balance water and ion fluxes with their environment.
- Kidney function: urine volume and concentration are adjusted to excrete wastes while maintaining osmotic balance.
- Evolution by natural selection: populations exposed to different environmental pressures evolve traits that increase fitness in those environments.

Step-by-step reasoning:
1. Freshwater fish live in water less concentrated than their body fluids (hyperosmotic internally). Water continuously enters their bodies by osmosis. To avoid dilution of body fluids, they excrete large volumes of very dilute urine to rid excess water and retain ions via active uptake in gills and kidneys.
2. Marine fish live in seawater more concentrated than their body fluids (hypoosmotic internally). They tend to lose water to the environment and gain salts. To conserve water, they drink seawater, excrete excess salts (mainly via gill chloride cells), and produce small volumes of concentrated urine to minimize water loss while disposing of nitrogenous wastes.
3. These physiological strategies are consistent, widespread, and align with selection pressures imposed by environmental osmolarity.

Why the correct answer is correct:
- The described urine patterns are adaptive responses that evolved in lineages exposed to different osmotic environments. Natural selection favored kidneys and osmoregulatory mechanisms that match the demands of freshwater versus marine habitats; thus differences arose during divergence from common ancestors.

Why other options are incorrect:
- Option 2: Kidneys did not evolve independently in separate lineages; vertebrate kidneys have a common origin and were modified by selection.
- Option 3: The differences are not accidental— they are functional adaptations to osmotic conditions.
- Option 4: While individual fish can physiologically adjust (euryhaline species), the broad freshwater vs. marine patterns reflect evolutionary divergence, not only within-generation plasticity.

Conclusion: The best explanation is that differences arose during divergence under different osmotic selective pressures, matching the provided correct answer.

Freshwater bony fish secrete large volumes of very dilute urine. Many marine bony fish secrete small volumes of concentrated urine. Which of the following best explains the differences in nitrogenous-waste excretion between freshwater and marine bony fish?单项选择题

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