Final answer:
Freshwater fish live in a hypotonic environment, causing them to be at risk of overhydration as water naturally moves into their cells. They adapt by excreting very dilute urine and actively taking in salts through their gills to maintain osmotic balance. Option c) Overhydration is the correct answer.
Step-by-step explanation:
Aquatic organisms, including freshwater fish, must regulate the osmotic pressure in their cells, a process known as osmoregulation. Osmoregulation is vital as it allows these organisms to maintain homeostasis despite the variations in environmental salt concentration. Freshwater environments are hypotonic compared to the bodily fluids of freshwater fish, meaning the surrounding water has a lower concentration of solutes (e.g. salts) than the fluid inside their cells.
In a hypotonic environment, water tends to move into the cells of the fish, possibly leading to overhydration. To cope with this, freshwater fish excrete very dilute urine and actively transport salts through their gills into their bodies. In contrast, saltwater fish reside in a hypertonic environment where the surrounding water is more concentrated with salts than their bodily fluids, causing water to potentially leave their cells. They address this by drinking seawater and excreting the excess salts through their gills and kidneys, which produce highly concentrated urine.
When considering the potential danger a freshwater fish might face, we can deduce that these organisms are in threat of overhydration due to the uncontrolled influx of water into their cells. Therefore, the correct answer to the question is: c) Overhydration