Q.How do freshwater animals cope with a hypotonic environment?

Life on land constantly risks water loss to the drier atmosphere; terrestrial animals counter this danger of desiccation through a combination of structural, physiological and behavioural adaptations that together minimise evaporative and excretory water loss.

1. Structural adaptations:

• Insects and other arthropods possess a waxy, chitinous cuticle over the exoskeleton that is nearly impermeable to water, drastically cutting evaporative loss through the body surface.
• Reptiles have dry, keratinised, scaly skin that resists water loss, allowing them to live in even very dry habitats.
• Mammals and birds have keratinised epidermis (skin/feathers) that reduces surface evaporation compared with the moist, permeable skin of amphibians.

2. Physiological adaptations:

• Insects, reptiles and birds excrete nitrogenous waste as uric acid, which is nearly insoluble and can be excreted as a semi-solid paste, conserving far more water than excreting urea or ammonia in solution.
• Mammals produce concentrated urine (via the kidney's loop of Henle and ADH-regulated water reabsorption), minimising water lost during excretion.
• Insects can close their spiracles (the openings of the tracheal system) to reduce respiratory water loss, opening them only as needed for gas exchange.
• Some desert animals (e.g. the kangaroo rat) rely substantially on metabolic water produced from the oxidation of food and can survive with minimal or no drinking water.

3. Behavioural adaptations:

• Many desert and terrestrial animals are nocturnal or crepuscular, being active during the cooler night/dawn/dusk hours and avoiding the desiccating heat of the day.
• Burrowing animals retreat underground, where the microclimate is cooler and more humid, during the hottest part of the day. …

Euryhaline and stenohaline are terms describing an aquatic animal's tolerance to changes in its surrounding salinity — euryhaline species tolerate a wide range, stenohaline species tolerate only a narrow range.

Euryhaline animals:

• Definition: animals capable of tolerating and osmoregulating across a wide range of external salinities.
• Mechanism: they possess efficient osmoregulatory machinery (e.g. specialised gill chloride cells, kidneys capable of producing very dilute or fairly concentrated urine) that lets their internal osmotic environment stay relatively stable even as the external salinity swings widely.
• Habitat/examples: species that move between fresh water and sea water during their life cycle, such as salmon (anadromous, breeding in fresh water and living at sea) and eels (catadromous), as well as many estuarine organisms that experience large tidal salinity swings.

Stenohaline animals:

• Definition: animals that can tolerate only a narrow range of salinity and cannot survive significant deviation from their normal habitat's salt concentration.
• Mechanism: limited osmoregulatory capacity means a large external salinity change overwhelms their ability to maintain internal osmotic balance, causing cell damage or death. …