8.3.3 Osmoregulatory Adaptations in Animals

Freshwater Animals

• Almost all of the freshwater animals are osmoregulators.
• These animals are generally hypertonic to their outer environment.
• These animals deal with these problems by producing large volume of diluted urine.
• Their kidney reabsorbs the salts that are required.
• Salts are also obtained from the food they eat.
• These animals also actively transport salts from the external dilute medium with the help of special salt cells called ionocytes.
• Ionocytes are found in the amphibian skin and gills of fishes.

Marine Animals

• Teleosts (bony fishes) are osmoregulators in marine environment which are hypotonic to their environment.
• So these fishes have tendency to lose water to the environment, especially across the gill epithelium.
• In order to replace the water loss, these fishes usually drink large amount of water unlike freshwater fishes.
• They also have problem of excess of salts in the body due to drinking of sea water.
• Among the excess salts, Na⁺, Cl⁻ and some amount of K⁺ are removed across the gill epithelium.
• Divalent ions like Mg²⁺, Ca²⁺ are excreted by the kidney.
• Some fishes also have special salt secreting glands in the wall of rectum called rectal glands that remove salts into the digestive tract which are then eliminated from the body during egestion.

Terrestrial Animals

• The successful groups of land animals are arthropods among the invertebrates and reptile, birds and mammals among the vertebrates.
• The presence of chitinous exoskeleton in arthropods and dead keratinized skin in vertebrates are adaptation to reduce water loss by their bodies.
• Desert mammals are very much resistant in this regard.
• They can tolerate against strong degree of dehydration by special metabolic and behavioural adaptation.
• This characteristic is called anhydrobiosis.
• Actually, these animals feed upon seeds of desert plants in which large amount of carbohydrate are stored.
• During the breakdown of these compounds; water is produced as byproduct that is utilized by these animals.
• Best example of such animals is kangaroo rat.
• They avoid day time heat and emerge at night.
• Ninety percent of the water that they use is metabolic water derived from cellular oxidation.

Summary Table of Osmoregulatory Adaptations

Intermediate Level

Osmoregulatory Adaptations in Animals

Detailed Description of Adaptations

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Freshwater animals are hypertonic to their environment.
• 2. Freshwater animals produce large volume of diluted urine.
• 3. Freshwater animals reabsorb salts.
• 4. Freshwater animals actively transport salts using ionocytes.
• 5. Ionocytes are found in amphibian skin and fish gills.
• 6. Marine teleosts are hypotonic to their environment.
• 7. Marine teleosts drink large amount of water.
• 8. Marine teleosts remove Na⁺, Cl⁻, K⁺ across gill epithelium.
• 9. Marine teleosts excrete Mg²⁺, Ca²⁺ by kidney.
• 10. Some fishes have rectal glands.
• 11. Rectal glands remove salts into digestive tract.
• 12. Terrestrial animals have adaptations to reduce water loss.
• 13. Arthropods have chitinous exoskeleton.
• 14. Vertebrates have keratinized skin.
• 15. Desert mammals tolerate dehydration.
• 16. Anhydrobiosis is the ability to tolerate dehydration.
• 17. Kangaroo rat is an example of anhydrobiosis.
• 18. Kangaroo rat uses metabolic water.
• 19. Ninety percent of water used by kangaroo rat is metabolic water.
• 20. Metabolic water is derived from cellular oxidation.

Remember

• - Freshwater animals = hypertonic; produce dilute urine; reabsorb salts; ionocytes
• - Marine animals = hypotonic; drink water; remove salts; rectal glands
• - Terrestrial animals = reduce water loss; chitinous exoskeleton; keratinized skin; anhydrobiosis
• - Ionocytes = salt cells in amphibian skin and fish gills
• - Rectal glands = salt-secreting glands in some fish
• - Anhydrobiosis = ability to tolerate dehydration
• - Kangaroo rat = example of anhydrobiosis

Common Mistakes

• 1. Confusing freshwater and marine adaptations — Freshwater = dilute urine; marine = drink water.
• 2. Mixing up the ion movement — Freshwater = salts in; marine = salts out.
• 3. Forgetting the role of ionocytes — Actively transport salts in freshwater animals.
• 4. Mixing up the salt removal sites — Na⁺, Cl⁻, K⁺ = gills; Mg²⁺, Ca²⁺ = kidney.
• 5. Forgetting the role of rectal glands — Remove salts into digestive tract.
• 6. Mixing up the terrestrial adaptations — Chitinous exoskeleton = arthropods; keratinized skin = vertebrates.
• 7. Forgetting the role of metabolic water — Used by desert animals.
• 8. Mixing up the examples — Kangaroo rat = anhydrobiosis.
• 9. Believing all marine animals drink water — Only marine teleosts drink water.
• 10. Thinking osmoregulation is not important — It is essential for survival.

Quick Revision

Almost all of the freshwater animals are osmoregulators. These animals are generally hypertonic to their outer environment. These animals deal with these problems by producing large volume of diluted urine. Their kidney reabsorbs the salts that are required. Salts are also obtained from the food they eat. These animals also actively transport salts from the external dilute medium with the help of special salt cells called ionocytes. Ionocytes are found in the amphibian skin and gills of fishes. Teleosts (bony fishes) are osmoregulators in marine environment which are hypotonic to their environment. So these fishes have tendency to lose water to the environment, especially across the gill epithelium. In order to replace the water loss, these fishes usually drink large amount of water unlike freshwater fishes. They also have problem of excess of salts in the body due to drinking of sea water. Among the excess salts, Na⁺, Cl⁻ and some amount of K⁺ are removed across the gill epithelium while divalent ions like Mg²⁺, Ca²⁺ are excreted by the kidney. Some fishes also have special salt secreting glands in the wall of rectum called rectal glands that remove salts into the digestive tract which are then eliminated from the body during egestion. The successful groups of land animals are arthropods among the invertebrates and reptile, birds and mammals among the vertebrates. The presence of chitinous exoskeleton in arthropods and dead keratinized skin in vertebrates are adaptation to reduce water loss by their bodies. Desert mammals are very much resistant in this regard. They can tolerate against strong degree of dehydration by special metabolic and behavioural adaptation. This characteristic is called anhydrobiosis. Actually, these animals feed upon seeds of desert plants in which large amount of carbohydrate are stored. During the breakdown of these compounds; water is produced as byproduct that is utilized by these animals. Best example of such animals is kangaroo rat. They avoid day time heat and emerge at night. Ninety percent of the water that they use is metabolic water derived from cellular oxidation.

60-Second Revision

• Freshwater animals = hypertonic; produce dilute urine; reabsorb salts; ionocytes
• Marine animals = hypotonic; drink water; remove salts; rectal glands
• Terrestrial animals = reduce water loss; chitinous exoskeleton; keratinized skin; anhydrobiosis
• Ionocytes = salt cells in amphibian skin and fish gills
• Rectal glands = salt-secreting glands in some fish
• Anhydrobiosis = ability to tolerate dehydration
• Kangaroo rat = example of anhydrobiosis
• Warning: Freshwater animals produce dilute urine; marine teleosts drink sea water

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