In fresh water environment: Fresh –water vertebrates have hypertonic body Fluids (osmolarity about 200-300mosmol per litre) then the surrounding water (less than 50 mosmol per litre).so these animals have to counter two Problems.

Gain of water due to osmotic gradient and Continuous loss of body salts

Such fresh-water animals prevent the net gain of water or net loss of body Salts in different ways.

In fresh–water fishes, it is achieved by:

No drinking of water.

Water uptake and salt loss are reduced by special body covering e.g. development of subcutaneous fat layer in scaleless fish. Take up salts from surrounding water by active transport with the help of special cells, called ionocytes or chloride cells, present In the gill membrane of their gill lamellae.

In marine environment: Body fluids of marine vertebrates is hypotonic(about 300 mosmol per litre) to sea water. The water loss in such animals is prevented by:

In taking large amount of water. Expel excess of monovalent ions like Na+, K+, Cl etc. from their body fluids to sea water with the help of ionocytes or chloride cells of membrane of gill lamellae. Long-trip migration by many birds like arctic tern, Siberian cranes, American golden plover, etc. Every year, thousands of migratory Birds of Siberia and other extremely cold northern regions reach the Famous Keol Dev Ghana National Park in Bharatpur (Rajasthan) during winter months. The Arctictern(sterna macrura) breeds on Northern most Coasts of Labad or in summers, then travels about 11,000 miles (about 17,600 km)  to reach Antarctica in winters. It is the longest distance migrant. Similarly, the golden plover covers a non-stop distance of about2400 km from its breeding sites in Himalayas to the feeding sites in Nilgiri hills.

Aestivation (Summer sleep): It is the period of dormancy during summer Months so as to escape from scorching heat of sun. It is commonly seen in Lung fishes (e.g. Protopterus remains surrounded in a cocoon of slime in a Tube-like burrow), lady-bird beetles, certains bugs, snails (e.g. Ariophante).

Diapause: It is a special type of aestivation in which morphological growth and development remain suspended to avoid summer-related Problems.


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