In this essay we will discuss about water and solute regulation in aquatic habitats.
1. In Fresh Water Environment:
Fresh-water vertebrates have hypertonic body fluids (osmolarity about 200-300 mosmol per litre) than the surrounding water (less than 50 mosmol per litre).
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So these animals have to counter two problems:
(a) Gain of water due to osmotic gradient.
(b) Continuous loss of body salts.
Such fresh-water animals prevent the net gain of water or net loss of body salts in different ways:
(a) Protozoans like Amoeba, Paramecium, etc. have contractile vacuole to expel excess of water.
(b) In fresh-water fishes, it is achieved by:
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(i) No drinking of water.
(ii) Water uptake and salt loss are reduced by special body covering e.g. development of subcutaneous fat layer in scaleless fish.
(iii) 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. These cells can transport Na+ and Cl– ions from surrounding water (less than ImM NaCl) to their body fluids (about 100 mM NaCl) in the presence of energy against concentration gradient.
2. In Marine Environment:
Body fluid of marine vertebrates is hypotonic (about 300 mosmol per litre) to sea water (osmolarity about 1000 mosmol per litre). So these have the problem of water loss from their body fluids through their permeable surfaces of gills, mouth and anal membranes.
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The water loss in such animals is prevented by:
(i) In taking large amount of water.
(ii) Expel excess of monovalent ions like Na+, K+, and CI– etc. from their body fluids to sea water with the help of ionocytes or chloride cells of membrane of gill lamellae.
(iii) Divalent ions like Mg++ and SO4— are eliminated with faeces.
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(iv) Certain fishes like hilsa, salmon, etc. show migration between sea water and fresh water. Such fishes can switch over their metabolic functions in different habitats and is regulated by a number of hormones.
(v) Elasmobranchs (sharks and rays) and coelcanth (lobe-finned fish) can accumulate certain organic substances, called osmolytes, in their body fluids to raise their osmolarity e.g. sharks can reabsorb 2-2.5% of urea from nephric filtrate, while coelcanth can accumulate trimethylamine oxide (TMAO) to make their body fluids slightly hypertonic to sea water. Retention of these osmolytes in their body fluids decreases their osmoregulatory problems.
(vi) Marine animals like ascidians (e.g. Herdmania) and hag fish are isosmotic to sea water.
3. Migrate:
It involves shorter or longer journey undertaken by the animals to resist the temperature extremes so as to move away temporarily away from the stressful conditions to a more favourable area and return to their original habitat when stressful conditions are over e.g.
(i) 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 Arctic tern {Sterna macrura) breeds on Northernmost coasts of Labador 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 about 2400 km from its breeding sites in Himalayas to the feeding sites in Nilgiri hills.
Migratory birds of Northern hemisphere migrate towards north during spring (increasing photoperiod) and towards south during autumn (decreasing photoperiod). It is so as with increasing photoperiod, more energy is available for gonadial activity which stimulates them to migrate north word for breeding.
(ii) Certain mammals like Bison, Reindeer, Caribou, etc. migrate from temperate coniferous forests to tundra biome during the summer months, so are short distance migratory animals.
4. Suspend:
Those poikilothermal organisms which are not able to migrate, to avoid stressful conditions of environment, escape such conditions in one of the following ways:
(i) Hibernation (Winter Sleep):
It is the period of dormancy during winter months. Poikilothermal animals like amphibians (e.g., frogs) and reptiles (e.g., lizards and snakes) and some homeothermal animals like insectivores (e.g., shrews), bats, ground squirrel, jumping mice and wood chucks undergo hibernation.
During this period, their body temperature, metabolic rate and energy requirements are greatly reduced and mostly reserve food is utilized. Polar bears of Tundra biome also undergo hibernation. Coccinella septempunctata (beetle insect) undergoes hibernation during different periods in different countries.
(ii) 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, certain bugs, snails (e.g. Ariophante).
(iii) Diapause:
It is a special type of aestivation in which morphological growth and development remain suspended to avoid summer-related problems. It has been reported in many insects, crustaceans, mites, snails and zooplankton species.