Dr. R.

Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan

Osmoregulation in Fishes
Osmoregulation is a type of homeostasis which controls both the volume of water

and the concentration of electrolytes. It is the active regulation of the osmotic

pressure of an organism’s body fluids, detected by osmoreceptors. Organisms in

aquatic and terrestrial environments must maintain the right concentration of

solutes and amount of water in their body fluids. The nature of osmoregulatory

problem is quite different in various groups of fishes in different environments.

There is always a difference between the salinity of a fish’s environment and the

inside of its body, whether the fish is fresh water or marine. Regardless of the

salinity of their external environment, fish use osmoregulation to fight the process

of diffusion and osmosis and maintain the internal balance of salt and water

essential to their efficiency and survival. Kidneys do play a role in osmoregulation

but overall extra-renal mechanisms are equally more important sites for

maintaining osmotic homeostasis. Extra-renal sites include the gill tissue, skin, the

alimentary tract, the rectal gland and the urinary bladder.

Stenohaline and Euryhaline Fishes:

 Stenohaline (steno=narrow, haline=salt): Most of the species live either in

fresh water or marine water and can survive only small changes in salinity.

These fishes have a limited salinity tolerance and are called stenohaline.

e.g., Goldfish

 Euryhaline (eury=wide, haline=salt): Some species can tolerate wide

salinity changes and inhabit both fresh water and sea water. They are called

euryhaline.

e.g., Salmon

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 Dr. R. Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan
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According to habitat, fishes can be distinguished as (i) Marine, and (ii) Fresh

water.

(i) The marine fishes fall into two distinct groups:

a) Those whose osmotic concentration is the same as or slightly above sea

water, e.g., hagfish, elasmobranchs, Latimeria etc. This group has no major

problem of water balance, because it’s inside and outside concentrations are

equal, there is no osmotic water flow.

b) Those whose osmotic concentrations are about one third of that of sea water,

e.g., lampreys, teleosts, etc. These are hyposmotic animals. They live in

constant danger of losing water to the osmotically more concentrated

medium.

(ii) The fresh water fishes, on the other hand, have internal concentrations greater

than that of their external medium. Thus, they are hyperosmotic to the medium.

Therefore, the osmotic problems and the means to solve them differ drastically

among fishes of different habitats.

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 Dr. R. Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan
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Osmoregulation in Freshwater Fishes

Water Balance:

 The osmotic pressure of body fluids depends on mineral and organic

compound content. The osmotic pressure of body fluids in fresh water fishes

is always higher than that of the surrounding water (hyperosmotic) and the

later diffuses into the body through oral membranes, gills and even intestinal

surfaces.

 In certain species of fish water may enter through the skin also. To counter

the continuous inflow of water through gills, a large amount of hypotonic

urine is produced by the freshwater fishes in general.

 The freshwater fishes possess more glomeruli (even more than 10000 in

number in the kidney). The kidney is also larger in size and is well

vascularized. Water excretion is the main function of the kidneys in these

fishes but small quantity of nitrogenous compounds, containing creatine,

creatinine, amino acids, ammonia and urea are also present in the urine.

 A freshwater teleost does not drink water as large amount of water enters the

body by osmosis and is more than necessary for the fish.

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 Dr. R. Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan
_________________________________________________________________________________

Salt Balance:

 As the osmotic concentration of the body fluids in freshwater fishes is

always higher than the surrounding water, some salts are lost into the water

by the process of diffusion which takes place mainly through surface tissues

like buccal epithelium, gills and skin. Some salts may be lost in the faeces

and urine also.

 The quantity of salts lost per day varies in different species of fishes. In

salmon loss of salts may be upto 17% of the body chlorides while in gold

fish, Carassius auratus, it is only 5%. In the proximal and distal part of the

renal tubule active reabsorption takes place and as such the loss of salt is

minimum in freshwater fishes.

 Salts are replaced by two ways: firstly through food and secondly by the

absorption of salt ions from the surrounding water. Absorption of salt ions

from the water takes place mainly through gills and oral membrane. The ions

of Sodium (Na+ ), Calcium (Ca++), Lithium (Li+ ), Cobalt (Co++),

Strontium (Sr++), Bromine (Br- ), Chlorine (Cl- ), Acid phosphate and

Sulphate are absorbed by chloride cells located at the base of the gill

lamellae. The entire mechanism of salt balance is under endocrine control.

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 Dr. R. Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan
_________________________________________________________________________________

Osmoregulation in Marine Fishes:

 In contrast to freshwater environment marine fishes live in the sea where salt

concentration is higher than that of the body fluids of the fish. Naturally,

water is lost through the semipermeable membrane by the process of

diffusion.

 Fishes have to drink sea water in order to make up the loss and thus salt

content of the body increases. Increase of salt in the blood is then eliminated.

 There is marked reduction in the urine output, for conservation of water and

the uriniferous tubules are modified for this purpose. In marine teleosts,

there is considerable reduction in the size of the kidney tubules. The distal

segment of the convoluted tubule is absent in most of the cases. The neck

segment also is either absent or constricted in certain species.

 Nitrogenous waste products are excreted mainly through the gills in the form

of ammonia. Traces of urea, ammonia and trimethylamine are present in the

urine.

 Water is lost through the gills and other tissues because of lower

concentration of blood in comparison to the sea water. The marine teleosts,

therefore drink water. Water is reabsorbed in the kidney tubules.

 There is physiological control over excess of urine formation and as such

large amount of water does not pass out through the glomeruli.

 In many teleosts the glomeruli become degenerate. Even aglomerular

kidney may be present in some marine teleosts.

 Salt secreting cells are present in the gills and oral membrane to help the

process of excretion because large amount of salt accumulates in the body

tissues because the fishes frequently drink sea water to avoid dehydration.

 The Hagfishes are isosmotic to their surroundings, so that there is practically

no movement of water across the semipermeable membranes. The Hagfishes

do not drink sea water and their requirement of water for urine formation is

met from the blood of host.

 The slime produced from their skin is high in Mg++, Ca++ and K+ content.

Their surplus salt is removed through the gills. Monovalent ions reach the

blood stream from the intestine and leave through the gills. But, the divalent

ions remain in the intestine where they combine with oxides and hydroxides.

Thus the insoluble compounds formed in the alkaline medium pass out along

with the faces.

 In Elasmobranchs there is no danger of dehydration because they maintain

their blood at a higher concentration in comparison to surrounding water.

Urine is formed as a result of filtration process in the glomerulus. The urea

present in the urine is reabsorbed by special segments of the kidney tubule.

 Moreover, the gills in elasmobranchs are impermeable to urea and as such,

the urea is retained in the blood in large quantities. Thus osmotic

concentration of the blood is raised and it becomes higher than that of the

sea water. The osmotic balance in these fishes is maintained by urea cycle.

Salts are excreted in the feces and in the urine. Some salts are reabsorbed in

the kidney tubules. Salts are never excreted through gills as there are no

special salt excreting cells. They achieve salt balance by secreting a fluid

containing higher concentration of sodium and chloride ions from the rectal

gland. There is high concentration of urea in the blood of elasmobranchs

living in fresh water.

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 Dr. R. Prasad, Department of Zoology, Eastern Karbi Anglong College, Sarihajan
_________________________________________________________________________________

Endocrine Control of Osmoregulation:

The endocrine glands are responsible for water-salt balance including urine

flow in fishes. The filtering rate of renal corpuscle is affected by hormones.

Diffusion and absorption of the gills have been found to be greatly influenced

by hormones. Cortisol is a sea water adapting hormone in fish and prolactin as

the fresh water adapting hormone. In salmon, growth hormone acts in

cooperation with cortisol to increase sea water tolerance. Cortisol under some

conditions may promote ion uptake and interact with prolactin during

acclimation to fresh water. In some species thyroid hormone support the action

of growth hormone and cortisol in promoting seawater acclimation. In Anguilla,

adrenaline has been shown to have strong vasodilator effect on the gill vessels.

It reduces or completely stops chloride secretion. In salmon, hypothalamus and

gonads have been implicated in the process of mineral balance. Thyroid gland

and suprarenal bodies secrete adrenocortical hormones which control

osmoregulation in fishes.

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