MSF 326/MSM 315 (FISH PHYSIOLOGY AND ANATOMY)

OSMOREGULATION IN AQUATIC ANIMALS

Osmoregulation is the process of regulation of osmotic (dissolved solute) concentration and water

in animals. This unit describes the need for osmoregulation, the physiological mechanisms

involved and the organs used for osmoregulatory responses among animals.

THE NEED FOR OSMOREGULATION IN ANIMALS

Water is a vital composition of an animal’s body, required for the maintenance of life and other

metabolic processes. It forms the primary medium as well as the most essential nutrients in all

animals. Water accounts for between 60% and 95% of the animal’s body weight. The water

within animals may be inside cells in intracellular fluid (ICF) or it may be outside cells in the

extracellular fluid (ECF). The ECF itself may be distributed between several smaller

compartments, such as blood plasma and cerebrospinal fluid. Dissolved in these fluids are variety

of solutes in form of ions and nutrients. Animals need to maintain appropriate and correct

amounts of water and solutes in their various fluid compartments. The ability to regulate water

and solute concentrations in animals is referred to as osmoregulation. Osmoregulation and

excretion are intimately linked together in animals as most animals utilize their excretory organs

for osmoregulatory functions.

THE PRINCIPLE OF OSMOSIS

Osmosis involves the movement of water across a selectively permeable membrane which

separates two solutions, from a region high concentration (i.e. a dilute solution) to a region of

lower concentration (i.e. a concentrated solution). When two aqueous solutions of different solute

concentrations are separated by a membrane permeable to water but impermeable to solute

molecules, water diffuses through the membrane from the solution. This process will continue

until equilibrium is established, at which point there is no further net movement of water and the

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concentrations of solution on either side of the selectively permeable membrane are equal. A

selectively permeable membrane is one which allows only water to pass through it and no other

substances, for example, solutes dissolved in the water

OSMOTIC RESPONSES OF ANIMALS

Animals may be classified into two broad categories on the basis of their osmotic responses- they

are either osmoconformers or osmoregulators.

Osmoconformers

Osmoconformers are animals whose body fluid concentration is exactly the same as that of the

immediate environment in which they live. Typical osmoconformers include marine

invertebrates, whose body fluid concentration is the same as that of salt water. This implies that

the two solutions (body fluid /sea water) are isosmotic. Although these animals may be in osmotic

equilibrium, they do not necessarily have to possess the same composition or be in ionic

equilibrium. In this regard, a great deal of energy is required for ionic regulation. Hence, for

osmoconformers, there is need for a corresponding change in the osmotic concentration of their

body as soon as the external environment changes in its osmotic concentration. Some

osmoconformers may be able to tolerate wide changes in the osmotic concentration of their

immediate environment. These are referred to as being euryhaline. Another group of

osmoconformers are those animals which can only tolerate much smaller changes in the osmotic

concentration of their immediate environment, and they are referred to as being stenohaline.

Osmoregulators

Osmoregulators on the other hand are animals which maintain a body fluid concentration that is

different from that of their immediate environment. If the osmotic concentration of body fluids is

maintained at a concentration greater than that of the immediate environment they are said to be

hyperosmotic regulators (e.g. crabs); if they maintain their body-fluid concentration below that of

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the immediate environment they are said to be hypoosmotic regulators (e.g. some crustaceans).

All terrestrial animals, by the very fact that they live on land are osmo-regulators.

OSMOTIC RESPONSE IN THE MARINE ENVIRONMENT

The marine environment is essentially characterized by high salinity, mineral concentration,

temperature, density, acidity and tidal action. These physical and chemical characteristics remain

fairly constant through the year except in some seasons where there are slight fluctuations. The

animals found in this environment have body fluid concentration similar to the salt water where

they live. They differ from the seawater they inhabit on the basis of their ionic composition.

These organisms overcome their osmotic challenges either as osmoconformers or osmoregulators

MARINE VERTEBRATES

Marine vertebrates show some remarkable differences in their osmotic responses when compared

with the saltwater invertebrates. Marine vertebrates are either osmotic conformers or osmotic

regulators. A typical example of osmoconformers who are in osmotic and ionic equilibrium with

seawater is the hagfish. Hagfish (cysclostomes) are the most primitive vertebrates. They show

some resemblance with the marine invertebrates in their osmotic response. Hagfish utilizes a kind

of osmotic and ionic conformation that has been used as physiological evidence that vertebrates

evolved in the marine environment. The majority of other marine fish, however, show varying

degree of osmotic and ionic regulation. The osmotic concentration of their plasma is

approximately one-third that of seawater, therefore they are hypoosmotic regulators.

Marine Elasmobranchs

The elasmobranchs (the cartilaginous fishes) are very successful osmoregulators, because they

have evolved a novel way of achieving this regulation. Given that their plasma is only one-third

as concentrated as the seawater in which they live, they face two problems – the loss of water and

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the gain of ions. The loss of water is minimized by the animals achieving osmotic equilibrium by

the addition of solutes to their plasma. The solutes added are urea and trimethylamine oxide

(TMAO). Urea is produced as an end-product of protein metabolism, whilst the biosynthesis of

TMAO is less clear. In many cases, more urea and TMAO is added to the plasma than is

necessary to produce osmotic equilibrium, thus making the plasma hyperosmotic to seawater. The

result of this is that the animal gains water across the surface of the gills. Gills are usually made

up of large surface area, thin walled and highly vascularized. They serve as sites for the gain and

loss of water and ions in aquatic animals. This gain and loss of water and ions is advantageous to

elasmobranchs because excess water can be used for the production of urine and the removal of

waste products, such as excess ions that diffuse into the animal which occurs across the gills.

Water gain also means that the animals do not need to drink seawater as a means to overcome

potential water loss, and in avoiding this they avoid ingesting large amounts of salt that is

dissolved in seawater, which would serve to further exacerbate the problems of ionic regulation.

Potentially, the biggest problem with the addition of large amounts of urea to the plasma is that

urea tends to denature and inactivate other plasma proteins. However, these animals have

overcome this problem to such an extent that proteins and enzymes are unable to function

correctly without urea. Similarly, another problem faced by the elasmobranchs is the gain of ions.

This is because their plasma has a different solute composition to saltwater, a concentration

gradient therefore exists that favours the movement of ions into the animals. For instance, there is

a massive influx of Na+ ions across the gills. Elasmobranchs overcome this kind of problem with

a special gland known as rectal gland. The rectal gland helps in the excretion of excess Na+ ions.

It is a specialized gland which opens out into the rectum and secretes a fluid which is rich in

NaCl. The small osmotic influx of water into these animals allow for the production of urine,

which is another route by which excess NaCl may be excreted.

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Marine Teleosts

Marine teleosts (bony fishes) face similar problems as elasmobranchs as their plasma is less

concentrated than seawater. Loss of water, particularly across the gills, is compensated for by

drinking large volumes of seawater. This solves one problem, but exacerbates another by adding a

further salt load to the animal. This means that the animal must somehow excrete large amounts

of NaCl. Since the kidney of telesost fishes is unable to produce concentrated urine, there be some

other organ that is able to excrete large amounts of NaCl. This organ is the gill, which has a dual

function in gas exchange and osmoregulation.

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The gills of marine teleosts contain special cells known as chloride cells which are responsible for

active transport of NaCl from plasma to seawater. Cl- ions are actively extruded form the blood

into the chloride cells, accompanied by the passive diffusion of Na+. Hence, Cl- moves passively

out from the gill into the surrounding seawater.

OSMOTIC RESPONSE IN THE FRESHWATER ENVIRONMENT

The organisms in the freshwater environment are unique because the osmoregulatory problems

faced by freshwater animals are the opposite of those faced by marine animals. Freshwater

animals, by definition, must be hyperosmotic to the water in which they live. It would be

impossible for any animal living in freshwater to be in osmotic and ionic equilibrium with it,

unless the body fluids were made of distilled water. This means they face two problems – they

tend to gain water from their immediate environment by osmosis and lose ions by diffusion due to

the presence of large concentration gradients as only a minimal amount of solutes are dissolved in

freshwater. Animals living in such an environment must be capable of significant osmotic and

ionic regulation.

FRESHWATER VERTEBRATES

Freshwater vertebrates face the same osmotic and ionic problems as freshwater invertebrates.

When considering freshwater vertebrates, it is only necessary to consider the osmotic and ionic

relations of the teleosts - there are very few elasmobranchs that are true freshwater species. Like

invertebrates, the major site of osmotic water gain in teleosts is the gills. The excess water is

removed by the production of large quantities of very dilute urine. Although the urine is dilute, it

does contain some dissolved solutes, and because large volumes of urine are produced, urine

excretion may result in a relatively large loss of ions. This in turn compromises the ion loss which

is already occurring by diffusion from plasma to water. Some loss of ions can be compensated for

by the gain of ions from food. However, the main source of ion gain is by the active transport of

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ions in the gills. It is thought that the transport of ions across the general body surface is

insignificant. The osmotic and ionic relationships of freshwater teleost

SUMMARY

An animal’s environment is constantly changing in its ionic composition, this results in varying

degree of fluctuations which the animal must always overcome. An animal may be either an

osmoregulator or osmoconformers. Osmoconformers are animals whose body fluid concentration

is exactly the same as that of the immediate environment in which they live while osmoregulators

are animals which maintain a body fluid concentration that is different from that of their

immediate environment.

The marine environment is very unique in its physical and chemical properties. The marine

environment is rich in dissolved oxygen, salinity and light penetration. The density of the salt

water is dependent on both temperature and salinity. The marine animals maintain body fluid

concentration similar to the salt water where they live. They differ from the seawater they inhabit

on the basis of their ionic composition. These organisms overcome their osmotic challenges either

as osmoconformers or osmoregulators.

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The osmoregulatory problems faced by freshwater animals are the opposite of those faced by their

marine counterparts. Freshwater animals have body fluids hyperosmotic to their medium. They

gain water from their immediate environment by osmosis and lose ions by diffusion due to the

presence of large concentration gradients as only a minimal amount of solutes are dissolved in

freshwater. Freshwater animals are capable of both ionic and osmotic regulation as no animal in

the freshwater environment is truly in ionic and osmotic equilibrium with the environment. They

conserve salts by producing urine which is generally less concentrated than blood.