Environments and animals can be classified in terms of salts and water

The salt concentration, or osmolarity, of ocean water is about 1,070 milliosmoles/liter (mosm/l), and fresh water is generally between 1 and 10 mosm/l. Aquatic environments grade continuously from fresh to extremely salty. Consider a place where a river enters the sea through a bay or a marsh. Aquatic environments within that bay or marsh range in os-molarity from that of the fresh water of the river to that of the open sea. Evaporating tide pools can reach an even greater osmolarity than seawater. Animals live in all these environments. Some aquatic species are osmoconformers; they allow the osmolarity of their tissue fluid to equilibrate with their environment. In contrast, osmoregulators maintain the osmolarity of their tissue fluid at a constant level as the environment changes.

osmoconformers. Over a wide range of environmental osmolarities, marine invertebrates simply equilibrate the osmolarity of their tissue fluid with that of the environment. There are limits to osmoconformity, however. No ani mal could survive if its tissue fluid had the osmolarity of fresh water; nor could animals survive with internal osmo-larities as high as those that may be reached in an evaporating tide pool. Such solute concentrations cause proteins to denature.

osmoregulators. All animals have solutes in their tissue fluids. Therefore, in fresh water, osmosis causes water to invade their bodies, so osmoregulation is essential. To osmoregulate in fresh water, animals must excrete water and conserve solutes; hence they produce large amounts of dilute urine.

In salt water, the opposite problem exists: For animals that maintain the osmolarity of their tissue fluids below that of the sea, osmosis causes a loss of water. To osmoregulate in salt water, animals must conserve water and excrete salts; thus they tend to produce small amounts of urine, and they have means of excreting salts.

Even animals that osmoconform over a wide range of environmental osmolarities must osmoregulate in extreme environments. The brine shrimp Artemia (Figure 51.1) can live in environments of almost any salinity. Artemia are found in huge numbers in the most saline environments known, such as Great Salt Lake in Utah and coastal evaporation ponds where salt is concentrated for commercial purposes (see Figure 27.20). The osmolarity of such water reaches 2,500 mosm/l. At these high environmental osmolarities, Artemia is capable of maintaining its tissue fluid osmolarity considerably below that of the environment, and therefore acts as a hypoosmotic regulator. Very few organisms can survive in the

A brine shrimp in dilute seawater actively transports ions into its body to keep the osmolarity of its tissue fluid above that of the environment.

.but over a wide range of seawater concentrations, it allows the osmolarity of its tissue fluid to equilibrate with the environment.

Hypoosmotic regulation

A brine shrimp in dilute seawater actively transports ions into its body to keep the osmolarity of its tissue fluid above that of the environment.

.but over a wide range of seawater concentrations, it allows the osmolarity of its tissue fluid to equilibrate with the environment.

Hypoosmotic regulation

Osmolarity Plasma

In highly saline water the brine shrimp actively transports ions out of its body to maintain the osmolarity of its tissue fluid below that of the environment.

Osmolarity of environment crystallizing brine in which Artemia thrives. The main mechanism this small crustacean uses for hypoosmoregulation is the active transport of Cl- from its tissue fluid out across its gill membranes to the environment. Na+ ions follow.

Artemia cannot survive in fresh water, but it can live in dilute seawater, in which it maintains the osmolarity of its tissue fluid above that of the environment. Under these conditions, Artemia behaves as a hyperosmotic regulator; that is, it maintains the osmolarity of its tissue fluid above the osmo-larity of the environment. It achieves this by reversing the direction of Cl- ion transport across its gill membranes.

ionic conformers and regulators. Osmoconformers can also be ionic conformers, allowing the ionic composition, as well as the osmolarity, of their tissue fluid to match that of the environment. Most osmoconformers, however, are ionic regulators to some degree: They employ active transport mechanisms to maintain specific ions in their tissue fluid at concentrations different from those in the environment.

The terrestrial environment presents problems of os-moregulation and ionic regulation that are entirely different from those faced by aquatic organisms. Because the terrestrial environment is extremely desiccating (drying), most terrestrial animals must conserve water. (Exceptions are animals such as muskrats and beavers that spend most of their time in fresh water.)

Terrestrial animals obtain their salts from food. But plants generally have low concentrations of sodium, so most herbivores must conserve sodium ions or obtain them elsewhere. Some terrestrial herbivores travel long distances to naturally occurring salt licks. By contrast, birds that feed on marine animals must excrete the excess of sodium they ingest with their food. Their nasal salt glands excrete a concentrated solution of NaCl via a duct that empties into the nasal cavity. Birds, such as penguins and seagulls, that have nasal salt glands can be seen frequently sneezing or shaking their heads to get rid of the very salty droplets excreted from their nasal salt glands (Figure 51.2).

In highly saline water the brine shrimp actively transports ions out of its body to maintain the osmolarity of its tissue fluid below that of the environment.

Osmolarity of environment

51.1 Environments Can Vary Greatly in Salt Concentration Animals such as the brine shrimp that live at the extremes of environmental osmolarities display flexible osmoregulatory abilities.They become hyperosmotic regulators in very dilute water and hypoosmotic regulators in very saline water.

Salt glands are located in depressions in the skull above the eyes.

The functional unit of a salt gland consists of secretory tubules that drain into a central canal.

Skull

Na+ ions are carried from the blood to secretory tubules via active transport. Cl- ions follow. The central canal carries salt solution to the nasal cavity.

Salt glands are located in depressions in the skull above the eyes.

The functional unit of a salt gland consists of secretory tubules that drain into a central canal.

Skull

Na+ ions are carried from the blood to secretory tubules via active transport. Cl- ions follow. The central canal carries salt solution to the nasal cavity.

Salt Gland Marine Birds
51.2 Nasal Salt Glands Excrete Excess Salt (a) Marine birds have nasal salt glands adapted to excrete the excess salt from the seawater they consume with their food. (b) This giant petrel has returned from a feeding trip at sea and is excreting salt through its nasal salt gland.

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Responses

  • Valentina
    How shrimps maintain body fluids hyposmotic to the environment?
    8 years ago
  • aino
    What molarity salt do brine shrimp need?
    8 years ago
  • larry
    What is the highest osmolarity that fresh water organism can survive in?
    7 years ago
  • ABBEY
    What osmolarity problem would a marine animal have in fresh water?
    5 years ago
  • Dion
    How environment can be classified?
    4 years ago
  • thomas
    Which animal inhibits metahaline environment of salt pens?
    2 years ago
  • simon ali
    Which animal inhabits metahaline environment of salt pens?
    2 years ago
  • ALLAN FINDLAY
    How terrestrial organism maintain osmorarity?
    2 years ago

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