Return of Fishes to the Sea
Return of Fishes to the Sea
Marine bony fishes maintain the salt concentration of their body fluids at approximately one-third that of seawater (body fluids = 0.3 to 0.4 gram mole per liter [M]; seawater = 1 M). They are hypoosmotic regulators because they maintain their body fluids at a lower concentration (hence hypo-) than their seawater environment. Bony fishes living in the oceans today are descendants of earlier freshwater bony fishes that moved back into the sea during the Triassic period approximately 200 million years ago. During many millions of years that freshwater fishes were adapting them-selves so well to their environment, they established an ionic concentration in the body fluid equivalent to approximately one-third that of seawater. The body fluid of terrestrial vertebrates is remarkably similar to that of dilute seawater too, a fact that is undoubtedly related to their ancient marine heritage.
By expressing concentration of salt in seawater or body fluids in molarity,we are saying that the osmotic strength is equivalent to the molar concentration of an ideal solute having the same osmotic strength. In fact, seawater and animal body fluids are not ideal solutions because they contain electrolytes that dissociate in solution. A 1 M solution of sodium chloride (which dissociates in solution) has a much greater osmotic strength than a 1 M solution of glucose, an ideal solute (nonelectrolyte) that does not dissociate in solution. Consequently, biologists usually express osmotic strength of a biological solution in osmolarity rather than molarity. A 1 osmolar solution exerts the same osmotic pressure as a 1 M solution of a nonelectrolyte.
When some freshwater bony fishes of the Triassic period ventured back to the sea, they encountered a new set of problems. Having a much lower internal osmotic concentration than the seawater around them, they lost water and gained salt. Indeed a marine bony fish literally risks drying out, much like a desert mammal deprived of water.
To compensate for water loss a marine fish drinks seawater (Figure 32-3). This seawater is absorbed from the intestine, and the major sea salt, sodium chloride, is carried by the blood to the gills, where specialized salt-secreting cells transport it back into the surrounding sea. Ions remaining in the intestinal residue, especially magnesium, sulfate, and calcium, are voided with the feces or excreted by the kidney. In this indirect way, marine fishes rid themselves of the excess sea salts they have drunk, and replace water lost by osmosis. Samuel Taylor Coleridge’s ancient mariner, surrounded by “water, water, everywhere, nor any drop to drink” undoubtedly would have been tormented even more had he known of the marine fishes’ ingenious solution for thirst. A marine fish regulates the amount of seawater it drinks, consuming only enough to replace water loss and no more.
The cartilaginous sharks and rays (elasmobranchs) achieve osmotic balance differently. This group is almost totally marine. The salt composition of shark’s blood is similar to that of the bony fishes, but the blood also carries a large content of organic compounds, especially urea and trimethylamine oxide. Urea is a metabolic waste that most animals quickly excrete. The shark kidney, however, conserves urea, allowing it to accumulate in the blood and raising the blood osmolarity to equal or slightly exceed that of seawater. With osmotic difference between blood and seawater eliminated, water balance is not a problem for sharks and their kin; they are in osmotic equilibrium with their environment.
The high concentration of urea in the blood of sharks and rays—more than 100 times as high as in mammals—could not be tolerated by most other vertebrates. In the latter, such high concentrations of urea disrupt peptide bonds of proteins, altering protein configuration. Sharks have adapted biochemically to the presence of urea that permeates all their body fluids, even penetrating freely into cells. So accommodated are elasmobranchs to urea that their tissues cannot function without it, and their heart will stop beating in its absence.
Marine bony fishes maintain the salt concentration of their body fluids at approximately one-third that of seawater (body fluids = 0.3 to 0.4 gram mole per liter [M]; seawater = 1 M). They are hypoosmotic regulators because they maintain their body fluids at a lower concentration (hence hypo-) than their seawater environment. Bony fishes living in the oceans today are descendants of earlier freshwater bony fishes that moved back into the sea during the Triassic period approximately 200 million years ago. During many millions of years that freshwater fishes were adapting them-selves so well to their environment, they established an ionic concentration in the body fluid equivalent to approximately one-third that of seawater. The body fluid of terrestrial vertebrates is remarkably similar to that of dilute seawater too, a fact that is undoubtedly related to their ancient marine heritage.
By expressing concentration of salt in seawater or body fluids in molarity,we are saying that the osmotic strength is equivalent to the molar concentration of an ideal solute having the same osmotic strength. In fact, seawater and animal body fluids are not ideal solutions because they contain electrolytes that dissociate in solution. A 1 M solution of sodium chloride (which dissociates in solution) has a much greater osmotic strength than a 1 M solution of glucose, an ideal solute (nonelectrolyte) that does not dissociate in solution. Consequently, biologists usually express osmotic strength of a biological solution in osmolarity rather than molarity. A 1 osmolar solution exerts the same osmotic pressure as a 1 M solution of a nonelectrolyte.
When some freshwater bony fishes of the Triassic period ventured back to the sea, they encountered a new set of problems. Having a much lower internal osmotic concentration than the seawater around them, they lost water and gained salt. Indeed a marine bony fish literally risks drying out, much like a desert mammal deprived of water.
To compensate for water loss a marine fish drinks seawater (Figure 32-3). This seawater is absorbed from the intestine, and the major sea salt, sodium chloride, is carried by the blood to the gills, where specialized salt-secreting cells transport it back into the surrounding sea. Ions remaining in the intestinal residue, especially magnesium, sulfate, and calcium, are voided with the feces or excreted by the kidney. In this indirect way, marine fishes rid themselves of the excess sea salts they have drunk, and replace water lost by osmosis. Samuel Taylor Coleridge’s ancient mariner, surrounded by “water, water, everywhere, nor any drop to drink” undoubtedly would have been tormented even more had he known of the marine fishes’ ingenious solution for thirst. A marine fish regulates the amount of seawater it drinks, consuming only enough to replace water loss and no more.
The cartilaginous sharks and rays (elasmobranchs) achieve osmotic balance differently. This group is almost totally marine. The salt composition of shark’s blood is similar to that of the bony fishes, but the blood also carries a large content of organic compounds, especially urea and trimethylamine oxide. Urea is a metabolic waste that most animals quickly excrete. The shark kidney, however, conserves urea, allowing it to accumulate in the blood and raising the blood osmolarity to equal or slightly exceed that of seawater. With osmotic difference between blood and seawater eliminated, water balance is not a problem for sharks and their kin; they are in osmotic equilibrium with their environment.
The high concentration of urea in the blood of sharks and rays—more than 100 times as high as in mammals—could not be tolerated by most other vertebrates. In the latter, such high concentrations of urea disrupt peptide bonds of proteins, altering protein configuration. Sharks have adapted biochemically to the presence of urea that permeates all their body fluids, even penetrating freely into cells. So accommodated are elasmobranchs to urea that their tissues cannot function without it, and their heart will stop beating in its absence.
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