World Builders™                                                                    Session Six  --  Microbiology           Blood

        The Gene Pool     

Living things must focus on two tasks, reproduction and energy capture.

    Sea water is saline, about 3.5% by weight of it is salt.  Animal life is rooted in the sea, and all the animals on earth can trace their ancestry back to early marine organisms.  We have evidence of this in fossils, and also in our blood.

     Our blood has salts in it

The various species of fish found in oceans, lakes, rivers and streams have evolved over millions of

years and have adapted to their preferred environments over long periods of time. Fish are

categorized according to their salinity tolerance. Fish that can tolerate only very narrow ranges of

salinity (such freshwater fish as goldfish and such saltwater fish as tuna) are known as stenohaline

species. These fish die in waters having a salinity that differs from that in their natural


Fish that can tolerate a wide range of salinity at some phase in their life-cycle are called euryhaline

species. These fish, which include salmon, eels, red drum, striped bass and flounder, can live or

survive in wide ranges of salinity, varying from fresh to brackish to marine waters. A period of

gradual adjustment or acclimation, though, may be needed for euryhaline fish to tolerate large

changes in salinity.

It is believed that when the newly formed planet Earth cooled sufficiently, rain began to fall

continuously. This rainfall filled the first oceans with freshwater. It was the constant evaporation of

water from the oceans that then condensed to cause rainfall on the land masses, which in turn,

caused the oceans to become salty over several billion years. As rain water washed over and

through the soil, it dissolved many minerals--sodium, potassium and calcium-- and carried them

back to the oceans.

Vertebrate animals (fish, birds, mammals, amphibians and reptiles) have a unique and common

characteristic. The salt content of their blood is virtually identical. Vertebrate blood has a salinity of

approximately 9 grams per liter (a 0.9 % salt solution). Almost 77 % of the salts in blood are

sodium and chloride. The remainder is made up primarily of bicarbonate, potassium and calcium.

Sodium, potassium and calcium salts are critical for the normal function of heart, nerve and muscle


If the salinity of ocean water is diluted to approximately one quarter of its normal concentration, it

has almost the same salinity as fish blood and contains similar proportions of sodium, potassium,

calcium and chloride. The similarities between the salt content of vertebrate blood and dilute

seawater suggest a strong evolutionary relationship among vertebrates and with the primordial


Indeed, it seems likely that vertebrate life evolved when the oceans were approximately one quarter

as salty as they are today. As the oceans became saltier and vertebrates evolved further, several

groups of vertebrates (birds, mammals, reptiles and amphibians) left the oceans to inhabit the land

masses, carrying the seawater with them as their blood. They maintained their blood salt

concentrations by drinking fresh water and absorbing salts from food.


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Wurts. William A.  (no date)
Why can some fish live in fresh water, some in salt water, and some in both?
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