How do fish survive in salt water?

[DCnDC]

A question for any ichthyologists among us.

I was curious about how ocean mammals like whales, dolphins, seals, etc. survived in salt water and my research yielded the answer that they extract their water from the food they eat (fish). That’s all well and good, but leads to the question of how do fish survive in salt water? Fish are animals, and being animals I would imagine they would have the same limitations of salt intake as mammals, so how do fish deal with the salt water problem?

[Anaglyph]

They use metabolic energy (ATP) to actively excrete salt through the gills and kidneys

[mutantmoose]

This is interesting. When we left the oceans we caried the seawater with us in the form of our blood. But blood is only one quarter as salty as the sea implying that we left the oceans when the sea was one quarter as salty as it is now.

http://www.ca.uky.edu/wkrec/vertebratefishevolution.pdf

It is believed that when the newly formed planet Earth cooled sufficiently, rain began to fall continuously. This rainfall filled the first oceans with fresh water. 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 percent salt solution). Almost 77 percent 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 tissue. 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 oceans.

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 freshwater and absorbing salts from food. But fish stayed in the aquatic environment. To adapt, they had to either remain in low salinity environments, such as bays and estuaries, or they had to evolve mechanisms to replace water lost through osmosis to the seawater and to remove salts absorbed from the increasingly saline oceans. To inhabit fresh water, fish had to replace salts lost through diffusion to the water and eliminate excess water absorbed from the environment. Kidney function had to be altered accordingly for fish to survive in these different habitats. Eventually, the gills developed the ability to excrete salts in seawater and absorb salts from fresh water.

[mutantmoose]

Hmm…

http://www.seashellsandsuch.com/articles/seawaterlikeblood.php

[Seawater has] even been used in transfusions in emergency situations when blood or plasma wasn’t available and the patient needed to have his blood volume maintained.

Sea water is remarkably similar to blood in many ways. When comparing the concentrations of various elements in seawater and blood, we find that the two most common elements in each are Chlorine (Cl) and then Sodium (Na). Together they make Sodium Chloride (NaCl), or common table salt. The concentrations are higher in seawater than in blood, though. When it’s been used for transfusions seawater needs to be diluted in fresh water first.

Experiments were run on dogs in the early 1900s to test diluted, filtered ocean water as a substitute transfusion liquid. An early experimenter named René Quinton and his team drained all of a dog’s blood and replaced it with the seawater solution. The dog lived, and on the second day after the transfusion, half of its normal blood components had returned, regenerated by its body. By the fourth day its blood was back to virtually normal and the dog was active and full of energy and happy. The dog went on to live for many years afterward, with no visible problems as a result of the experiment.

[Smeghead]

That shouldn’t be terribly surprising. Saline solution, which is constantly pumped into people in hospitals, is basically just salt water.

[CookingWithGas]

@mutantmoose

This is interesting. When we left the oceans we caried the seawater with us in the form of our blood. But blood is only one quarter as salty as the sea implying that we left the oceans when the sea was one quarter as salty as it is now.

http://www.ca.uky.edu/wkrec/vertebratefishevolution.pdf

[relevant part of quote extracted by CWG]…The similarities between the salt content of vertebrate blood and dilute seawater suggest a strong evolutionary relationship among vertebrates and with the primordial oceans.

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 freshwater and absorbing salts from food.

I have no qualifications in this subject area but I find this explanation highly speculative at best. One could also argue that the proportions of these minerals are similar in animals because those are the proportions required for these life forms, and the fact that they are similar to the proportions found in the oceans and on land are what made it possible for life to arise. One could also argue that it’s coincidence.

The argument breaks down with the explanation that sea animals had to evolve to excrete the excess salt. If we follow the original argument, what we should be seeing instead is animals evolved have higher concentrations of salt, to reflect their environment.

I note that the referenced paper has no cites whatsoever, nor describes any original research by the author.

[Anaglyph]

The argument breaks down with the explanation that sea animals had to evolve to excrete the excess salt. If we follow the original argument, what we should be seeing instead is animals evolved have higher concentrations of salt, to reflect their environment.

Some marine animals (osmoconformers) such as starfish do indeed have the same internal salinity as sea water. However, this makes them very sensitive to changes in salinity, as their tissue cell organelles swell or shrink, and their cell membranes get damaged if the salinity changes. This is why echinoderms are not found in estuaries, or river mouths where fresh and salt water meet and the salinity fluctuates greatly.

[CC]

In the 1950’s, the Bell Labs produced (using Frank Capra, by the way) a series of hour long tv programs on science. One of them, Hemo The Magnificent, was written around the notion (maybe just receiving currency) that “in the beginning, blood WAS sea water.” While the movie, still available on DVD, is extremely dated in some ways, the explanation of the connection between life on earth and the chemistry of sea water still holds, as does the fabulous, and elegantly simple, animation of the structure and function of the circulatory system. For anyone who teaches and who still has the latitude to show a movie once in a while, this is a must-see for kiddos. Still.

[glowacks]

In my 9th grade biology class, the teacher said that salt water fish “pissed like a race horse”. Never heard that turn of phrase before, or since, but it’s stuck with me. I’m guessing the was that they were able to filter out the salt and excrete liquid saltier than they took in.

[njtt]

As fish almost certainly first evolved in the sea, a more pertinent question would be “How do (some) fish survive in fresh water?”

[CookingWithGas]

@glowacks

In my 9th grade biology class, the teacher said that salt water fish “pissed like a race horse”. Never heard that turn of phrase before, or since, but it’s stuck with me. I’m guessing the was that they were able to filter out the salt and excrete liquid saltier than they took in.

I’ve heard it lots of times but never about a fish…

[Hi_Medlo]

If I remember some of my college biology courses from long, long ago correctly, njtt is right. The real feat is surviving in fresh water. Freshwater fish have a more advanced ability to regulate their internal salinity that marine organisms do.