Reef Chemistry Question of the Day #230 Osmoregulation by Marine Fish

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #230

Marine fish have a big problem. Their blood is less salty (lower osmotic concentration) than seawater, but their gills are fairly permeable to water. Consequently, water moves through the gills from the blood to the higher saltiness seawater, and the fish dehydrate.

Marine fish have a large number of ways of dealing with this issue.

Which of the following is NOT a PORTION of the process whereby fish prevent dehydration?

A, Fish drink seawater

B. Fish raise the alkalinity of the water in their GI tract and precipitate calcium and magnesium carbonate, reducing the osmotic concentration of the fluid, allowing the fish to more easily take up pure water from it.

C. Fish lower the pH of the water in their GI tract, reducing the osmotic concentration of the fluid, allowing the fish to more easily take up pure water from it.

D. Fish have a single transporter in some of the cells that line their GI tract that takes up a sodium ion, a potassium ion, and two chloride ions, reducing the osmotic concentration of the intestinal fluid, allowing the fish to more easily take up pure water from it.

 

[jsker]

I have no idea, but very interesting.

 

[Randy Holmes-Farley]

I know this is a hard one. I had no idea until I looked into it a bit.

 

[ndrwater]

B. I think fish precipitating Calcium and Magnesium would be counted productive to their biological function.
But that's just a guess...

 

[MnFish1]

B ( I think it at the opposite)

Edited - I meant C - I think it is the opposite they raise alkalinity/pH

 

[TherealplexiG]

It's A
Fish doesn't drink seawater.

 

[TherealplexiG]

@Randy Holmes-Farley
Answer it please?

 

[Randy Holmes-Farley]

lol OK.

 

[Randy Holmes-Farley]

And the answer is...

Marine fish have a big problem. Their blood is less salty (lower osmotic concentration) than seawater, but their gills are fairly permeable to water. Consequently, water moves through the gills from the blood to the higher saltiness seawater, and the fish dehydrate.

Marine fish have a large number of ways of dealing with this issue.

Which of the following is NOT a PORTION of the process whereby fish prevent dehydration?


C. Fish lower the pH of the water in their GI tract, reducing the osmotic concentration of the fluid, allowing the fish to more easily take up pure water from it.


One of the fundamental processes is that fish take in somewhat salty water from their GI tract, and then excrete some of the salts elsewhere in their bodies (such as gills).

For this to happen, they first:

1. Drink seawater (so answer A is wrong).

2. Remarkably, the do pump alkalinity into their lower GI tract, precipitating calcium and magnesium, and reducing the osmitic concentration, making it easier to absorb some of the water. (so answer B is wrong)

3. The have a transporter in their GI tract that does take up several ions (a sodium ion, a potassium ion, and two chloride ions), reducing the osmotic concentration of the intestinal fluid. Why would they do this? It decreases the osmotic concentration further, to the point where water flows on its own through the cell membranes lining the GI tract and into the blood. The ions are then dumped out elsewhere (such as the gills). Why go to all thetrouble to basically take in salt water? Because they can control the process, where a big pore wouldn't be easy to control.

There are potentially many other transporters and processes involved, some of which vary by species, but answer C involving acidification of the stomach is not one of them. In that process, a potassium ion is taken up and swapped for a proton that is pumped into the stomach (people do this too). That process has no net impact on the osmotic concentration or the uptake of water, since the total ions in the GI tract are unchanged.

This article has some extensive discussion of this complicated process:

http://agrifs.ir/sites/default/files/Fish_Osmoregulation.pdf#page=346

and


http://jeb.biologists.org/content/209/15/2813

from it:

"The formation of carbonate precipitates, which is a direct consequence of the high HCO3- concentration and alkaline conditions, account for the majority of the Ca2+ lost from the intestinal fluids and has important consequences for the osmotic pressure of the intestinal fluid. An estimated reduction of ∼70 mOsm resulting from the precipitation of Ca2+ and CO32- in the intestinal fluids (Wilson et al., 2002) is an obvious benefit for intestinal fluid absorption and has also been demonstrated to be important for Ca2+ homeostasis "