Freshwater V Salt Water

[andywg]

OK, I came up with this idea, so I thought I should post

Here is an extension of a topic currently going on in the marine section.

Are marine fish really more susceptible to pollutants, or imperfections, in their water?

We are often told (especially anyone moving from FW to SW) that SW fish are more delicate and need better water conditions than FW due to their osmoregulatory functions making them take in vast amounts of water. I am less sure.

First up I shall provide the background information for anyone not already comfortable with it, or perhaps unsure of the exact ascience of it. The bulk of the information has been gleaned from Fishes: An Introduction to Ichtyhology 5th Edition, by Joseph J Cech Jr and Peter B Moyle (in particular pages 89-99)

Osmoregulation in Marine Teleosts

Marine teleosts maintain a total ionic concentration in the plasma of approximately one third that of sea water. Most of the ions the fish will want are contained in sea water, so the principal feature of osmoregulation is actively to secrete sodium and chloride based ions into the water while "drinking" (ingesting into the stomach) large amounts of sea water to counteract the continual loss of water from the fish's blood at the gills.

The fish have special chloride cells in the gill filament and opercular skin epithelia to eliminate much of the excess salts through the aforementioned active transport. Marine teleost kidneys cannot produce a urine more salty than the blood (Schmidt-Nielson, 1975) and as such cannot utilise the marine elasmobranch method of osmoregulation which involve retaining a high concentration of organic salts to prevent the inorganic salts flooding the body. These organic salts are primarily urea and secondarily trimethylamine oxide at a ratio of 2:1.

Osmoregulation in Freshwater Teleosts

Freshwater teleosts tend to maintain an internal level of salts between a quarter and a third of the salt concentration of salt water (so less than marine fish, but not by a huge difference). However, the water they live in tends to have a Total salts level of 1-10 mOsm/L instead of sea water with around 1,000.

As can be seen, the problem for freshwater teleosts is not retaining water, but rather retaining salts and preventing the water entering the body from pushing out the salts to a point where the fish can no longer operate at a cellular level (internal drowning).

Freshwater telosts manage this through producing continual low salt dilution urine which is nigh on constantly being passed by the fish. The active transport in freshwater teleosts is in uptake of salts, rather than the expulsion seen in marine teleosts.


The General Statement

As mentioned above, we are often told that marine teleosts actually drink water, rather than freshwater teleosts which are fighting to retain salts and resist water "flooding" their bodies. Therefore it is supposed that the marine water are retaining their water more and thus will be affected more by nitrates/ammonia etc.

My problem with this is that both types of fish are taking on large amounts of water. Marine teleosts are taking the water in through the mouth and into the stomach, but losing it in large quantities at the gills. Freshwater teleosts are taking water in at the gill and passing it out through urine.

Thoughts

The marine fish are mostly just wanting to extract the water from sea water and pass many of the dissolved salts out at the gills where possible. The freshwater teleosts are taking the water in at the gills and trying to retain the dissolved salts and pass the water out.

Surely such a path could conceivably lead to the freshwater teleost maintaining any pollutant dissolved in the water while the marine teleosts could be passing the dissolved problems out at the gills?

Would it be more correct to say that both types of fish are equally susceptible to the pollutants, as both have large amounts of the water flowing in and out of the body at all times, just by different processes?

I would be interested to see if there are any papers that discuss the relative toxicity of compounds to FW vs SW fish. I can't help but feel that the end result will be that in a closed loop system such as an aquarium they will all be pretty equally toxic, but I would like to hear, nonetheless.


Secondary Point

Tommy Gunn came up with an interesting point on the potential toxicity of nitrite in relation to a FW/SW combination:

To the best of my understanding right now, nitrites are toxic to freshwater fish because they bond with the hemoglobin in the blood which inhibits the amount of oxygen the blood can carry and one method of helping defeat that is to add some salt to the water since it inhibits the ability of nitrite to bond with the blood....so what is the case in saltwater fish? I understand that there are huge differences between the salt being suggested for a FW tank (NaCl) and marine salt, but I am curious to know if nitrites play a different role in SW or if that remains the same.

Now the main difference between "FW" or "Aquarium" salt and marine salt is that marine salt has the 70+ other chemicals and salts found in sea water, but is there any difference?

 

[Tommy Gun]

Ok Andy. Let me go back over what I have found so far, and the assumptions I am making based upon them.

*Please let me preface the following by saying that, in order to save time, I will skip paraphrasing by copying and pasting directly from resources, but will certainly note when I have done so.

Are Saltwater fish more sensitive to water quality than FW fish?

IMHO yes, saltwater fish are more sensitive to water quality than FW fish because the role that osmosis plays in each type of fish.

Key Terms and Definitions

****Osmosis****
According to an on line resource designed as a teaching aid for high school level biology students, the definition of osmosis contains three main parts:

1. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
   
2. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
   
3. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

(Gondar Design Biology, 2000)​

While this science can apply to other items (i.e. the semi-permeable membrane in an RO unit), for this specific topic, it will most focus on osmosis as it related to animal (fish) cells.

That same website also points out this...

"Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through" (Gondar Design Biology, 2000).

...which if nothing else brings some support to the notion that an ammonia molecule can pass through a cell.

****Osmoregulation****

According to K. O'Driscoll et. al., apparent 'experts' on the subject of osmoregulation and excretion who have provided this same information during presentations for college students in at least a few different colleges, osmoregulation can be defined as:

"Osmoregulation is the regulation of water and ion concentrations in the body. Keeping this regulation precise is critical in maintaining life in a cell. Balance of water and ions is partly linked to excretion, the removal of metabolic wastes from the body" (K. O'Discoll, et. al., n.d.)

In my mind, the important part of this definition is that Osmoregulation pertains to the processes of maintaining life in a cell - which I realize can be argued to imply an entire animal since it is basically a collection of cells, but in this case, I would like to stick to the cellular function and how it pertains to water quality.

Hypertonic

#1) "having a higher osmotic pressure than a surrounding medium or a fluid under comparison" (www.m-w.com, keyword: hypertonic)

#2) "If the concentration of salts and other solutes in the fish is greater than the outside, watery world, the fish takes on water (natural attempt to 'dilute the fish to what it's like outside' - bad for the fish). In scientific terms the fish is hypertonic to its surroundings (freshwater fish, e.g.). " (Agro, J., 11/19/96)

Hypotonic

#1) "having a lower osmotic pressure than a surrounding medium or a fluid under comparison" (www.m-w.com, Keyword: hypotonic)

#2) "If a fish is hypotonic to its environment (marine fish), then the seawater has a higher solute concentration than the fish; the fish loses water to its surroundings (also bad for the fish)." (Agro, J., 11/19/96)

Other tidbits of useful knowledge

1. "Animal cells are in danger of swelling and bursting if they are placed in a solution which is too dilute" (Gondar Design Biology, 2000)

2. "Waste products generated in metabolic processes are often toxic, and therefore must be eliminated before they can harm the organism. The major metabolic wastes produced by animals include carbon dioxide, metabolic water, and nitrogenous wastes. Small aquatic organisms are able to get rid of wastes by simple diffusion across membranes. More complex animals with circulatory systems rely on kidneys to filter wastes out of the blood and eliminate them from the body" (K. O'Discoll, et. al., n.d.)

3. "Carbon dioxide and metabolic water produced in respiration easily diffuse into the environment from respiratory surfaces. Nitrogenous waste excretion is more difficult, yet necessary. " (K. O'Discoll, et. al., n.d.)

The "heart" of my argument that osmosis plays some role in sensitivity to water in SW fishes

First, let me point out that this conversation came about during a discussion which focused upon what would really constitute overstocking in a saltwater tank as well as the reasoning behind most 'inch per gallon' guidelines suggesting that we should stock our marine tanks with around 1/2 as many inches of fish per gallon versus a freshwater tank. (Legal Disclaimer: I don't agree with any 'per gallon' guideline or rule; however, that is a different topic).

My contention here is based upon the fact that, as already alluded to, because freshwater fish live within a hypertonic situation (meaning, they are more salty that the surrounding water, in general), they are required to severely limit the amount of water within their bodies due to the rules of osmosis = because there is a higher content of salt within the cell than within the water, osmotic pressure if formed which tries to balance that situation until it is equal on both sides of the cell (in and out), thus removing some salt from the cell. Conversely, since marine fish live within an hypotonic situation (meaning the water is more salty than the fish), they need to utilize more of the surrounding water within their bodies due to the rule of osmosis = because there is a lower content of salt within the cell than in within the water, osmotic pressure if formed which tries to balance that situation until is it equal on both sides of the cell.

The main problem = should a cell become too dilute, or too salty, it is harmed, or killed; hence fish are in a particularly constant struggle due to the fact that they live within water.

======================================================================

Freshwater teleosts tend to maintain an internal level of salts between a quarter and a third of the salt concentration of salt water (so less than marine fish, but not by a huge difference).

Even though I realize that the sentence after this one explains the same thing, because I am assuming that the comment in quotes is your own, I would point out that this similar salt content within the body, if true, shows why each type of fish (FW and SW) approach osmosis differently: one needs to keep water out of its cells in order to keep the salt within (because the water is less salty) and the other needs to bring in more water to dilute the over abundance of salt within their body by flushing larger amounts of water within themselves.

As can be seen, the problem for freshwater teleosts is not retaining water, but rather retaining salts and preventing the water entering the body from pushing out the salts to a point where the fish can no longer operate at a cellular level (internal drowning).

Exactly....which is why they need to keep the majority of the water they live in outside of their body (read: cells); hence, while they may be passing water over their gills in order to remove dissolved oxygen from it, most of that water is expelled (water moves in the mouth and out the gill flaps, not in the mouth, through the stomach, into the bloodstream, and then out). This is not to say that they do not contain water within their bodies, but to a much lower extent, or at least for a much short period of time than in marine fish.

Marine teleosts maintain a total ionic concentration in the plasma of approximately one third that of sea water. Most of the ions the fish will want are contained in sea water, so the principal feature of osmoregulation is actively to secrete sodium and chloride based ions into the water while "drinking" (ingesting into the stomach) large amounts of sea water to counteract the continual loss of water from the fish's blood at the gills.

Exactly....however, the point I think you are not considering here is this:

The information and ideas that are outlined in this blurb:
[URL="http/www.madsci.org/posts/archives/dec96/847855291.Zo.r.html"]http/www.madsci.org/posts/archives/dec96...55291.Zo.r.html[/URL]
Particularly the points made regarding the role of kidneys in marine fish...which are either very low (however, intentionally) or non-existent in order for the animal to push salts out of its body in the manners which you explain, while almost literally trying to 'trap' water inside. As I pointed out before though, it is hard for these animals to excrete nitrogen based wastes via respiratory surfaces AND the fact that ammonia, a nitrogenous chemical can pass across the semi permeable surfaces of the cell (still trying to find that same idea pertaining to nitrites)

Here is perhaps a better explanation:

"The freshwater animals are generally hyperosmotic to their environment. The problems that they face because of this are that they are subject to swelling by movement of water into their bodies owing to the osmotic gradient, and they are subject to the continual loss of body salts to the surrounding environment (which has a low salt content). They way these animals deal with these problems is to produce a large volume of dilute urine. The kidney absorbs the salts that are needed, and the rest of the water is excreted" (K. O'Discoll, et. al., n.d.)

What this says to me, and as you also state, freshwater fish take in water, remove the salts, and basically excrete the unneeded water in an almost constant fashion....hence, any water which is inside them only stays for a very short time and due to the kidney function, they are able to excrete nitrogenous wastes (ammonia) in a more efficient and less-energetic manner.

Conversely:

"Marine animals do not need to expend much energy in regulating the osmolarity of their body fluids. There is a tendency for mairne fishes to lose water to the environment through the gill epithelium. The net result of combined osmotic work of the gills and kidneys in the marine teleosts is a net retention of water" (K. O'Discoll, et. al., n.d.)

What this says to me is that, unlike freshwater counterparts, marine animals which are osmoregulators (versus osmoconformists) are in fact, trying to keep the water inside their body. Now, what I feel is the real 'proof in the pudding':

"The body fluids of marine teleosts, like those of higher vertebrates, are hypotonic to seawater, so there is a tendency for these fishes to lose water to the environment, especially across the gill epithelium. To replace the water, they drink salt water and actively secrete the excess salt ingested with the seawater back into the environment. By absorption, 70% to 80% of the ingested water enters the bloodstream, along with most of the NaCl and KCl." (K. O'Discoll, et. al., n.d.)

The important part of that, in my mind, is the statistic right there at the end....

References

Agro, J. (1996). RE: Do fish drink water? [electronic version]. Retrieved on November 25, 2007 from [URL="http/www.madsci.org/posts/archives/dec96/847855291.Zo.r.html"]http/www.madsci.org/posts/archives/dec96...55291.Zo.r.html[/URL]

Gondar Design Biology (2000). An account of osmosis for GCSE biology students [electronic version]. Retrieved on November 25, 2007 from [URL="http/www.purchon.com/biology/osmosis.htm"]http/www.purchon.com/biology/osmosis.htm[/URL]

Gondar Design Biology (2000). Osmoregulation [electronic version]. Retrieved on November 25, 2007 from
[URL="http/www.purchon.com/biology/osmoregulation.htm"]http/www.purchon.com/biology/osmoregulation.htm[/URL]

K. J. O’Driscoll, L. K. Staniels, and D. E. Facey (n.d.). Osmoregulation and Excretion [electronic version]. Retrieved on November 25, 2007 from [URL="http/www.cartage.org.lb/en/themes/Sciences/Zoology/"]http/www.cartage.org.lb/en/themes/Sciences/Zoology/ AnimalPhysiology/Osmoregulation/Osmoregulation.htm[/URL]

 

[andywg]

What this says to me, and as you also state, freshwater fish take in water, remove the salts, and basically excrete the unneeded water in an almost constant fashion....hence, any water which is inside them only stays for a very short time and due to the kidney function, they are able to excrete nitrogenous wastes (ammonia) in a more efficient and less-energetic manner.

It stays in for a short time, but they are constantly taking on the water. Therefore the pollutants in the water are going into the fish at a faster rate and the fish will quickly have a similar profile of ammonia and nitrites and nitrates as the surrounding water, in much the same way as the SW fish.

I have not doubted the different way that the fish approach osmoregulation, but merely your belief that somehow the SW fish are affected more by the water as they retain the water they drink but lost it at the gills rather than FW fish which gain it at the gills and pass it as urine. While much of the water will pass through, it is still going into the body, being absorbed into the blood and then it is passed out after visiting the kidneys.

As a result of this I feel that both FW and SW fish are affected by any nitrogenous waste in a broadly similar fashion as each will have a very swift uptake of water with the pollutants in and the body will soon reflect these parameters.

What this says to me is that, unlike freshwater counterparts, marine animals which are osmoregulators (versus osmoconformists) are in fact, trying to keep the water inside their body. Now, what I feel is the real 'proof in the pudding':

But FW fish are not osmoconformists, they do not have an identical level of salts to their environment. The only fish I am aware of that are osmoconformists are marine hagfish.

"The body fluids of marine teleosts, like those of higher vertebrates, are hypotonic to seawater, so there is a tendency for these fishes to lose water to the environment, especially across the gill epithelium. To replace the water, they drink salt water and actively secrete the excess salt ingested with the seawater back into the environment. By absorption, 70% to 80% of the ingested water enters the bloodstream, along with most of the NaCl and KCl." (K. O'Discoll, et. al., n.d.)

The important part of that, in my mind, is the statistic right there at the end....

But FW fish absorb the water around them into the blood at the gills (with some going in through the skin). It passes to the kidneys in the blood, meaning the FW fish have absorbed the water into the blood just like the marine fish do, and as such will be equally susceptible to the effects of the nitrogenous waste. In fact, 100% of the water entering the body in FW fish goes into the blood.


The urine production rate of freshwater teleosts is up to 14ml per hour per kg (Hickman and Trump 1969), so one can assume
that the absorption rate of a fish is going to be similar. The flow of water in and out of the FW fish is described as "copious" by Bond in Biology of Fishes 2nd Edition, page 404.

So while some of the water may only be in the fish for a short time, the quantity that flows through, and the fact that it is constant means that the FW fish (IMO) will maintain the same levels of nitrogenous waste as their marine counterparts.

 

[Tommy Gun]

It stays in for a short time, but they are constantly taking on the water. Therefore the pollutants in the water are going into the fish at a faster rate and the fish will quickly have a similar profile of ammonia and nitrites and nitrates as the surrounding water, in much the same way as the SW fish.

However, due to kidney functions, the fish are able to expell those nitrogenous chemicals more easily.

I have not doubted the different way that the fish approach osmoregulation, but merely your belief that somehow the SW fish are affected more by the water as they retain the water they drink but lost it at the gills rather than FW fish which gain it at the gills and pass it as urine. While much of the water will pass through, it is still going into the body, being absorbed into the blood and then it is passed out after visiting the kidneys.

Like that one resource I provided states though, it is harder and less efficient to rid the body of nitrogenous chemicals via resperatory functions; hence I assume it takes more time to do so. While I am not familiar with the circulatory system of fish, I wonder if the more highly functioning kidneys of freshwater fish are able to remove the unwanted or toxic chemicals from the blood earlier in the process of that blood being carried throughout the body.

As a result of this I feel that both FW and SW fish are affected by any nitrogenous waste in a broadly similar fashion as each will have a very swift uptake of water with the pollutants in and the body will soon reflect these parameters.

I also do not doubt that any type of aquatic life is affected by its environment (the water) and am starting to feel that the issue at hand here is not the amount of water a fish takes into its body, but rather, how it deals with the water once it is there - meaning, freshwater fish may be more able to expell toxic chemicals than a marine counterpart via kidneys or other processes - which may lend to the notion that fish like the molly are great for handling a saltwater cycle and arguably better than the more hardy saltwater fish such as various damsels.

But FW fish are not osmoconformists, they do not have an identical level of salts to their environment. The only fish I am aware of that are osmoconformists are marine hagfish.

I did not mean to imply that they are....I was simply trying to make the exception for some marine animals.

But FW fish absorb the water around them into the blood at the gills (with some going in through the skin). It passes to the kidneys in the blood, meaning the FW fish have absorbed the water into the blood just like the marine fish do, and as such will be equally susceptible to the effects of the nitrogenous waste. In fact, 100% of the water entering the body in FW fish goes into the blood

Again, the kidneys may be the large difference here. As has been stated in many of the resources I provided, marine fish have evolved in such a way which means they can transport the salts out of their body but attempt to keep the water within and kidney function goes against this need so many have no kidneys, or if they do, they do not function as well as they otherwise might.

(Hickman and Trump 1969),

Please don't take any offense to this, however, it might be good to point out that you are using resources which are almost 40 years old and so the science and theory could have evolved greatly since then. Obviously, keeping marine tanks has come a long long way since those days - which can certainly be attributed to the research done in the 1960s - and I am sure that there are huge differences in the thoughts, ideas, and information surrounding water chemistry and such today.

Again, this is not meant as a 'rip' on any book or author - past, present, or future - however, this is perhaps one of the problems with written text....it cannot change if the science changes. In fact, I will be honest and say that I am surprised no one has said that about my one reference which was written in 1996 since that is already 11 years ago now. The reason I bring this up is becuase I have, over the past year and a half, come up short in some collegiate debates which focus on business strategy, another rapidly evolving subject, by citing some of the more "old school" theories.

The urine production rate of freshwater teleosts is up to 14ml per hour per kg (Hickman and Trump 1969), so one can assume
that the absorption rate of a fish is going to be similar. The flow of water in and out of the FW fish is described as "copious" by Bond in Biology of Fishes 2nd Edition, page 404.

I have found at least a few resources which sound like they differ on the definition of 'copious' amounts of water that FW utilize within their bodies. In fact, thinking about the rules of osmosis, it would seem as though 'copious' amounts of water would only serve to remove salts from a fish since, unless some other form of protection is being used, the salt is going to move out of a cell and into water if it is the more salty of the two (osmotic pressure).

As a sidenote, I am curious to know how someone could measure the amount of urine that is being produced by a fish since that seems like it would be a hard thing to do. Certainly a body of water isn't going to get larger because of urine produced by fish if that urine is being generated mostly from the water that is already there. I would think that an ammonia test wouldn't help since some is released via gills (unless that is considered urine as well). For whatever reason, trying to think of how that might be possible is escaping me right now.

So while some of the water may only be in the fish for a short time, the quantity that flows through, and the fact that it is constant means that the FW fish (IMO) will maintain the same levels of nitrogenous waste as their marine counterparts

IMHO, regardless of the amounts of water being taken into any fish, the difference may be the differences between how a FW fish deals with wastes and nitrogenous compounds versus a SW fish.....which those differences are neccessarily evolved due to the rules of osmosis. For example, a freshwater fish can utilize highly functioning kidneys to remove wastes from its body without worry of dehydrating itself where as a marine animal must find other methods, possibly less effective/efficient or slower methods of removing wastes because, even though it can remove salts from its body, it cannot do so quickly enough to survive if it was to use kidneys to the same extent as a FW counterpart.

So, in the end, I realize that you understand osmosis and such, but I am trying to say that this is the root cause which requires different bodily function in each type of fish, including those which deal with removing wastes and other toxic items from the body.

 

[andywg]

As I pointed out elsewhere, the nitrogenous waste is mostly taken out of FW fish at the gills and not in the kidneys. The amount is around 80% gills and 20% but I can't find the source of said comment. I know bignose uses teh statement often, but cannot recall if he has posted teh source. Hopefully he will pop in to help me with that, but basically the kidneys function more to take water out than filter out nitrogenous compounds from the blood. These tend more to go out at the gills meaning they have come in, done a "circuit" of the body and then pass out at the next pass of the gills along with any fresh ammonium produced.

Remember that osmosis is the passage of water from high concentration to low concentration, meaning the water is constantly entering the body of the fish. This will mean that the levels of nitrogenous waste, even if removed fairly well at the kidneys there is still constantly going to be that same level of nitrogenous waste in the fish as in the water due to the influx of water.

With regards to the research I was quoting, it is still considered relevant by Cech and Moyles in their 5th Edition which was a 2003 print. If you feel the science has moved on and made this redundant, please provide a newer paper which comes to different conclusions. Until someone produces experiments to show that old experiments did not provide correct results, those papers will be true. It is some ten years since Dr Hovanec proved that it is Nitrospira and Nitrobacter bacteria which aid in the filtration of FW tanks. just because his work is old does not mean it is now incorrect.

 

[Tommy Gun]

Remember that osmosis is the passage of water from high concentration to low concentration, meaning the water is constantly entering the body of the fish.

Actually, osmosis is the passage of water from high concentration to low concentration through a semi permeable membrane (e.g. a cell wall), so it's not like water is entering a fish's body uncontrollably....

As I pointed out elsewhere, the nitrogenous waste is mostly taken out of FW fish at the gills and not in the kidneys. The amount is around 80% gills and 20% but I can't find the source of said comment.

Ok.....I have found reference to this notion within marine fish here:

[URL="http/reefkeeping.com/issues/2007-02/rhf/index.php#11"]http/reefkeeping.com/issues/2007-02/rhf/index.php#11[/URL]
(Particularly under the sub-heading of: Mechanisms of Ammonia Excretion by Marine Fish)

However, this article doesn't get into the specific differences between how marine fish differ from freshwater fish, but states that there are important differences....which I am hot on the trail of right now.

Still, this article brings up other great points. For example:

"The predominant source of ammonia in marine aquaria is excretion by fish and other heterotrophs (organisms that live by consuming organic materials). Fish excrete large amounts of ammonia (and ammonium) from their gills, and possibly smaller amounts from their urine, which is why all aquaria must contain some mechanism to prevent the excreted ammonia from rising to toxic levels. " (under the sub-heading: Sources of Ammonia in Reef Aquaria: Biological Processes)

Why I feel this is interesting is because, seeing as how the article is focused on Marine life, not freshwater or both, it may be one reason why stocking a saltwater tank is usually suggested to be done at a much lower rate than FW tanks. HOWEVER, I completely realize that there is no mention of how or if this might be different in a FW tank so I can't really base any opinion on it....but I wanted to bring it up since I am sure someone will look deeper into this point and maybe find good info before I do.

With regards to the research I was quoting, it is still considered relevant by Cech and Moyles in their 5th Edition which was a 2003 print. If you feel the science has moved on and made this redundant, please provide a newer paper which comes to different conclusions.

I do not feel as though this is the case, and only brought it up because I know how easily these debates can become diluted (pun intended) by research which isn't all that pertinent anymore. It was certainly not meant to be accusational in any way. However, I have found myself steeping in that 'species of beneficial bacteria' debate and don't want to go back. For example:

"BIO-Spira, containing several newly discovered patented and patent pending species of the actual nitrifying bacteria found in closed, freshwater aquatic systems is "biological filtration" in a bottle"
[URL="http/www.marineland.com/products/mllabs/ML_biospira.asp"]http/www.marineland.com/products/mllabs/ML_biospira.asp[/URL]

....again, not getting into that, but showing that some debate does exist apparently.

 

[andywg]

While acknowledging that this is over a year dead, whilst looking into the digestive system of goldfish for another post I came across the below:

Acute toxicity levels (lethal concentration to 50% of test orrganisms after a 96 hour exposure) range from 0.068 to 2.0 mg/l for freshwater and from 0.09 to 3.35 mg/l for marine fish (from Handy, R D and Poxton, MG 1993. Nitrogen pollution in mariculture: Toxicity and excretion of nitrogenous compounds by marine fish. Rev. Fish Biol. Fisheries 3:205-241).

This would indicate that there is very little in it between SW and FW but that FW have a slightly lower tolerance short term for ammonia levels. Could this support a theory that SW fish are more hardy in an initial tank than FW?

 

[OldMan47]

Andy, I am far from an expert on this subject and have no other sources to cite. I have been reading your thoughts and the underlying information and am forming an opinion based solely on what has been cited. The biggest difference that I see between FW and SW is that the FW fish are dealing with an uptake of water, not water and all of its pollutants. They use a strategy to remove water efficiently but are dealing primarily with water only using those strategies. The SW on the other hand are taking in SW and all of its component parts in order to deal with having a tendency to lose water. The strategy they use means taking in all that is contained in the SW by drinking it and needing to deal with not only the water but also its contaminants that arrived with the water. This would lead me to the simplistic conclusion that the SW fish are dealing with not only water but all the components that arrived with that water while the FW fish deal only with the water itself that comes across the cell membranes at the gills. I am left with a feeling, I know that is a bit loose, that SW fish will end up with the need to deal with a more complex situation to deal with any pollutant that they encounter in their environment because it will cross into the fish by them drinking the water. Drinking does not tend to separate any of the components based on osmotic pressure.
I am sure that as a newbie to this I have missed some key factor but that is what I see from the citations you two have used.

Edit: spelling

 

[andywg]

My reply to the above would be that osmosis occurs at the gills of FW fish in relation to more than just water, meaning that a higher ammonia concentration in FW will cross straight into the bloodstream of a FW fish. Indeed, in FW the fish is doing its best to take up almost everything except water but the water still flows in and must be dealt with.

One of the ways FW fish deal with losing salts to the water is the active exchange of nitrogenous waste for sodium. Once you have a level of nitrogenous waste in the water it will lower the ability of the fish to excrete the waste as well as prevent the uptake of sodium leading to health issues.

The water around a FW fish is constantly forcing itself into the fish and flushing through. In a SW fish the water is swallowed and is processed in the gut with the water retained for the body and the undesireds being sent out through either the gills or the urine.

I think the end result is that for our purposes both FW and SW are as close as equally susceptible to nitrogenous waste but that test results indicate a slightly higher tolerance in SW fish. This would appear to be evidential support that SW fish are at least as hardy, but it does not (with the info provided) provide an indication as to why the SW fish might perform better in the test carried out.

 

[reportingsjr]

Just like to point out something about the osmosis which might have something to do with it.

I think that you guys are mistaken about the whole osmosis process from what I read, that the water wants to go from an area of high concentration to an area of low concentration possibly through a semi-permeable membrane.

Now, I know that in the human body the transfer of water is largely controlled by salt, which if that data correlates with fish then would explain something there.

You see, the body takes a salt molecule (NaCl) and takes the two ions it's made of (Na+, Cl-) and uses those to transfer water. If you remember your chemistry class you'll remember that H20 is a polar molecule, meaning one side is one charge, the other the opposite charge, that makes it attract to ions. (hmm, maybe a Na+ or a Cl-? hehe)

All cells have what are called sodium pumps, and move Na+ either in or out of the cell. When a cell does this it changes the charge of the cell to +1, meaning the outside of the cell has a -1 charge. The negative pole of the H2O molecule is thus attracted to the sodium ion, thus being pulled through the cell wall, along with a Cl- ion to even out the charge.

Voila!

Now, since ammonia (NH4+), nitrate (NO3+), and nitrite (NO2+) are all ions they might very well be pulled through too. Understanding what I said before, the more salt in the water the more salt the fish will have, and thus the more osmosis will be going on which could make for a higher rate of other polyatomic ion transfers which would attribute to SW fish being more sensitive.

Just my 2 cents, probably wrong though, heh.