Switching Freshwater To Salt Or Vice Versa

[mr130gallon]

i dont know why saltwater fish only survive in saltwater, or why freshwater fish only survive in freshwater but...

do you think it would be possible to take a fish such as a guppy which breeds every 28 or so days and slowly through about 50 generations, add more and more salt to each generation untill you could successfully have a guppy (or any FW fish for that matter) in a marine aquarium?

i watched a documentary about some Russian scientists who took a couple hundred foxes and took the most docile pups and eventually after about 20-30 generations the had a domesticated fox which looked a bit different, (the ears were floppy and its tail was curled) i know thats quite a bit different but it shows with time you can change an animal.

do you think it would be possible to have a marine guppy or a freshwater clownfish? i know i would like a freshwater clownfish.

 

[Mr Bliss]

All fish have the same internal salt content. With marine fish, that content is less than in the surrounding environment, and in freshwater it's more. Thus both types of fish need to deal with osmotic pressure, only in different directions. In saltwater, fish need to constantly expel excess salt from their bodies while retaining as much water as they can. Conversely, in freshwater, fish need to pump out excess water while retaining salt in their bodies. So there's a pretty fundamental difference between the two situations.

There are fish that can cope in both marine and freshwater environments, though, as they have the physiological mechanisms in place for efficient osmoregulation in both directions. Salmon, for example. And there are both marine and freshwater fish that can tolerate brackish water (but can't go beyond that). You could probably selectively breed some degree of increased tolerance to salinity (or lack thereof) to these types of fish, because they already have much of the required genetic machinery in place. But keep in mind that the reproduction of any vertebrate organism is a complicated matter compared to the "simple" task of adult fish osmoregulation. Most euryhaline (can tolerate a wide range of salinities) fish can only breed under specific conditions, either marine or freshwater.

But how about strictly freshwater of strictly marine fish? I think here it'd depend on the species in question. How long ago did it evolve into a marine or freshwater fish? If it's been very long, it's unlikely that it still has the genes required to adapt to different salinity fast (within dozens or hundreds of generations). Unnecessary genes tend to disappear or find a new function during evolution. Now, you'd think if related species can be found in fresh- and saltwater, they might be able to adapt to each others' conditions, but this isn't necessarily true. For example, freshwater Mediterranean gobies diverged from marine gobies 10 - 40 million years ago. Could you selectively breed them back into saltwater within a human lifetime, undoing tens of millions of years of environmental specialization? I don't know, but I'd put my money on "no".

For further perspective, cyprinids are thought to have been around at least 160 million years, yet there are only a few brackish species and only one marine species in this order, and even that one (the pacific redtail) needs freshwater to breed. The other major fish order found in our tanks, characins, have been around for about as long, there are about 1700 species of them, and they're all exclusively freshwater. Africa and South America have drifted apart during this time, and the Atlantic Ocean has opened up between them, yet not a single species of characins has adapted to saltwater or even brackish life (in, say, estuaries) during that time. To me that suggests that the border between the freshwater and saltwater lifestyle is a pretty major evolutionary leap once you've crossed a certain treshold of specialization and a few dozen generations of breeding in a salinity gradient probably wouldn't do the trick.

 

[Im a beginner]

yea but guppies can live in salt water tho and freshwater

 

[Mr Bliss]

Yeah I kinda went with the "or any FW fish" part of the question and forgot that the original question involved guppies specifically. Guppies are euryhaline fish, native to brackish waters, that can give birth in saltwater, and apparently even their fry can live in saltwater for a while. Do they grow and develop normally though? Can they mate in a saltwater environment? I don't know. All reports I've read about guppies in saltwater tanks end with the guppies dying within a couple of months.

 

[mr130gallon]

wow... i wish i knew that much about something lol.

i didnt know guppies where euryhaline fish i didnt know that the word existed, but you make a good point, and it would probably take a long time to change a freshwater to saltwater, but say it were possible wouldnt it be cool to be the person who did that? and i understand the problem with them actually breeding in the salt or fresh water because some fish are picky about where they breed. do you think it would be possible to 'help' a brackish species turn fully saltwater or fully freshwater in a human lifetime?

 

[Mr Bliss]

I think it'd be interesting to find out.

At first I figured that if this could be done (with guppies), someone would have done it already. But then, marine aquaria are more expensive to set up, and there are so many stunning fish and invertebrates to choose from, with colors that put the fanciest of fancy guppies to shame, so there probably isn't a commercial incentive. Then again, any marine fish that can be readily bred in captivity is probably a good thing, to reduce the amount of reef-caught fish. And in smaller reef tanks you're pretty limited in your choice of fish, so I can see why someone might want marine guppies, as weird and out of place they might seem.

 

[mr130gallon]

i agree that they would be put to shame, but if they would breed as well as they do (every 28 or so days) then they could be good inexpensive fish to help establish a tank. ive seen prices on some tank bred saltwater fish and even those are expensive compared to freshwater, it would be pretty cool if it were possible.

 

[mr130gallon]

i re-read this topic, and started thinking. Rift valley lake cichlids adapt to situations impressivly fast, alot faster then most other fish, which is why they did so well in the great african lakes. and they seem to breed fairly quickly, i wonder if it would be possible to switch these fish to saltwater, or high salinity brackish water...