Internal Parasites Fresh to Salt

[taylored_projects]

I have been acclimating some Molly's from freshwater to saltwater. I've seen other threads mention how freshwater sicknesses don't survive in saltwater. Is this the same for internal parasites? Like one's that cause sunken bellies etc.

 

[subaru watxher]

parasites from freshwater including internal parasites shouldn't transfer over to saltwater, so your other pre existing saltwater fish wont catch them.

 

[Jay Hemdal Verified Community Expert]

I have been acclimating some Molly's from freshwater to saltwater. I've seen other threads mention how freshwater sicknesses don't survive in saltwater. Is this the same for internal parasites? Like one's that cause sunken bellies etc.

Yes - internal parasites can be transferred from freshwater fish to saltwater fish as all fish have essentially the same internal salt levels in their blood.
That said, you would have to be using live feeder fish for this to happen, (the parasites have to get inside your other fish) and it isn’t super common.
The risk with mollies is if they were kept in brackish water, they may have external parasites that can survive being moved to saltwater, and those can infect other fish more easily.

Jay

 

[taylored_projects]

Yes - internal parasites can be transferred from freshwater fish to saltwater fish as all fish have essentially the same internal salt levels in their blood.
That said, you would have to be using live feeder fish for this to happen, (the parasites have to get inside your other fish) and it isn’t super common.
The risk with mollies is if they were kept in brackish water, they may have external parasites that can survive being moved to saltwater, and those can infect other fish more easily.

Jay

So the Molly's from freshwater to salt could still die from their own internal parasites even in a saltwater system. Because it won't kill the parasites?

 

[lion king]

While I'm not going to argue and say it is not possible, what I will say is that I have used mollies as feeder fish for dozens of predators over 20 years and have never seen it.

 

[Jay Hemdal Verified Community Expert]

So the Molly's from freshwater to salt could still die from their own internal parasites even in a saltwater system. Because it won't kill the parasites?

Yes, physiologically, all fish have pretty much the same salt content internally. If I recall correctly, it is around 1.017. Some fish are “euryhaline” and can tolerate a change in water salinity, but any internal parasites would be isolated from that change through the fish’s osmotegulatory system.
External parasites are a different matter - many of those are “stenohaline” and will die if the salinity bathing them changes too much. There are euryhaline parasites though, like velvet, that can survive from marine to almost pure freshwater.
I should add though that farm raised mollies do not have large internal parasite loads (but wild ones do).

Jay

 

[taylored_projects]

Yes, physiologically, all fish have pretty much the same salt content internally. If I recall correctly, it is around 1.017. Some fish are “euryhaline” and can tolerate a change in water salinity, but any internal parasites would be isolated from that change through the fish’s osmotegulatory system.
External parasites are a different matter - many of those are “stenohaline” and will die if the salinity bathing them changes too much. There are euryhaline parasites though, like velvet, that can survive from marine to almost pure freshwater.
I should add though that farm raised mollies do not have large internal parasite loads (but wild ones do).

Jay

Thank you!