I wonder if anyone knows how fresh-water fish species populate separated river basins. They cannot take the sea route and they cannot jump the mountains in between, do they? How do they manage? I guess some were introduced by humans, like carps, trouts or koi, but that cannot be the answer for the species that are not bred for human consumption.
Tiny eggs sticking to bird legs.
Also, what are separate basins now weren’t necessarily separate basins in the past. Rivers can change their course and even direction, and landforms can change their shape.
And sometimes the recent past. Massive areas of land can become submerged during periodic floods creating a common lake for numerous waterways.
Yeah, I read somewhere that the Amazon once started in what is now the Sahara and flowed in the opposite direction into the Pacific, but since then evolution has made the species in the Niger and the Amazon diverge. Those are different species now, I was wondering about the same species.
Let me ask the question differently: imagine Antarctica melted and got a climate where rivers flowed most of the year. Would fresh-water fish get there? How? Would there be separate species in different parts of Antarctica, would they diverge or would they converge? Or would it be more likely that a sea-water species evolved to colonize the new fresh-water habitat? How long might that take?
Different kinds of freshwater fish have different levels of tolerance to salt water. What are known as “secondary freshwater fish” have some saltwater tolerance and can pass between mouths of separate rivers or cross narrow ocean barriers. Primary freshwater fish have little or no tolerance for saltwater and their dispersal is by other means.
That may happen in a few species, but it is not the general means of dispersal of primary freshwater fish between different basins. In particular, this is not a likely method of transfer for fish that live in mountain streams.
The main method of the spread of primary freshwater fish between different river basins is the process of stream capture. In particular, in steep mountain terrain sometimes erosion can cut into the watershed of the next river over, resulting in what’s known as headwater capture. This allows fish to spread between basins, but it’s a pretty slow process. Fish of South American origin have gradually spread northward through the Isthmus of Panama since its closure about 3 million years ago, but most haven’t spread that far even over that length of time.
There’s some geographical confusion here. You’ve gotten the Amazon and Congo mixed up in some fashion. As far as I known the Congo never originated in the Sahara, or was connected to the Niger.
The Amazon on the other hand once did flow into the Pacific. The rise of the Andes redirected it to take its present course to the Atlantic.
Primary and secondary freshwater fish would be unlikely to colonize Antarctica. It’s just too far.
There are plenty of saltwater fish that have tolerance for freshwater. No doubt some would specialize for the newly available habitat. Some colonization would take place immediately. It might take tens to hundreds of thousands of years before speciation took place.
Thanks!
I think I remember reading that the origin of the Amazon was in the Sahara before Africa and South America broke apart and that when the continents separated one part remained the Niger and the other became the Amazon, but I cannot find it. When you search for anything concerning the Amazon River’s origin you get books sold by Amazon about the River or hagiographies about the origin of Mr. Bezos’ firm 
My recollection is old and probably confused anyhow so never mind that. The rest of your explanation is just what I was looking for, you have been very helpful. Thanks again!
On checking, I see that you are right. Apologies. There are references to a proto-Amazon starting in Africa before the breakup of Gondwanaland and flowing westward (although I wouldn’t call that ocean the Pacific at that time). Possibly what is now the Niger was related to the proto-Amazon. However, the present Amazon originated in the Miocene, after the continents divided. The present Niger is definitely a composite river, with the upper Niger originally having drained into the Sahara, but later being captured by the lower Niger (which accounts for the river’s strange course).
Some primary freshwater lineages go back before the breakup of Gondwanaland, notably the lungfishes, which today are found only in Australia, South America, and Africa. (They also once occurred on the northern continents, but went extinct there long ago.) They very likely occurred in Antarctica as well until it froze over.
No, they’ve gotten the *Niger *and Congo confused. A proto-Amazon-Congo has been theorized. But it’s just speculation, I don’t know/can’t easily find any work confirming it. And in any case, it would not have flowed out of the Sahara, even if it did exist.
There are plenty of Farmed salmon along the southern half of Chile that could easily make the journey over to Antarctica if the habitat were suitable; it’s only about 500 miles away and salmon regularly migrate twice that distance. Being anadromous they are already evolved to tolerate (and require) fresh water for part of their life stage.
During an ice age say in North America, ice will slowly migrate south and freeze up the fish habitat, pushing the freshwater fish living in them further south year by year; they can end up hundreds of miles away from where they were a few thousand years earlier. Eventually the glaciers start melting and retreating, which forms huge temporary freshwater lakes along the toe that allow fish to mingle around and swim up other streams and start colonizing them, spreading further and further north as the ice retreats. There have been dozens of ice ages that have basically pushed all the fish down into big lakes and let them find their way back up over the eons.
We find fossils of cold water species like arctic grayling waaaay further south of where they can survive today, and we know from geology how glacial activity forms vast lakes and river systems that change over time. Fish can move through them much faster than the systems themselves actually open up or close, so it’s little wonder that you find the same fresh water fish species in river basins that today may have no connection to each other, but 10’s of thousands of years ago did.
There was a stink between Canada and the USA a few years ago over Devil’s Lake in North Dakota. IIRC this is a lake in its own basin, that used to be part of the Mississippi river system now isolated. Then there was a time of heavy precipitation and the lake started growing too large, and the most convenient drainage was into the north-bound Red River system into Hudson Bay. Canada was upset because there was a likelihood that Mississippi ecology had different types of creatures that might become invasive species and cause problems in the northbound watershed.
So there are some fairly separate watersheds with not necessarily equal inhabitants.
Should also point out that at the height of the last Ice Age, there was Lake Agassiz which built up against the glaciers, presumably draining partially into the southbound rivers of the Mississippi. So aquatic wildlife would have migrated north into the system maybe 20,000 years ago before they became separate.
Salmon and other anadromous (breeding in freshwater, adults living in saltwater) and catadromous (breeding in saltwater, adults living in freshwater) are considered to be peripheral freshwater fish, that is, highly tolerant of saltwater. These would be the kind of fish that might initially colonize a place like Antarctica. But “true” freshwater fish (primary and secondary divisions) wouldn’t be able to.
Turns out there has been a study on the lack of studies on bird-egg dispersal.
Not fish, but I once had an above-ground basin outside my house that had filled with rain water that grew a colony of ostracods. It was pretty cool.
It also depends how long you are talking about for fish to get into new fresh water habitat. You’re not gonna get true fresh water fish if they can’t migrate through salt water over just a few hundred years. But if you have millions of years and the new habitat remains then a few salt water species will eventually evolve into fresh water fish, which is where we got our fresh water species today from.
This page talks about a study from a while back suggesting that most of the salt water fish today are descended from fresh water species, who of course originally evolved in salt water. So the longer you have, the more ways to get fresh water fish into previously unavailable habitats.
Or sport. People don’t tend to eat large mouth bass, but it has been a popular “bucket biology” species.
Thanks for that. That idea has so generally been accepted that I was not aware that it wasn’t based on any actual studies.
There has, however, been a recent study showing that swans can transport viable fish eggs through fecal matter. (I will say that killifish are among the most durable life forms on the planet. They were common in a creek in my neighborhood in the Bronx that I think must have been nearly half petroleum.)
Just a guess based on experience but I wonder if birds incompletely catching fish is one factor in their dispersal. I know that in my work parking lot I see several dead fish per year so a bird dropping a struggling fish happens quite often if you add up the square miles and years. I just don’t know how common it would then be for the fish to fall into water and then survive.
I doubt that it’s a regular way that fish get transported between separate watersheds. Birds aren’t normally going to transport fish far enough from where they are caught for that.
When I was a kid, the three flat next door burned down. They demolished what was left and dug out the foundation. They did what the could to fill and level it but there was a depression that eventually filled with rainwater. Sure enough, I spotted some minnows within a year. I always wondered where they came from. It was really small, more puddle than pond and I doubt any ducks would have bothered landing there. It had recently been a basement so no redirected rivers or other bodies of water nearby. I finally decided that someone might have dumped some live bait in after fishing.
…and some people by exotic fish species as pets and then dump them in local lakes, etc.