One thing that really fascinated me when I first learned of it is that freshwater fish distributions depend on past geology. Excluding those that migrate to or from saltwater, each exclusively freshwater species must have evolved in a single river system. This is why there is so much endemism among freshwater fishes. Yet there are some exclusively freshwater fish species that are found in several river systems. This comes about through “stream capture”; that is, a stream is connected to one river system, but then erosion over time causes it to connect to another river system and lose its connection with its original river system – bringing its fishes with it. Freshwater fish species inhabiting more than one river system thus indicate that those river systems exchanged streams at some point in the past through stream capture.
Yeah the “river capture” phenomenon is fascinating. There are several instances of it in Central American fish like this toadfish:
http://watlfish.com/species/batrachoididae/archives/2014/01/30/daector-quadrizonatus/
Apparently all the Daector sp. are Pacific marine and brackish species, except for this one which is found in an Atlantic slope freshwater river. It seems that the ancestor of the species was “stranded” in that river when the Panamanian isthmus was uplifted and it slowly evolved over that time into a freshwater toadfish derived from marine ancestors. As is the case in many taxa on either side of Panama, the genus Daector went extinct in the Caribbean after the uplift and persists only in the Pacific and in this one river on the Atlantic slope. The “uplift trap” theory makes more sense than “invasion from the sea” given the fact that there is no Atlantic Daector toadfish to invade the river from the Atlantic side.
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