slow and still water darters

The talk of the darters of slow water on the Ethoetoma Gracile post got me wondering.

Exactly how uncommon are still water darters like swamps and slow current darters like tesselates? I keep hearing about riffle darters, does this reflect the riffle darters population, their ease to catch, or popularity due to color, or is this an accurate view on their population relative to others. (my own state has no riffle darters, only still and slow water ones, despite lots of riffles).

Also how did the tesselate darter (found from NH to Florida) and swamp darter (found from maine to florida) get such a huge range? Most widespread darter species are explainable by beiong in a large watershed such as the mississippi or great lakes drainage but not so much with these species. Are their preference for slow or still water a factor that let them spread their range where other darters could not?

I short, yes, their preferences for slow moving, lowland habitats probably did help in their dispersal. Consider the sea level changes of the Pleistocene. As the seas retreated, many coastal drainages became connected (on land that is now underwater). That's one way for lowland fishes to move from one drainage to another. Also, lowland streams meander quite a bit. They are promiscuous, geologically speaking. Avulsions and stream captures (look for "elbows" in current streams) have moved lowland fishes all over the place throughout history. Finally, flooding in lowlands can traverse drainages, passing fish from one to the other. I might've missed something here, but those are the major natural ways fishes go across drainages.

So yes, lowland fishes generally have larger ranges than highland fish just because of the relative ease of lowland cross-drainage movement. Fish found in higher gradients are usually isolated from crossing drainages by the lower frequency of highland streams being captured by other drainages, and life history restrictions that prevent them from moving in large enough numbers into slow, sluggish areas of a drainage to be captured by another drainage and begin colonization.

I did not mean to imply that there are few individuals of those species, only that there are relatively few species of lentic darters. The great majority of darter species are inhabitants of streams. Only a handful are pond and swamp fishes (though several stream darters also occupy the wave zones of lakes in the north).

In addition to all rjmtx said above, there may also be some bias in the delineation of lowland versus upland species. My impression is that systematists in the last several decades have concentrated on fauna of upland streams, in part due to the Central Highlands Vicariance hypothesis, which has to do with the distribution and relationships of species in the hilly uplands of the Appalachian-Piedmont-Interior Plateau region, Ozark-Ouachita region, and to a lesser extent, the Edwards Plateau of Texas. Lowland species have been almost ignored. This is changing, and I suspect many of these very widespread coastal plain species will be chopped up in years to come.