1 80 Branley A. Branson 



ferent subgenera of darters — four if we accept Nanostoma (Page and 

 Burr) — (Table 6), perhaps resulting from various impulses or cycles of 

 invasion from extra-limital stream basins, have markedly diversified 

 within these river systems, doubtless abetted by the notable niche and 

 habitat variability from stream to stream and within drainages. 



Actually, the Low Plateau endemic species of the percid subgenera 

 Nothonotus, Catonotus and Ulocentra (including Nanostoma) and their 

 nearly 30 species, pose a biogeographic and evolutionary problem of 

 considerable importance that has been inadequately studied. These three 

 groups, in my estimation, are species swarms that have developed in 

 response to mechanisms similar to those proposed by Pflieger (1971) to 

 account for stepwise fish dispersal through the Mississippi and Ohio 

 rivers: "Aggradation subsequent to the last glacial stage produced the 

 environmental conditions now prevailing in the Embayment, restricting 

 further dispersal by upland fishes. All the glacial and interglacial peri- 

 ods were accompanied by alternate entrenchment and aggradation in 

 the Mississippi Embayment, and this would seem to provide an ade 1 

 quate mechanism for the alternate dispersal and isolation of populations 

 east and west of the Embayment." 



Thus, this mechanism may have been accompanied by cycles of 

 isolation, adaptation, and speciation, aided by extralimital stream cap- 

 tures that brought congeners back into contact to heighten competition 

 and perhaps establish new patterns of variation and divergence. What- 

 ever the mechanisms and processes, the species swarms are real and the 

 whole problem is deserving of detailed analysis. 



Another group of fishes of considerable interest and importance is 

 the ictalurid genus Noturus. The fact that most species of Noturus avoid 

 cold water (Taylor 1969) indicates a southern origin for the group. The 

 center of greatest abundance of species encompasses Kentucky and 

 Tennessee to Virginia and North Carolina, with a derivative secondary 

 speciation center in the Ozarks of Arkansas and Missouri. Tennessee 

 has the largest number of species, mostly associated with the Tennessee 

 River basin, Kentucky is second followed by Alabama, and the number 

 decreases peripherally. Noturus gyrinus (Mitchill), a distinctive lowland 

 species that is widespread and common in the Lower Green and Trade- 

 water systems, gets into Low Plateau streams in western Kentucky and 

 adjacent Indiana and Illinois (Wabash drainage). Noturus exilis Nelson 

 is absent from southern Indiana and most of Kentucky, but occurs in 

 extreme southwestern Illinois, much of the Low Plateau of central Ten- 

 nessee and northern Alabama, and peripheral areas to the north (post- 

 glacial) and trans-Mississippian in Oklahoma, Kansas, Missouri, and 

 Arkansas. In the Low Plateau, Noturus nocturnus Jordan and Gilbert 

 occurs in the Tennessee River drainage of western Tennessee, northern 



