47 
Freshwater Fish Zoogeography 
Both Micropterus notius and Ictalurus serracanthus are well defined 
species, with the former regarded as the most generalized form of black 
bass (Bailey and Hubbs 1949). Both have achieved a higher level of 
taxonomic differentiation from their closest congeners than have the 
endemic peninsular populations of Micropterus salmoides and at least 
four other species. Inasmuch as peninsular endemism in the last five 
species is believed to have been achieved during the Pliocene, it may be 
assumed that M. notius and /. serracanthus evolved earlier, and that 
species differentiation was well under way at least by the end of the 
Miocene. 
As previously indicated, average Pliocene sea levels were higher than 
those in the latter half of the Miocene. At its maximum incursion, about 
5 million years ago, the sea stood at or near the present Cody Scarp, as 
evidenced both by the presence of the scarp itself and by the existence of 
marine deposits from the upper Suwannee River, in Hamilton County, 
Florida. Later, as Pliocene sea levels gradually dropped, there was a 
corresponding increase in the amount of land exposed (Robertson 
1976). Considering that much of the freshwater habitat present today 
and during pre-Pliocene times in northern Florida and southern Geor- 
gia was absent during the Pliocene, the question may be asked as to 
how and where Micropterus notius and Ictalurus serracanthus (as well 
as other freshwater fishes) survived. 
It is reasonable to assume that both of the above-named species 
occupied much of their present ranges during the latter half of the Mio- 
cene. During the Pliocene, however, a considerable amount of dis- 
placement must have occurred. One possibility is that these species 
essentially remained in place during the marine invasion, and found 
refuge in the many freshwater springs located throughout the Suwannee 
River valley. It appears likely, in fact, that the populations of troglobitic 
crayfishes living in the region survived in this way, although Franz and 
Lee (1982) did not address this particular point in their discussion of 
Florida cave crayfish zoogeography. It seems, however, that the physi- 
cal and ecological resources of these springs would be much too limited 
for M. notius and /. serracanthus, particularly considering that neither 
is today intimately associated with such areas. Furthermore, neither 
species is known to go underground. A more likely possibility is that 
they simply occupied more confined ranges upstream during the Plio- 
cene, and at times may have been found only above the scarp. Although 
upstream habitat today seems marginal for these species (particularly 
for M. notius ), this may not always have been the case. The distributions 
of both species in the Suwannee and Ochlockonee rivers, together with 
their near absence from all intervening drainages (/. serracanthus is 
present in the St. Marks), strongly suggest that headwater stream 
transfer was involved (Gilbert 1980g, Yerger 1980). This presumably 
