124 U.S. NATIONAL MUSEUM BULLETIN 2 59 



Evolution 



Distribution and Origin 



Distribution of stygonectes. — The presently kno\vn distribution 

 of Stygonectes is shown by the map in figure 30. Despite several 

 sizeable disjunctions wdthin the range, some of which may be attributed 

 to lack of collecting, the northern and western limits of distribution 

 appear to be generally well established. The most conspicuous dis- 

 junction occurs along a broad stretch of Piedmont and Coastal Plain 

 that extends south-southwestward from Virginia to Alabama. Addi- 

 tional records and possibly even undescribed species may be expected 

 from potential ground-water biotopes of this area. On the other 

 hand, the extensive cavernous limestone area extending from the 

 southern Appalachians across the Interior Low Plateaus, although 

 biologically explored almost continuously for the last ten years, has 

 not yielded any Stygonectes, and it is concluded from these negative 

 data that the genus does not occur in ground waters of this vast karst 

 region. 



Notmthstanding the fact that three major cavernous areas of North 

 America lie mthin the range of Stygonectes, only in the Edwards 

 Plateau of central Texas does this genus make up a conspicuous part 

 of the aquatic cave fauna, and even there only about three out of the 

 nine recognized species of the region can be considered abundant in 

 terms of numbers of individuals per cave. Only the central and 

 northern sections of the cavernous Appalachians contain a representa- 

 tive stygonectid fauna, and with few exceptions, cave populations of 

 this region are extremely rare and often ephemeral. Similarly, cave 

 populations of Stygonectes in the Ozark Plateau region are also usually 

 small and apparently occur sporadically. 



The rarity of most cave popidations of this genus and their charac- 

 teristically small size may be more apparent than real, since the bulk 

 of these seemingly depauperate populations may inhabit associated 

 solution channels, and related bedrock interstices rather than open 

 cave waters; however, a few caves do contain relatively large, ap- 

 parently stable populations. Certain species, at least, appear able to 

 build up rather large populations in open cave habitats and maintain 

 them over extended periods of time. Explanations of fluctuation in 

 the density of other cave populations as well as for the rather sporadic 

 occmrence of these animals throughout theu' range must await 

 careful ecological studies. Nevertheless, distributional data indicate 

 that species of Stygonectes, although seldom abundant anywhere 

 throughout their range, have been able to invade and successfully 

 colonize a variety of ground-water biotopes ranging from interstices 



