no. 3602 CRAYFISHES — HOBBS, HOLT, AND WALTON 11 



earlier draft of this paper, we wrote concerning the Potts Creek 

 locality, "Here, in the single known locality, it [0. juvenilis] was found 

 with C. b. bartonii and C. I. longulus but occurring in an area that 

 might well have been inhabited by C. acuminatum. This is perhaps 

 significant in light of the account of the occurrence of 0. juvenilis in 

 Mountain Lake." With the apparent expulsion of most of the C. I. 

 longulus population by 1965, the supposed eradication of C. acuminatum 

 prior to 1960, and the apparent inroad on the C. b. bartonii population 

 during the intervening 5-year period, the possibility of a rapid spread 

 of 0. juvenilis in the James drainage system might well be anticipated. 

 Also worthy of note is the fact that, as in Mountain Lake, C. acumina- 

 tus was supposedly supplanted first, and C. b. bartonii is now very 

 poorly represented in the crayfish population. (See further remarks 

 under 0. juvenilis.) 



In 1966, Hobbs and Walton again visited this locality and collected 

 71 specimens of 0. juvenilis, 6 of C. I. longulus, and 2 of C. b. bartonii. 

 Concentrated efforts in selected riffle areas were required to obtain the 

 specimens of C. I. longulus, and it seemed obvious to the two of us that 

 there can be no question that the C. I. longulus population has been 

 reduced and that the C. b. bartonii population has either been reduced 

 or has largely retreated to burrows along the banks of the creek. We 

 do not yet know the range of 0. juvenilis within Potts Creek but 10 

 miles downstream from Waiteville, where Route 17 crosses the 

 Creek, the large riffle area there is largely populated by C. I. longulus, 

 but 0. juvenilis is also present. Cambarus b. bartonii and C. acuminatus 

 were not found. With the absence of the latter, it is probable that 

 even that far downstream, 0. juvenilis is vicariating for C. acuminatus. 



It seems unlikely that in any part of the region a lack of food is 

 responsible for depauperate populations, and even in waters low in 

 calcium the crayfishes do not seem less abundant than in those high in 

 calcium. One of the chief limiting factors seems rather to be available 

 cover in the way of loose rocks (as opposed to bedrock), debris, and 

 suitable banks into which the crayfish may burrow. For example, in 

 suitable riffle areas within the range of C. I. longulus (James and 

 Roanoke drainages), populations proportional to the size of the riffle 

 and number of rocks of diameters greater than 10-100 mm have been 

 invariably encountered. In contrast, in areas where the water flows 

 rapidly over scoured bedrock, not a specimen of C. I. longulus or any 

 other crayfish is to be found. Except in the upper portions of streams 

 that are subject to being converted to a series of pools in dry seasons 

 (such as the upper portion of Craig Creek, sta. 122), the oxygen 

 content (sometimes a limiting factor) of most of the streams in this 

 area approaches saturation throughout the summer. 



