12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



The only known instance of actual fluctuation of a population in 

 the area is one of C. sciotensis, which occupied the riffle area on 

 Sinking Creek just below the bridge on State Route 700. Since this 

 is a readily accessible area for obtaining crayfishes for experimental 

 work at the Mountain Lake Biological Station, hundreds of specimens 

 were removed during one summer, and the following spring, hours of 

 work in the riffle revealed very few adult individuals. It was not 

 until early fall that the population seemed to have recovered. 



The ostracods. — Although the population size of ostracods on a 

 single crayfish is unpredictable, there are few crayfishes in this area 

 that do not harbor a number of ostracods. A single female specimen 

 of the burrowing crayfish, Cambarus carolinus, was infested with 238 

 ostracods (Dactyl ocythere chalaza and Ascetocythere asceta) ; however 

 other individuals of the same species from the same locality had as 

 few as a dozen. Thirty-five ostracods were recovered from a specimen 

 of C. b. bartonii with a carapace length of only 14 mm. 



It is not known at what age the young crayfish may become infested 

 but apparently they are susceptible to harboring the ostracods within 

 a short time after leaving the parent. The smallest crayfish observed 

 to be infested was C. b. bartonii with a carapace length of 12 mm 

 carrying a single individual of Donnaldsoncyihere scalis. 



Repeated observations of infestations of "soft" crayfishes examined 

 shortly following a molt indicate that the ostracods respond to the 

 molt by leaving the old exoskeleton and making their way to the new 

 one. This reinfestation is undoubtedly more nearly assured because 

 of the fact that in this area most of the crayfishes devour their old 

 exuviae, thereby bringing any ostracod remaining on it close to the 

 vulnerable gnathal appendages of the freshly molted host. 



It is not known how crayfishes become infested nor are data available 

 to indicate that the population of ostracods increases with time 

 following a molt or that population size diminishes with a molt. 



In some portions of the area, a single crayfish may be infested with 

 only one species of ostracod, but, more frequently, at least two species, 

 and often more, occur on a single host; in such instances, they may 

 tend to be concentrated on one area of the host's exoskeleton but 

 there are other associations in which one of the ostracods may occur 

 with equal frequency on the setae of the gnathal, ambulatory, or 

 natatory areas. 



Many of the ostracods are infested with peritrichous ciliates; 

 colonial (Epistylidae) and solitary stalked forms are found on the 

 appendages between the valves, and the external surf aces of the valves 

 are often inhabited by members of the family Urceolariidae. 



The branchiobdellids. — Branchiobdellids may occur in large 

 numbers on a single adult crayfish and there are few crayfish in the 



