70 



Julian J. Lewis 



Fig. 2. Palmar margin of propodus of male first pereopods of Caecidotea bicren- 

 ata whitei from Roaring River Shrimp Pools, Mammoth Cave, KY: (a) 8 mm 

 individual, C. meisterae form; (b) 6 mm individual, C. whitei form; (c) 4 mm 

 individual, immature. 



Caecidotea whitei is redefined as a subspecies of Caecidotea bicre- 

 nata due to the slight morphological differences that distinguish them, 

 and the lack of dispersal barriers that might provide reproductive isola- 

 tion along the contact between C. whitei and C. bicrenata. Apparently 

 the ranges of C. b. bicrenata and C. b. whitei contracted at some time in 

 the past, then range expansion followed. If this occurred, some secon- 

 dary contact phenomenon might be expected, either hybridization or 

 character displacement. Along the contact between the two subspecies, 

 occasional populations occur in which the lateral process of the male 

 second pleopod endopod tip (Fig. 3) is either vestigial or absent. Con- 

 sidering the close morphological similarity of the two subspecies, it 

 seems more likely that this phenomenon is caused by breakdown of any 

 isolating mechanisms developed, rather than reinforcement. However, 

 without experimentally crossbreeding individuals taken from popula- 

 tions of each subspecies it is impossible to say with certainty that 

 hybridization is occurring. Furthermore, the presence of specimens lack- 

 ing the lateral process in one stream in Mammoth Cave (Mystic River) 

 complicates the situation. Mammoth Cave is relatively distant from 



