SUBTERRANEAN AMPHIPOD STYGONECTES 157 



jority of collections of phreatobitic amphipods have been made. 

 Johnston (1964) stated further that no water-bearing structural 

 feature m the Washington area had a continuous areal extent of more 

 than a few miles. Moreover, in the neighboring Coastal Plain to 

 the east, ground water from shallow water-table wells originates as 

 precipitation in or near the local watershed or basin in which the 

 wells are situated. 



Collecting data indicate that populations of *S'. t. potomacus are 

 abundant and widely distributed in these shallow ground waters and 

 that diu-ing wet weather, when seeps and springs are rejuvenated, 

 ground-water amphipods are either forced out or possibly migrate 

 out of shallow subterranean biotopes. In consideration of the above 

 conditions, one must suppose that dispersal of the interstitial am- 

 phipod fauna of this area takes place near the surface and occasion- 

 ally, during periods of extremely wet weather, over short distances 

 on the surface. In this manner, contiguous populations of phreato- 

 bitic amphipods would be able to maintam at least minimal gene 

 flow over an extensive area. That this does take place is strongly 

 implied by distributional data. 



S. hayi, which is presently poorly known from a single spring in 

 Washington, D.C., is allied rather closely with the tenuis-allegheniensis 

 complex both morphologically and geographically. Similarities be- 

 tween these three species are illustrated by the possession of ventral 

 margmal spines on the fu'st abdommal side plates (a character which 

 has not been found in any other species of the tenuis group) and 

 possession of a proportionately long and somewhat enlarged second 

 antenna in larger males. In regard to the latter character, S. hayi 

 more closely resembles S. allegheniensis, since the second antenna of 

 this species, while rather long, never exceeds the first antenna in 

 length. Three structural differences, however, indicate divergence 

 of S. hayi away from the tenuis-allegheniensis line: more convex pos- 

 terior margins of the pereopod bases, lack of distal tines on the outer 

 apical spines of the telson, and more apical spines on the telson than 

 in any other species of the tenuis group, with the exception of S. 

 clantoni and S. ozarkensis. Despite these differences, S. hayi must be 

 regarded as evolutionarily closer to S. allegheniensis and S. tenuis 

 than to any other species in the group. The possibility of hybridiza- 

 tion between S. hayi and S. tenuis, alone, strongly suggests that the 

 ancestry of these species is not far removed genetically. If hybrid- 

 ization has occurred between these two species, as tentatively inter- 

 preted (see Systematics, re: population from spring between Suitland 

 and Forestville, Md.), it has undoubtedly been effected by the rapidly 

 expanding urbanization in the Washington, D.C. area in recent years. 

 Habitat disturbance could conceivably have led to the breakdown of 



