Morphological and Habitat Variability in 
Gammarus minus Say (Amphipoda: Gammaridae) 
James L. Gooch 
Department of Biology, Juniata College, 
Huntingdon, Pennsylvania 16652 
AND 
Jeffrey S. Wiseman 
Department of Biology, Muhlenberg College, 
Allentown, Pennsylvania 18104 
ABSTRACT. —The surface dwelling ecotype of Gammarus minus Say in 
the central Appalachian Mountains varies in morphology partly in con- 
formance to a habitat scale from fully open to borderline hypogean. 
Characters investigated were eye facet density and relative lengths of eye 
and three appendages to body length. There also is considerable inter- 
demic variability not related to habitat scale. This is inferred to be due 
to isolation of some demes, which have undergone differentiation, and 
to gene flow among others, which has prevented full ecotypic dif- 
ferentiation. 
INTRODUCTION 
Geographical variation in morphology within species of gammarid 
amphipods is known to occur in Gammarus minus (Cole 1970, Minckley 
and Cole 1963, Holsinger and Culver 1970); G. bousfieldi (Minckley and 
Cole 1963); G. oceanicus (Croker and Gable 1977); G. pulex (Pinkster 
1971, 1972); and Crangonyx antennatus (Dickson 1977). Intraspecific 
variation in freshwater species has been linked to diet, light, current 
velocity, substratum, competing and predatory species, and genetic drift. 
This study examines interdemic morphological variation in epigean 
populations of Gammarus minus Say. This species ranges from the mid- 
Atlantic piedmont westward through the middle Appalachian Moun- 
tains, Interior Low Plateaus, and portions of the Mississippi Valley and 
Ozarks uplift, reaching peak abundances in caves, springs, and springfed 
streams underlain by carbonate rocks (Holsinger 1976). Nine popula- 
tions were examined, seven from Huntingdon and Centre counties in cen- 
tral Pennsylvania, and one outlier population each in Virginia and West 
Virginia. Our investigation was directed to the question: is morphological 
variation systematically related to habitat variation? Habitats vary in 
substratum, velocity, macrofauna, and many other attributes. This study 
focuses on variation in habitats grading from open surface, i.e. fully 
epigean, to cave associated or borderline hypogean. The study by 
Holsinger and Culver (1970) indicated that the morphology of Gammarus 
minus is particularly sensitive to this habitat spectum. All future 
references to Holsinger and Culver will be to this 1970 paper. These 
authors reported differences in eye shape and size and ratio of appendage 
to body length in cave and surface dwelling populations in the mid- 
Appalachians. Shoemaker (1940) earlier recognized a distinct cave form 
Brimleyana No. 4; 133-147. December 1980. 133 
