Systematics of the freshwater amphipod genus Crangonyx (Crangonyctidae) in North America

The systematics of the amphipod genus Crangonyx of North America (north of Mexico) is revised, based on available collections (ca. 2240) and literature dealing with the genus. A grand total of 42 North America species of Crangonyx are recognized in the present study, 24 of them new to science. All species are described or redescribed and figured, utilizing external morphological features. Keys to both species groups and individual species are given. Phylogenetic trees are built using computer programs (PAUP, Hennig86, MacClade) based on 26 characters. Wagner parsimony produced 18 parsimonious trees and Fitch parsimony produced 45 trees. The consensus tree of both methods and a hypothesis phylogenetic tree of the North America species of Crangonyx are developed along with a discussion of Crangonyx phylogeny and character evolution. Six monophyletic species groups are recognized.; With respect to the origin of Crangonyx, putative ancestors of the genus probably originated as very old freshwater lineage dating back at least to the Mesozoic and possibly earlier. The ancestor of modern Crangonyx species possibly resembled C. forbesi and separated from the common ancestor of Synurella and Crangonyx somewhere on Laurasia. Species of Crangonyx have exploited a wide variety of surface and groundwater habitats in North America. Many of the spring/seep dwellers are apparently predated to life in subterranean waters. A detailed distribution map is given for each species with an interpretation of its biogeography. Almost all species of the genus occur in eastern North America, east of the Rock Mountains. The species of Crangonyx with distribution in glaciated areas also occur south of the southern extent of Pleistocene glaciation. Distribution patterns are analyzed and compared with phylogeny in an attempt to evaluate extrinsic barriers, dispersal limits and other geographic and geological considerations that might have a bearing on species ranges.; Eleven species of Crangonyx in North America are troglobites, many of which are found in karst areas in the Appalachians and Interior Low Plateaus regions. The origin of troglobitic members of the genus is attributed to active or passive invasion and colonization of subterranean waters by preadapted epigean ancestors, sometimes under climatic constraint and sometimes by adaptive shifts into new niches. Perhaps a few troglobitics species originated through peripheral isolation in subterranean waters from previously established troglobites.; Human introduction of pseudogracilis in Europe and C. floridanus in Japan is explained by the ballast water theory, which suggests that small aquatic organisms, such as certain species of amphipods, are transported in the ballast water of ocean-going ships.