Systematics of the Subterranean Amphipod Genus \u3ci\u3eStygobromus\u3c/i\u3e (Crangonyctidae) in Western North America, With Emphasis on Species of the \u3ci\u3eHubbsi\u3c/i\u3e Group

This dissertation presents a study of the systematics of the subterranean amphipod genus Stygobromus of family Crangonyctidae in western North America. A total of 53 species of Stygobromus from western North America (including one species from Wisconsin) are recognized, 28 of which are new to science. All species are described and illustrated using external morphological features. A key to the western North America species of Stygobromus is given, as well as a summary of habitats of the species. Cladistic analyses are conducted using 32 external morphological characters and two categories of outgroup choices: “all-zero” theoretical ancestor and three non-Stygobromus species. The consensus trees of unordered/unweighted and ordered/partially weighted are developed. The results of both categories support one monophyletic species cluster, which includes most species in California. Two loose constant congruent species clusters were also recognized in the two category runs: a California-Oregon-Washington species cluster includes S. imperialis. S. rudolphi, S. gallawayae, S. latus, S. grahami, S wengerorum, S. harai, S. gradyi, S. elliorti, S. mysticus , and S. hubbsi; a Colorado-Idaho-Montana species cluster includes S. urospinatus, S. fontanalis, S. pennaki, S. coloradensis, S. canadensis, and S. idahoensis. The number of species in each of the two loose constant clusters of the two categories is variable. The species diversity of Stygobromus decreases from east to west in North America, possibly resulting, from geologically more recent landforms in western North America. In addition, many habitats presently occupied by western species of Stygobromus may not be older than the Pleisiocene. Some lava tubes in Oregon and Washington are even more recent. This explanation may also account for close morphological similarity of hubbsi group species in western North America