The western North America leech distributions was studied to assess this aquatic fauna diversity in widely dispersed and arid habitats during the last 10 Ka (thousand years). The leech distributions today may also reflect habitats some 10 Ma (million years) ago, with different drainages, climate, and topography, lacking precise geological events for leech barrier crossings, and correlated with other aquatic fauna distributions.
Western North America fish species have been extensively studied to determine aquatic connectivity among these dispersed habitats. The recession of the last continental glacier (10 – 15 Ka) revealed colonization from the southern Mississippi River to the mouth of the Mackenzie River and from the Columbia River and the Mackenzie River to British Columbia coastal drainages. The leeches, as fish, illustrate similar colonization of these same post-glacial habitats. Each regional section discusses the regions geology, the geological effects on aquatic fauna, and the geological effects on the leech distributions.
This study examines the geological processes to understand more ancient distributions as applied to mollusks and fish. The most classic is the Pliocene and Miocene “Fish hook” distribution between Bonneville Basin and Snake River and the western Great Basin and Pacific Coast drainages. Equally old as the “Fish hook”distributions, the southern route includes the western Great Plains, Rio Grande River and northern Mexico through southern Arizona to southern California. The Bonneville Basin, Snake River, and upper Green River was intertwined with aquatic colonization by fish with selective barriers for the fish host with mussel glochidia. The leech distributions are similar to the fish and mollusk distributions, with different sets of selectivity. Within the late Miocene time, the upper Colorado River adjoined the Gulf of California through the Grand Canyon, the upper Green and the lower Green through the Uinta Range, and the Snake adjoined the Columbia River.
With purported leech fossils found in Jurassic Europe and Silurian North America, one can suggest that leeches at one time or another have been on every continent, their distribution is a result of continental drift. Sister taxa have distributions with genetic based clade distributions on Euro-North American (Erpobdellidae) and South-North America (Helobdella) continents, suggesting isolation by continental separations. The present leech populations and distributions are a result of geological and climatic changes, with widespread abundant populations, widespread and restrictive populations, and populations isolates with possible extinctions.
The leech distributions will be discussed within these geological patterns. Leech taxonomy and Nearctic continental distributions are discussed in Section I. Postglacial mobility of the leeches to high elevations (Section II) and into northwest North America (Section III) are discussed. Section IV describes the western United States drainage basins distributions: the United States Pacific Coast and Columbia-Snake Rivers drainages (Section IVA), the Great Basin (Section IVB), the Colorado River basin (Section IVC), and the western Great Plains (Section IVD). Each section will have a discussion of the how geography and geology has affected aquatic fauna distribution. A summary discussion (Section V) of western United States leeches concludes this paper, illustrating the different distributional patterns within a geological frame.
Key Words: Hirudinida, leeches, western North America, paleogeography, drainage basin distributions, aquatic fauna