Freshwater crayfishes are taxonomically distributed among three families; two Northern Hemisphere families, Astacidae and Cambaridae and one Southern Hemisphere family, Parastacidae. There are two centers of species diversity for freshwater crayfishes. The first is located in the Southeastern United States where some 80% of the cambarid species can be found. The second center of diversity is in Victoria, Australia; housing a large proportion of the parastacid species. Freshwater crayfishes naturally occur on all of the continents except Africa (Figure 1). The Astacidae are distributed West of the Rocky Mountains in the Northwest United States into British Columbia, Canada and in Europe. The Cambaridae are found in the Eastern United States and south through Mexico. The Parastacidae are distributed in Australia, New Zealand, South America, and Madagascar.
Evolution and Systematics
Discussion of Phylogenetic Relationships
The phylogenetic relationships among the freshwater crayfish families and theirrelationships to lobster-like ancestors has been of considerable debate for at least 100years. Two alternative hypotheses have been proposed for the origins of crayfishes. Thefirst supposes a diphyletic origin of astacoids and parastacoids suggesting independentinvasion of the freshwater habitat (Huxley, 1880). This idea is supported by the twocenters of diversity in the northern and southern hemispheres and by a number ofmorphological features (Hobbs, 1974). However, Ortmann (1902) argued for a monophyleticorigin of the crayfishes. This position has recently been supported by spermultrastructure characteristics (Jamieson, 1991) and by embryonic characters (Scholtz,1993). Because of this ongoing debate, the positioning of Parastacidae is shown asunresolved.