More than 50 families of crab, numbering thousands of species, thrive in practically all parts of the globe. Most crab species are marine and live in salt water or the brackish waters of bays, lagoons and river deltas. A relatively small number have adapted themselves to completely freshwater life-cycles and even fewer varieties have become solely land-dwelling.

Of the over 200 freshwater crab species found throughout Asia, Japan hosts only two: the sawagani (marsh crab) and the mokuzugani (mitten crab).

The sawagani — written with the character sawa, for marsh or mountain stream, and the character kani, meaning crab — is a true freshwater crab found all over the Japanese archipelago, on the Korean peninsula and the island of Taiwan. The diminutive 3-cm adult sawagani have also been spotted as far away as a lake in Las Vegas, Nev., where the U.S. Geographical Survey reported the first sighting early this year. (The origin of these nonnative crabs was eventually traced back to the tank of a local sushi establishment.)