Crayfish, crawfish, and crawdad are interchangeable terms for a large group of freshwater crustaceans (not fish) resembling small lobsters and living in many regions throughout the world. Crayfish and crawfish are renderings of regional pronunciations of the same word, descended from the Middle English crevise (-vise became –fish), which in turn has Old French and Germanic origins.
Crawfish is preferred in the U.S., while crayfish is preferred in most other English-speaking areas. Crawdad is prevalent in parts of the U.S., and cray is frequently used in Australia and New Zealand.
Examples
Without the guest workers, job losses would trickle to the alligator and crawfish farmers. [Wall Street Journal]
The white clawed crayfish, which lives in freshwater, has suffered a significant decline since the 1970s. [BBC News]
There is no reverential silence. We finish the “snacks”, all of which are shared with a plate of freshwater crayfish. [Irish Times]
Maybe wild-caught snakes would be gamier, more like alligator or crawdad than tough, chewy fish. [NY Times]
Female crayfish are in berry, but packhorse crays are turning up in some catches. [New Zealand Herald]