@farmerman,
Muddies (mudcrab) are tropical salt water/tidal species FM.
What many in aust would call a crayfish or simply a cray
Freshwater crayfish:
A large species - Murray cray is the second largest freshwater crayfish in the world. To greater than 150 mm carapace length.
(The largest freshwater crayfish is the giant Tasmanian freshwater crayfish Astacopsis gouldi which grows to at least 400 mm overall and 3.6 kg and has been recorded at over 6 kg.)
http://www.nativefish.asn.au/spiny.html
Yabbie (Freshwater): There several types of yabbie but really they are just small freshwater crayfish. I think its just a regional name, they grow to abiout the length finger tip to heel of palm.
All the above are good eating. Many Aussies regulaily enjoy them. Yabbies are farmed in an adhoc way from farm dams.
The freshwater crays have a size and bag limit applicable. Seasonal catch applicable. fishing licence required.