jakeb, in all likelihood the species you got from Wal-Mart was P. paludosus. They are the most common species raised for use as feeders. They are a solely freshwater shrimp but cannot tolerate cold temperatures as they are native to the deep south only.
I found this site...
http://www.fishhobbysupply.com/invertebrates.php Scroll to the bottom and they list grass shrimp and the supplier claims they are P. kadiakensis. They are 12 for $10.00. I have not bought from him, but if I cannot find another source I may. Hard to tell if they really are P. kadiakensis. Even harder to tell if like your Wal-Mart shrimp they are handled roughly and will mostly die. If you do buy some, let me know how they turn out...
