i think thay are FW and breed in brackish. tho i could be wrong. Never heard of turning them 100% marine tho.
That is true of
Caridina japonica, which is not labeled as Ghost Shrimp but Amano Shrimp.
The common name Ghost Shrimp covers variety of shrimp in the Palaeomonidae family. In this family there are 4 genus (whats the plural of genus?) which are Macrobrachium, Palaeomonetes, Palaemon, and Leander. Palaemon, and Leander are marine water shrimp. Macrobrachium are freshwater and do not make there way into the aquarium trade often. Palaeomonetes often makes there way into the aquarium trade. The Palaeomonetes group are a freshwater to brackish shrimp, and two shrimp in this genus are most common in your LFS.
Palaeomonetes kadakensi is exclusive to freshwater and is usually found in the United States while
Palaeomonetes pugio is brackish to marine and is usually found in England and the rest of Europe. There are exceptions of this "rule" as many
Palaeomonetes pugio are found in the United States and some
Palaeomonetes kadakensi are found in Europe.
This is why many brackish hobbyists have ghost shrimp survive in their aquariums despite their owners thinking they are freshwater.
EDIT: Note that Palaemon and Leander
sometimes make their way into the aquarium trade, but chances are the ghost shrimp in your LFS (Regardless of location) are either of the two most common Palaeomonetes.