A prawn is a shrimp. Prawn are predators and eat most anything they can catch. There are many species of prawns, just like crayfish. The type I've raised are M. rosenbergii. They are the most common ones bred in the US. There are also saltwater varieties, like the tiger shrimp.

Because they fill the same niche as the crayfish, they are natural enemies. Fortunately for the crayfish, they need warm water to survive, so their northern range is limited to the deep south. They also require saltwater for the fry to survive, so they don't live far from the coast unless farmed.

There is one species that can reproduce in fresh water. that is M. amazonicum, the Amazon River Prawn. Here is a pic. I was not successful at breeding them. I had no males.