Because there are so many packages of ground turkey roaming around in the wilds of Africa… 
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A crayfish is just an aquatic bug when you get down to brass tacks
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I do not see a crayfish posing any kind of hazard to a sav. The crush power of their jaws is not insignificant. They also have a pretty tough hide.
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The possibility of parasites is probably pretty low as commercial ones are farmed. It is not an impossibility, but then again, any time you are feeding raw meat of any kind you run that same risk.
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I would imagine that an occasional crayfish would be a nice addition to a diverse diet, but I would not use them as any kind of staple.