http://www.thekitchn.com/fresh-or-frozen-shrimp-buy-fro-146270
"In the display case, fresh shrimp always look more enticing — and, well, fresher — than frozen, but according to the LA Times Test Kitchen, frozen shrimp is actually the better choice.
Because nearly all the commercial shrimp we buy are individually quick-frozen shortly after being caught, the "fresh" shrimp in the seafood case is actually just thawed frozen shrimp. Non-frozen shrimp are less convenient and usually more expensive than frozen and because there's no telling when they were thawed, they can also be less fresh. Oh, the irony."