Limit to 12 oz./week. Authorities advise limiting most seafood to 12 oz per week to minimize mercury exposure, but there are benefits to moderate seafood consumption too. Seafood high in mercury like tuna should be limited even more, while seafood with the lowest mercury like wild salmon & sardines can be eaten more. See http://www.Babycenter.Com/0_eating-fish-during-pregnancy-how-to-avoid-mercury-and-still_10319861.Bc.
Doubt it's been stud. I strongly doubt it's been studied scientifically, but like anything else (particularly during pregnancy) moderation is the key. Don't do anything that could endanger your fetus, because you only get one chance with no "do-overs". After delivery you can go hog (or crab meat) wild.