I just got the current Gourmet mag and there was a paragraph (a sad, small, lonely paragraph - I wanted a whole article!) on 'Sashimi' grade fish.
They mentioned that if the flesh of the fish is iridescent (opalescent...) it is *dangerous* and indicates poor handling.
We bought some frozen white tuna steaks and when I thawed them (not to use as sashimi... they were to be grilled) sure enough they had this strange sheen.
Is it really *dangerous*? They were purchased from a very reputable source. I would hate to conclude that the store is selling *dangerous* fish.