I know someone that was part of a group of marine zoologists from University of Hawaii that were the first to study the marine life in Bikini and Marshall Islands in the mid-70's. I cannot find anything published publicly about this study but was told that even then they didn't find any evidence of mass die-offs or mutations. Certainly the bombings effected corals just from the physical damage, but the types of extensive radiation expected were not noted. Granted, this is only second-hand information but I have reason to have confidence in their retelling.