Attack Of The Crab Monsters (1957)

Starring Richard Garland, Pamela Duncan, Russell Johnson, Leslie Bradley, Mel Welles
Directed by Roger Corman
(actor & director credits courtesy IMDB.com)

When a team of scientists investigating atomic fallout on an island go missing, a new team is sent to continue their work, and discover the fallout has mutated land crabs into powerful giants. 

Although it's hard to take this film seriously, it's an entertaining ride, with a wonderful score by Ronald Stein, a likable cast, and good direction by Corman.  Charles B. Griffith's screenplay, in giving the crabs incredibly fantastic powers, and attempting to explain them scientifically, comes off at times as preposterous nonsense, but that's all part of the fun, and the giant crab mockups do look fairly convincing when crawling out of the sea.  Leslie Bradley, playing the film's nuclear physicist, really ties everything together with such a serious and urgent delivery of his lines, we can't help but believe in the crabs' dangerous abilities.  The rest of the cast, including future Gilligan's Island castaway Russell Johnson, Mel Welles (who affects an entertainingly thick French accent for his role), and young lovers and biologists Garland and Duncan give us characters worth caring about, and Corman and cinematographer Floyd Crosby frame the crab attacks effectively without revealing the limitations of their creature mockups.  It's one of my favorites among Corman's many productions.