Not to take anything away from the tragedy and severity of Three Mile Island, Tjernobyl, and Fukushima, but there's plenty of other toxines in the seas that cause mutations.
I live in Stockholm, which is just south of the area which was hardest hit by Tjernobyl downfall (by rain), except for the immediate surroundings to the plant itself. For years wild game and berries were off-limits for human consumption, but we haven't really seen much of an increase in mutation levels in wildlife or vegetation. Likewise, wildlife has taken over the vacated Tjernobyl area, and visible mutations are rare even in moderately contaminated areas.
Moreover, deformed claws among crayfish and crabs is quite common. Quite possibly what you see in the pic is from a fight or predator attack rather than birth defect. Crabs can regrow a lost limb, so it could be a not yet complete regrowth of the right claw.
If you want to point fingers, perhaps take a look at modern chemicals as well.
And the probability for mutation is always higher than zero even in the absence of radioactive and chemical pollution, so a single occurence isn't enough to draw any conclusion. A study of next year's crabs will be much more relevant (and I'm sure there will be plenty of studies in the aftermath of the Fukushima accident).
