It depends on whether the restaurant knew you had a problem with crab and knew or should have known they were serving you crab. If the answer to both questions is yes, then they could be liable.
Assuming you had an allergic reaction to the crab and that is the reason for the hospital visit, the restaurant might be liable for negligence if they served a food different than what you ordered. You do not indicate what the long term consequences of the reaction are, so their liability may be for the medical expenses and any harm caused.
It depends why they served you the wrong food; did you tell them no crab, did they just mix the order and you did not notice it was different, etc. You probably have a case but it is not worth much more that the medical bills, which they might be willing to pay even if they deny fault.
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