Well, on a zero-carb/carnivore diet, the carbohydrate content of meat and seafood is so small that it can be ignored. The point is to avoid plants and stick to animal foods, and any incidental glycogen is not a concern.
As far as a ketogenic diet is concerned, I just checked various types of crab in the U.S. Department of Agriculture’s database, and the amount of carbohydrate per 100 g of crab is minuscule (0.04 g). There is considerably more carbohydrate in scallops, apparently (2-3 g/100 g), but are you really going to be eating a kilo of scallops, that you need to worry?
If it is a concern to you, simply count the carbohydrate as part of your daily limit, just as you would any other carbohydrate.