Differences in dietary restrictions?

There is actually a difference of opinion on shellfish, and seafood in general, it is agreed upon that all fish with scales are Halal, As for fish without scales and shellfish it's debateable, and ranges the spectrum between Only fish and shrimp, to everything, minus Eels and Sharks(For all groups). Depending, on which madhab one follows. However, since it is haram to eat anything that is Boiled or electrocuted to death, and lobsters and some other shellfish are at the very least disliked for that reason and are therefore not eaten very much. The point is there is nothing in the Qur'an or the Sunnah that says that eating a crab or shellfish will give you bad deeds, but if you make it suffer to kill it you will take bad deeds.