Answered by Sidi Mostafa Azzam
In a previous answer, Sheikh Amjad mentioned that eating aquatic animals (meaning those that do not live outside of water) is permissible. He also mentioned that it is not permissible to eat crab. What, then, is the ruling on eating crab that lives only in water, as is the case with many crabs that are eaten? For example, the Blue Crab, as far as I know, is the most commonly eaten crab and is entirely aquatic.
As Sheikh Amjad mentioned, animals that are entirely aquatic are permissible to eat (this means whether crabs, lobsters, prawns, sharks, or anything else), except those that are poisonous (or harmful). The impermissibility to which Sheikh Amjad’s answer is referring is regarding crabs that do live outside of water (for example, shore crabs and land crabs), as is understood by the word saratan in the Arabic.
As for other crustaceans that live only in water, never have I heard from Sheikh Amjad, or any other Shafi’i, say that they are forbidden.