Answer
(Fatwa: 442/372/B=1431)
According to the Hanafi fiqh only fish is lawful among sea animals, while all other sea animals and insects are unlawful. Crab is also unlawful according to Hanafi fiqh. Almighty Allah says in the holy Quran:
حُرِّمَتْ عَلَيْكُمُ الْمَيْتَةُ وَالْدَّمُ وَلَحْمُ الْخِنْزِيرِ . [المائدة : 3]
Prohibited for you are: carrion, blood and the flesh of swine.
The holy Quran makes no differentiation between terrestrial and nautical animals. And the holy Prophet Muhammad (صلى الله عليه وسلم) has explained two types of dead animals.
احلت لنا الميتتان والدمان ، السمك والجراد .
The holy Prophet Muhammad (صلى الله عليه وسلم) said: Two types of dead animals and two types of blood have been made lawful for us. The types of dead animals are fish and locusts.
While crab, frog and snake all these are impure things, hence Almighty Allah declared them unlawful. The holy Quran says:
وَيُحَرِّمُ عَلَيْهِمُ الْخَبَآئِثَ . [الأعراف : 157]
And he makes unlawful for them impure things.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband