Answer
(Fatwa: 2325/1915=B/1429)
No sea animal is allowed except fish; therefore it is not lawful to eat crabs. If fish is given unlawful things to eat, then it is allowable to eat such fish but it is better to avoid.
الدود الذي يقال: جهينگا حرام عند بعض العلماء لأنه يشبه السمك فإنما يباح عندنا من صيد البحر أنواع السمك وهذا لايكون كذلك وقال بعضهم حلال لأنه يسمى باسم المسك (مجموعة الفتاوى 2:297)
ان الذي يعيش في الماء فجميع ما في البحر يحرم أكله (عالمگیری 5:357)
There are different opinions about eating prawns, some say it is a kind of fish, so according to them, it is lawful to eat it, while some regard it a kind of crab, so they do not allow eating it, therefore it is better to avoid it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband