Seafood in the four madhabs
In the name of Allah, the most Beneficent, the most Merciful.
According to Imam Ahmed Ibn Hanbal (RA) And Imam Shafi (RA) all living creatures that live in the sea are halal whether they are in the form of fish or not. Imam Malik (RA) has said that all living creatures are halal except for a eel. Based on this oyster, shrimp, lobsters and crabs will be permissible to consume. (Hidayah p.442 v.4 & Al Lubab p.533 v.7)
Their evidences are the following verse in which Allah (SWA) has said; “The game of the sea and its eating has been made lawful for you.” (Surah Maaidah v.96)
Furthermore, Saaiduna Abu Hurairah Radiallahu Anhu narrates that a man asked the Prophet of Allah Sallallahu Alahi Wasalam “O Prophet of Allah Sallallahu Alahi Wasalam, we travel on the sea and take a small quantity of water with us. If we use this for ablution, we would suffer from thirst. Can we perform ablution with seawater? The Prophet of Allah Sallallahu Alahi Wasalam replied; its water is pure and what dies in it is lawful food.” (Sunan Abi Dawud p.13 v.1)
In the Hanafi Fiqh, animals that are born and live in the water are unlawful to consume with the exception of a fish.
Saaiduna Abdullah Ibn Umar Radiallahu Anhu narrates that the Prophet of Allah Sallallahu Alahi Wasalam said; “Two types of dead meat and two types of blood have been made lawful for our consumption; the two dead meats are: fish and locust and two types of blood are liver and spleen.” (Sunan Ibn Majah)
Furthermore, there is no mention of the Prophet of Allah Sallallahu Alahi Wasalam or the companions consuming sea animal besides a fish. (Dars Tirmizi p.280 v.1)
However, with regards to prawns in the Hanafi fiqh there are differences of opinions. The question is whether they are considered from the fish family or not.
According to the Arabic Language (lughat) prawns are considered to be like a fish. Therefore, Maulana Ashraf Ali Thanwi (RA) has said it is permissible to consume prawns. (Imdadul Fatawa p.103 v.4)
However, contemporary zoologists do not consider prawn be a fish, as it does not have a spine (Aapke Masaail p.250 v.4)
Rulings of such issues are not based on technical research but upon the understanding of the common people and as it was mentioned earlier that it will be according to the understanding of the scholars of the Arabic Language and as they have considered it to be a fish thus, lawful to consume. (Fiqhi Maqalat p.217 v.3)
With regards to your question, the advice given by my respected teachers is that she can continue eating lobsters, crabs etc… However, I would like to stress this is a special ruling for this particular situation and is not for every individual.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham