Crabs live in the water and on land.
The jurists differed as to whether it is permissible to eat them. The Hanafis and Shafa`is prohibit that, whereas the Malikis and Hanbalis permit it.
1.The Hanafis prohibit it on the basis of their guideline that nothing from the water is permissible except fish only, so it is permissible to eat fish, except those that die and float on the surface. In their view, everything except fish is filthy.
2.The Shafa`is prohibit it because it is regarded as filthy in their view, as ad-Dumayri and al-Khatib ash-Sharbini said in Mughni al-Muhtaj, 6/146.
An-Nawawi (may Allah have mercy on him) said in Rawdat at-Talibin (3/275): Frogs and crabs are prohibited according to the most well-known view. End quote.
It says in Al-Majmu` (9/32): The second type is that which lives in the water and also on land; this includes waterfowl such as ducks, geese and the like. They are permissible as noted above, but they are not permissible if found dead, with no difference of scholarly opinion regarding that; rather it is stipulated that they be slaughtered properly..
Shaykh Abu Hamid and Imam al-Haramayn included frogs and crabs in this category, but they are prohibited according to the correct view, as stated in the religious texts, and this was stated definitively by the majority. There is a weak view concerning them which says that they are permissible; al-Baghawi narrated that concerning crabs from al-Halimi. End quote.
3.The Malikis say that it is permissible to eat them (crabs) without slaughtering them.
Ibn Buzayzah (may Allah have mercy on him) said: Aquatic creatures are of two types, one of which spends a great deal of time on land, such as turtles and crabs. There is a difference of opinion concerning them: are they to be treated like aquatic creatures, because the water is their place of origin, or are they to be treated like land animals, because they spend a great deal of time on land? According to Malik they are pure and permissible, and there is no need to slaughter them.
Ibn Nafi` said: They are prohibited and impure, if they die of natural causes.
End quote from Sharh at-Talqin, 1/243.
4.The Hanbalis are of the view that it is permissible to eat crabs, if they are slaughtered properly.
Al-Bahuti (may Allah have mercy on him) said in Kashshaf al-Qina` (6/204): Whatever lives in the sea and on land, like otters, seabirds, turtles, crabs and so on, do not become permissible when captured except by being slaughtered, because they live on the land, so they should be treated like land animals, to be on the safe side. Ahmad said: We slaughter otters [for food] and I do not see anything wrong with slaughtering turtles. End quote.
Slaughtering crabs means doing to the animal that which will kill it, by stabbing it in any part of its body, such as its neck. End quote.
It was narrated from Ahmad (may Allah be pleased with him) that it is permissible to eat them without slaughtering them.
It says in Al-Mubdi` (8/22): It was narrated from him concerning crabs and other aquatic creatures that they are permissible without being slaughtered, because crabs have no blood. Ahmad said: There is nothing wrong with crabs. He was asked: Should they be slaughtered? He said: No. That is because the point of slaughter is to drain the blood and make the flesh more edible by removing the blood. But in the case of that which has no blood, there is no need to slaughter it.
Conclusion: whatever lives mostly in the water and also lives on land, such as seabirds, turtles, and otters, it is not permissible to eat them except after slaughtering them. This is the correct view according to our madhhab.
However, the more correct view regarding crabs is that it is not permissible to eat them except after they are slaughtered.
End quote.
What is most likely to be the case is that it is permissible to eat crabs, because of the general meaning of the texts which say that what is in the water is permissible, and we do not accept the view that they are to be regarded as filthy.
According to the view that they are permissible, to be on the safe side they should be slaughtered, as is the view of the Hanbalis, so one should do whatever will kill the animal.
The easiest and quickest method should be chosen, so if that means freezing the crab, there is nothing wrong with that, otherwise it may be stabbed in its neck.
It is not appropriate to cut off any part of it when it is still alive.
If it is not found alive, there is nothing wrong with eating it, based on the view of the Malikis and what was narrated from Ahmad (may Allah have mercy on him).
And Allah knows best.