CRABS.
Generally the scholars include this crab problem as an animal that lives in two realms. Animals that live in two realms are called barma`i animals. Like frogs, turtles, crabs, snakes, crocodiles, seals and the like.
In this case, the type of crab and others are indeed different opinions. There are at least three differences of views:
Ash-Syafi`iyyah and Al-Hanafiyah (Syafi'i and Hanafi schools)
They argue that these animals should not be eaten. Because it is considered to include the category of khabaits (dirty animals). One of the arguments they use is that the Messenger of Allaah 'alaihi wa sallam forbids to kill frogs. If it could be eaten, it would not be forbidden to kill it. This hadith was narrated by Abu Daud, Ahmad, Ishaq, Alhakim from Abdurrahman bin Utman at-Tamimi.Al-Malikiyah (Maliki School)
They argue that eating crabs and frogs, insects, turtles and other laws is permissible. And as long as there is no text / argument that clearly prohibits it, we cannot forbid a type of food or animal to eat.
And categorizing these animals as khabaits (dirty) cannot be done by the standards of each individual, because it will certainly be subjective. Because maybe there are people who don't feel that the animal is disgusting or dirty and there is also the opposite. So to prohibit it is not enough with it, but there must be a clear text. And according to Al-Malikiyah, there are no texts that prohibit explicitly eating these animals.Al-Hanabilah (Hambali School)
Whereas the scholars from Al-Hanabilah distinguished the problem. That all marine animals that can live on land are not lawfully eaten except by slaughtering them. Like water birds, turtles and seals. Except if the animal doesn't have blood like crabs.
Crabs according to Imam Ahmad bin Hanbal may be eaten because as sea animals that can live on land, crabs have no blood, so they do not need to be slaughtered. Whereas if the two natural animals have blood, then to eat it is obligatory by slaughtering it.