There has been a khilafiyah opinion about the law of crab among scholars'.
The ulama 'who forbid it generally come to the conclusion of the understanding that animals that live in two realms, are haram to eat. For example, frogs, turtles and others. Usually people mention the term amphibian, or in terms of fiqihnya called barma''i. The problem of this amphibious animal prohibition we get one of them in the book of Nihayatul Muhtaj by Imam Ar-Ramli. There it is expressly mentioned the haram of animals that live in two realms. But actually, the problem of animal prohibition that lives in two realms, is also still disputed. Because there is no verse or hadith that mentions the prohibition of animals that live in two realms.
The second opinion is about the halality of crabs, either because they view the prohibition against animals that live in two realms is weak, also partially ensure, that crabs are not amphibious animals. this is the opinion of the scholars' scholars such as Atha'dan Imam Ahmad (see Al-Mughni 13/344 by Ibn Qudamah and Al-Muhalla 6/84 by IbnuHazm).
Conclusion
Opinion that the crab is not a two-nature animal proposed by many experts in the field of perkepitingan. Generally they make sure that crabs are not amphibian animals like frogs. Frogs can live on land and water by breathing with the lungs and skin.
But not so with crabs. Crabs just breathe with gills. Crabs can survive on land for 4-5 days, because the gills save water, so it can still breathe. But if there is no water at all, he dies. So the crabs can not get out of the water.
