Assalamualaikum ustaz. What is the ruling on eating stone crab? Thank you.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
We begin with the statement of Allah SWT:
وَيُحِلُّ لَهُمُ الطَّيِّبَاتِ وَيُحَرِّمُ عَلَيْهِمُ الْخَبَائِثَ
“And makes lawful for them the good things and prohibits for them the evil,” 
This verse shows that it is made permissible anything that is good and delicious naturally by people, except if there is evidence that stated its prohibition. Likewise, anything that is considered naturally disgusting is prohibited, such as carcass and blood, and those who eat them are considered fasik. In other words, everything that is considered filthy and disgusting is prohibited, for it is the cause of a disease. Then, anything that is harmful is prohibited. Thus, everything that is naturally considered disgusting is originally prohibited except if there is evidence stating otherwise. 
Definition of Stone Crab
Crabs are animals that are covered by hard shells and have jointed legs. It lives in shallow waters along the coast as well as in deep waters. There are about 4500 different types of crabs.  Usually, stone crab refers to a type of crab found in Florida. It can also be found in the middle of the ocean and coastal waters from Belize to North Carolina. This rock crab consists of two species namely Menippe mercenaria and M. adina. However, M. adina is often seen as a subspecies of M. mercenaria. The hard shell or carapace of a rock crab is about 4 inches wide, while its length is about 3 to 3.5 inches. In addition, this carapace is also smooth, oval and convex. Female crabs have a larger carapace than male crabs, while male crabs have larger claws than female crabs. These crabs live at the bottom of bays, rock jetties and oyster reefs. It can also be found in limestone and seagrass beds. These crabs love to dig and seek shelter from their predators. Yet the baby crab does not spend much time digging, instead, it hides in the seaweed or in the middle of the rocks. Baby crabs at the larval stage feed on small zooplankton. Later, it grows up with opportunistic carnivore behaviour but by switching to larger food sources. While adult rock crabs usually feed on hard-shelled clams, small mollusks, acorn barnacles, conch and various types of crustaceans. 
Before discussing this matter further, we should know that some of our previous scholars named this crab in their books as water scorpion (عَقْرَبَ الْمَاءِ) and this crab is also named the Father of the Sea (أَبُو بَحْرٍ). 
Aquatic / Sea animal
Basically, an aquatic animal is an animal that cannot survive outside of water. When it is not in the water, it is just like a slaughtered animal or it can survive only for short time – on land.  All sea animals are permissible to be eaten without being slaughtered except those where there is evidence stating otherwise or are harmful. Allah SWT states:
أُحِلَّ لَكُمْ صَيْدُ الْبَحْرِ وَطَعَامُهُ مَتَاعًا لَّكُمْ وَلِلسَّيَّارَةِ
“Lawful to you is game from the sea and its food as provision for you and the travellers,” 
Likewise, there is a narration from Abu Hurairah RA, where Rasullullah PBUH said:
هُوَ الطَّهُورُ مَاؤُهُ، الحِلُّ مَيْتَتُهُ
“Its (the sea) water is purifying and its dead (animals) are lawful (to eat).” 
Furthermore, Syeikh al-Islam Zakaria al-Ansari cited the opinion of Imam al-Mawardi that become the dhawabit (guideline) in determining the category of a certain animal when both land and sea coincides are:
مَا يَجْمَعُ مِنْ الْحَيَوَانِ بَيْنَ الْبَحْرِ وَالْبَرِّ إنْ كَانَ اسْتِقْرَارُهُ بِأَحَدِهِمَا أَغْلَبَ وَمَرْعَاهُ بِهِ أَكْثَرَ غَلَبَ عَلَيْهِ حُكْمُهُ
“Animals where both land and sea coincide (they can live in both) if it can remain (continuously) alive in one of them according to the norm and it is alive and spends more of its time there. Then, the ruling follows the norm.”
Hence, any animal that couldn’t live except in water or when it exits the water, it is just like a slaughtered animal, then it is permissible to be eaten, no matter its situation it will still be considered as an aquatic animal. Then, if an animal isn’t inclined to any of the two (land or water), then its ruling is just like land animal according to the sahih opinion and it is impermissible to eat it except if it is slaughtered if it is included as an animal that can be slaughtered. 
Moreover, Imam al-Syafi’i also stated that in determining an animal is in the category of which hunted animal, the look whether it lives more on land or in the sea. Also included in the category of aquatic animals are animals that live in wells, swamps, freshwater, saltwater, shallow water and deep water and others. 
The Ruling of Eating Stone Crab
Ibn Hajar al-Haitami stated that some scholars are of the opinion that crabs are permissible to be eaten because they can’t survive for a long time on land, however, the sahih opinion- according to madhhab Syafi’I – is it is prohibited.  The reason is it is considered filthy and disgusting and harmful (poisonous). 
Moreover, Imam al-Nawawi stated that there is a weak opinion that ruled frogs and crabs permissible. Likewise, al-Baghawi cited from al-Halimi regarding the permissibility of crabs. Then, Imam al-Nawawi stated that the sahih and final opinion is that all aquatic animals’ carcass is permissible – to be eaten – except frogs. Hence, it can be understood that what is stated by scholars of madhhab Syafi’I or some of them said that turtles, snakes and nasnas are prohibited that live in water other than the sea.  In other words, there is a possibility that there are other factors that leads to the prohibition such as evidence that prohibits it, it is harmful to the person that consumes it or others.
This is clearly understood through the statement from Dr Muhammad al-Zuhaili who said that frogs are prohibited to be eaten because there is evidence that prohibits killing it. While snake and crabs are due to them being poisonous and both are considered disgusting. Furthermore, crocodiles are prohibited to be eaten because they hunt using their incisors. 
Some scholars who ruled crabs permissible are scholars from madhhab Maliki  who stated that it is permissible to eat frogs, insects, crabs and turtles because there is no evidence that prohibits them from being eaten. As for scholars who prohibit them is because the animals are considered disgusting and filthy must base their opinions on syarak evidences. Thus, certain animals can’t be prohibited just because they are naturally considered filthy and disgusting when there is no evidence supporting this. According to madhhab Hanbali, every living animal on land that originates from aquatic animals is impermissible to be eaten except after it is slaughtered such as waterfowl, turtles, seals, except creatures that don’t have red blood such as crabs, according to the opinion of Imam Ahmad. 
Thus, we are inclined to the opinion that states crabs are included as permissible animals to be eaten as long as there is no clear evidence that prohibits them and isn’t harmful to anyone who eats them. Furthermore, after we analyze the opinion of some scholars who prohibit them, there are two reasons for it, which are that they are harmful (poisonous) and considered disgusting. Thus, if these two reasons don’t exist, then it returns to its original ruling which is it is permissible. This is in accordance with the Islamic legal maxim:
إِذَا زَالَتِ العِلَّة زَالَ الحُكُم
“When the ‘ilah (reason) is gone, then the ruling also cease to exist.”
Especially, when the matter is strengthened by another opinion of Ibn Qasim who states that Umum al-Balwa occurs  that danilas (a type of snail) is eaten in Egypt and crabs are eaten in Syria.  It is the same in Malaysia, our own country where crabs are aquatic animals that are normally eaten by the locals and aren’t considered disgusting.
According to the above discussion and deliberation, we conclude that the Stone crab is included as an aquatic animal that is permissible to be eaten according to the general evidence that permits all aquatic animals as long as there isn’t any evidence that shows its prohibition and it isn’t harmful. Furthermore, the ‘uruf and custom in our country show that crabs are eaten and aren’t considered disgusting.
However, if the stone crab meant are crabs that are usually found in rock crevices at the beach and others, then we are also inclined towards the opinion which ruled it permissible on the condition that there is no evidence that shows the consequence, such as it is poisonous and others.
Alhamdulillah, recently, the issue of eating crabs is not something that is odd or weird, however, it has been the norm for most people. Interestingly, although we are in Malaysia, but now, we can also get to taste crabs that live near the Arctic and Antarctic oceans. For example, the Alaskan crab. Thus. Issus such as this has been applied in the lives of most societies.
May Allah SWT grant us all a clear understanding in religion. Amin
 Surah al-A‘raf : 157
 See Mafatih al-Ghaib, 15/381.
 Translated by Dewan Bahasa dan Pustaka, Ensiklopedia Dunia, (Malaysia: DBP, First Edition, 2005), 11/183.
 See https://a-z-animals.com/animals/stone-crab/. Accessed on 16th November 2021.
 See Tuhfah al-Muhtaj, 9/378; Mughni al-Muhtaj, 6/146.
 See Nihayah al-Muhtaj, 8/150.
 Surah al-Ma’idah: 96.
 Narrated by Abu Daud (83); al-Tirmizi (69); al-Nasa’i (59)
 See Asna al-Matalib, 1/566.
 See al-Umm, 2/130; 2/199.
 See Tuhfah al-Muhtaj, 9/378.
 See Mughni al-Muhtaj, 6/146; See Tuhfah al-Habib ‘ala Syarh al-Khatib, 4/326; Hayah al-Hayawan al-Kubra, 2/28.
 See al-Majmu‘ Syarh al-Muhazzab, 9/32-33.
 See al-Mu‘tamad fi al-Fiqh al-Syafi‘i, 2/548-549.
 See Hayah al-Hayawan al-Kubra, 2/28.
 See al-Fiqh al-Islami wa Adillatuh, 4/2799-2800.
 Something that has become the norm.
 See Mughni al-Muhtaj, 6/147.