Question: What is the ruling on sea food? Are crabs and lobsters allowed?
    Answer: At the outset, it needs to be understood that the basic objective of Islam is purification -- both physical and spiritual. Therefore, it only prohibits things that are unclean and hinder purification. Certain edibles are also prohibited because of this very reason. 
    The second thing which needs to be appreciated is that in case of most edibles, human nature is well equipped to know for itself which food stuff is inedible^*. It is only in a very small sphere that human nature is unable to pronounce a clear verdict on edibles that are unclean, and it is in this sphere in which the Shari‘ah has guided man. Consequently, where this guidance is stated, the words of the Qur’an subtly allude to the fact that this guidance is being provided about edibles. The prohibition of animals or stuff which is non-edible is not mentioned for the simple reason that human nature has already pronounced them to be inedible:

Say: ‘I find not in the Message received by me by inspiration forbidden to a person who eats things which are edible, unless it be dead meat, or blood or the flesh of swine .’ (6:145)

    Human nature has always remained averse to eating beasts. No one generally eats carnivores (animals that feed on meat) like lions, tigers, elephants etc. Such animals were never liked by man for eating. On the other hand, herbivores (animals that feed on plants) like goats, cows and camels have always been relished by human nature. The meat of such animals has always been eaten. According to the Qur’an, the following four are expressly forbidden food items and about the rest one must decide keeping in view human nature and instinct:

Believers! Eat of the good things that We have provided for you and be grateful to Allah if it is Him you worship. He has only forbidden you dead meat and blood and the flesh of swine and that on which any name other than Allah has been invoked. (2:172-3) 

    So based on this principle, one can decide about sea food also. Barring carnivorous fish and sea animals like sharks and other sea animals whose abhorrence is found in human nature, one can eat anything. Crabs and lobsters, on this very basis would be okay to eat. In other words, except for the above mentioned restrictions, all sea food is left to the choice and taste of the eater. In this regard, the real judge is one’s instinct and nature. Of course, there are differences in the taste of people also in this regard which should be tolerated.