I am interested in the reasons why in Shi'i fiqh shrimp are halal but lobsters and crabs are haram. [All three belong to the order Decapoda.] I have read that shrimp are regarded as "locust" of the sea, which makes them halal for food. From a biological point of view, this idea does not make any sense to me.
As far as sea food is concerned, only that fish which has scales may be consumed. In addition, it must be alive when caught from the water. Crabs are not classified as fish and may not be consumed. There is some differences amongst certain ulama' regarding lobsters but the general consensus seems to be that it should not be consumed. See Ayatullah Seestani's Islamic Laws p. 489.