The sanctity and sanctity of aquatic animals are important and practical issues in Islamic jurisprudence, which has gained additional importance due to the expansion of the fisheries industry and the diversity of consumption of marine products. Imamiyyah jurists, based on numerous narrations, consider the presence of “shell” or “scale” to be the main criterion for the sanctity of fish. However, the ambiguity in the exact meaning of the word “shell” and its examples has created challenges in applying the ruling to different species of aquatic animals. This research, using a descriptive-analytical method and citing library sources, examines the lexical, customary, and jurisprudential meaning of “shell” and its synonyms such as “scale” and “scale”. In this regard, by examining the views of linguists, jurists, and also biological findings on the structure of the skin and types of scales in fish, an attempt is made to provide a comprehensive and practical definition of this criterion. The present study, in addition to explaining the jurisprudential evidence for the legality of fish with scales and the prohibition of fish without them, examines the specific status of some aquatic animals such as shrimp (shrimp) and sturgeon (outer crustaceans). refers to a separate, visible (even if carefully or at certain stages of growth) covering the body of the fish, and has a structure similar to scales. This definition provides a clear criterion for distinguishing halal from haram aquatic animals and can be a guide for those involved in the halal food industry and the general public.