Final answer:
Many commercial fish and shellfish rely on estuaries for part of their life cycle due to the unique mix of fresh and salt water. Estuaries are areas where a river meets the ocean, creating a brackish water environment. These environments provide protected areas for the development of the offspring of crustaceans, mollusks, and fish.
Step-by-step explanation:
Many important commercial fish and shellfish spend at least a portion of their life cycle in the waters of an estuary. Estuaries are biomes that occur where a river, a source of fresh water, meets the ocean. These areas provide a unique environment where both fresh water and salt water are present, making it an ideal habitat for the young offspring of various marine organisms.