Nacreous mollusks are a group of marine animals known for their ability to produce nacre, commonly called mother-of-pearl. This iridescent substance forms the inner layer of their shells and is created by species such as oysters and abalones. Nacre serves as a protective layer, helping to shield the mollusk from parasites and environmental threats. These mollusks belong to the class Bivalvia or Gastropoda, depending on their shell structure. Nacreous mollusks play a vital role in marine ecosystems, contributing to biodiversity and serving as a food source for various predators. Their shells are also valued in jewelry and decorative arts.