Mother-of-Pearl is found in Oyster shells
Bivalve crustaceans such as oysters have pearls.
oyster
The pearls aren't in the Sea of Monsters. They are in the first book.
Natural Pearls,Cultured Pearls, Freshwater Pearls, Saltwater Pearls, South Sea Pearls, Tahitian Pearls
Sea urchins do not produce pearls in the same way that oysters do. While oysters create pearls from irritants by secreting layers of nacre, sea urchins have a different defense mechanism. They can form structures called "urchin pearls," which are calcified deposits, but these are not true pearls and are generally not valued like those from mollusks. Instead, sea urchin pearls are often irregular and not sought after in jewelry.
The places to purchase South Sea pearls online are many and varied. A good place to start would be Pearl Paradise, Raw Pearls, Premium Pearls and Pearl Gallery.
Imitation pearls made from sea shells; very popular on Okinawa.
Fish and eels were eaten. Some hand painted books like the Book of Kells had sea creatures in the artwork of the book. Pieces of jewelry often featured a fish or another sea animals. ( The Pics saw a salmon as a fish of luck and knowledge)
Oyster
clams
clams
Some sea creatures have ansetors of dinosaurs