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.
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.
Oyster
clams
clams
In medieval times, sea creatures had various uses. Fish and other seafood were important sources of nutrition and were consumed by people including the nobility. Whales were hunted for their blubber, which was used to make oil for lamps and in industrial processes. Additionally, shells and pearls from sea creatures were used for jewelry and decoration purposes.
Some sea creatures have ansetors of dinosaurs
If you mean, "are all under water creatures, sea creatures?" then no. The term SEA is for the ocean and there also can be creatures in freshwater. Also some creatures that live in water such as some snakes and sea turtles come out of the water every so often, so they are no total sea/water creatures. Fish are underwater creatures though.
Oysters, but very very rarely they are found in clams.