Which of the following is not one of the three major structures of coral reefs?

a. planulae

b. fringing

c. barrier

d. atoll

A planulae is not one of the three major structures of coral reefs as it is one of the stages in the life cycle of a jellyfish known as the free swimming larvae stage. Coral reef structures are made up of fringing reefs that grow directly from the shore, atoll reefs that grow upward usually in a lagoon, and barrier reefs are separated from the land and shore by open water.