Coral reefs are part of the marine and oceanic ecosystem that are hard and made up of calcium carbonate. Planulae is not the major structure of coral reefs.
<h3>What are coral reefs and Planulae?</h3>
Coral reefs are the hard calcium structures that are of three major types barrier, fringing and atoll. They feed on the planktons and algae that are the small structures inhabiting them.
Planulae is a freely swimming larvae of the jellyfish and other species of Ctenophores. They are solid, flat and have cilia over their body.
Therefore, option a. Planulae is not a structure or type of coral reef.
Learn more about coral reefs here:
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