Those relationships developed relatively recently in geological terms, scientists at the ARC Center of Excellence in Coral Reef Studies said.
Early coral reefs, 300 million to 400 million years ago, were much simpler than today's colorful and complex systems, researcher Peter Cowman said, as fish had not yet become specialized to live on or among corals.
"By 200 million years ago we are starting to see fish with jaws capable of feeding on corals, but the real explosion in reef diversity doesn't occur till about 50 million years ago when we see fishes very like today's specialist coral feeders emerging," he said.
The ever-increasing complexity of the relationship between corals and fishes over the last 20 million or 30 million years means each has become more critical to the survival of the other as their lives have become more interwoven, he said.
"The study of the past tells us that reefs are all about relationships and, like a family, for them to survive those relationships need to remain strong," Cowman said. "In coming years it is probable reefs will be subject to relentless pressures that may cause them to change fundamentally. Those with the best long-term prospects of survival will be the ones where the relationships between fish and corals are healthiest."