Almost like a blank slate for fishes to diversify on
Fifty million years ago, in the aftermath of a planetary warming event, certain fish discovered how to bite food from hard surfaces — and in doing so, unlocked an evolutionary door that would fill the world's coral reefs with thousands of species. A UC Davis study tracing 9,560 fish species across 350 million years of history has found that this single behavioral innovation accelerated reef fish diversification by 1.5 to 1.7 times, while fish dwelling in open water evolved at a steady, unremarkable pace. The finding reminds us that biodiversity is not a gradual accumulation but a series of leaps — moments when a new way of living opens into an entire world of possibility.
- Coral reefs hold 6,000 to 7,000 fish species while the open ocean remains comparatively barren — and for decades, science lacked a clear explanation for that disparity.
- The Paleocene-Eocene Thermal Maximum, a 200,000-year warming spike 56 million years ago, reorganized ocean ecosystems so thoroughly that it effectively handed surviving fish a blank evolutionary slate.
- Fish that evolved the ability to scrape algae and shellfish from rock and coral gained access to a textured, niche-rich world that rewarded specialization — parrotfish, butterflyfish, surgeonfish, and angelfish all diversified explosively as a result.
- Crucially, corals themselves did not diversify at the same accelerated rate after the warming event, pointing to fish behavioral innovation — not coral abundance — as the true engine of reef biodiversity.
- Mid-water reef feeders like snappers and groupers, which never adopted hard-surface feeding, show no comparable burst of speciation, confirming that habitat complexity alone is not enough without the behavioral key to unlock it.
Fifty million years ago, fish that learned to bite food directly from hard surfaces — scraping algae, snails, and shellfish from rock and coral — began diversifying at a pace that would reshape the underwater world. A study published in the Proceedings of the National Academy of Sciences, led by UC Davis graduate student Nick Peoples, traced this explosion across 9,560 fish species and seven habitat types, mapping speciation and extinction rates over 350 million years. The finding was clear: around 50 million years ago, new species emerged in reef and lakebed environments at 1.5 to 1.7 times the previous rate. Fish higher in the water column evolved at a steady, unremarkable pace throughout.
The timing connects to a pivotal disruption. The Paleocene-Eocene Thermal Maximum, a warming event roughly 56 million years ago, drove global temperatures up by 5 to 8 degrees Celsius, overturning plankton communities and shallow-water ecosystems. For fish, the aftermath was something close to a blank slate — old dominants gone, new niches open. Hard-surface feeding, already emerging as a capability, suddenly had an entire reorganized reef world to exploit.
The researchers found that corals themselves did not accelerate their diversification after this event, which is a telling detail. The fish explosion was not simply a response to more coral variety — it was driven by how fish themselves changed, by the innovation of biting into a complex, textured habitat that rewarded specialization. Parrotfish, butterflyfish, angelfish, and surgeonfish diversified explosively. Snappers and groupers, which feed in open water above the reef, did not.
The broader lesson is about how biodiversity is built — not steadily, but in lurches, when environmental upheaval coincides with a species finding a new way to live. One behavioral innovation, amplified across millions of years, explains why a coral reef today is among the most species-rich places on Earth, while the vast open ocean remains, by comparison, almost empty.
Fifty million years ago, something shifted on coral reefs. Fish that had learned to bite food directly from hard surfaces—scraping algae, snails, and shellfish from rock and coral—began to diversify at a pace that would reshape the underwater world. Today, that ancient innovation explains why coral reefs teem with roughly 6,000 to 7,000 fish species, while the open ocean remains comparatively sparse.
A study published in April in the Proceedings of the National Academy of Sciences, led by UC Davis graduate student Nick Peoples, traced this explosion back to a specific moment in evolutionary time. The researchers analyzed 9,560 fish species across seven different habitat types—from the bottoms of freshwater lakes to the open ocean to coral reefs—and mapped how quickly new species emerged and old ones vanished over the past 350 million years. What they found was striking: around 50 million years ago, the rate at which new fish species appeared in coral reefs and on lake and ocean bottoms jumped by 1.5 to 1.7 times. Fish living higher in the water column, by contrast, continued evolving at a steady, unremarkable pace.
The difference comes down to habitat complexity. Open water offers little friction, little structure, few opportunities for a fish to develop novel behaviors or physical traits. A reef or a lakebed is crowded, textured, full of niches and problems to solve. When a fish develops the ability to bite hard surfaces, it gains access to an entirely new food source—and with that access comes the possibility of specialization, of splitting into different species that each exploit that resource in slightly different ways.
But the timing matters. The acceleration in fish diversification began shortly after the Paleocene-Eocene Thermal Maximum, a 200,000-year warming event that occurred roughly 56 million years ago. During that period, global temperatures climbed 5 to 8 degrees Celsius. The shock reorganized the ocean. Plankton and shallow-water corals turned over rapidly. Species that had dominated vanished or shifted. For fish, Peoples said, it was almost like being handed a blank slate.
When the researchers looked at whether corals themselves diversified after the PETM, they found no such acceleration. This detail is crucial: it suggests that the explosion of fish species was not simply a response to more coral diversity, but rather to how fish themselves evolved and began interacting with their environment in new ways. The innovation—hard-surface feeding—was the key.
Not all reef fish benefited equally. The groups that underwent the most dramatic speciation were those that bite the reef itself: parrotfish, butterflyfish, angelfish, surgeonfish, rabbitfish, and triggerfish. These fish diversified explosively. Meanwhile, reef-dwelling fish that feed in the open water above the reef—snappers, groupers, sea basses—showed no comparable burst of speciation. They remained, in evolutionary terms, stable.
The pattern reveals something fundamental about how biodiversity gets built. Evolution does not proceed at a constant rate. It lurches forward in response to environmental upheaval or when a species stumbles upon a new way of living. A fish that learns to bite hard surfaces gains not just a meal, but a pathway to becoming many different kinds of fish. That single innovation, amplified across millions of years, explains why a coral reef today is one of the most biodiverse places on Earth, while the open ocean—vast as it is—remains comparatively empty of species.
Citas Notables
When we account for the entire evolutionary history of fishes in different habitats, we find that this immense and unparalleled acceleration of diversification happened on coral reefs.— Nick Peoples, UC Davis graduate student and lead author
On reefs, it's the fish that eat parts of the reef that really took off and diversified. The groups that really stand out are parrotfish, butterflyfish, angelfish, surgeonfish, rabbitfish and triggerfish.— Peter Wainwright, UC Davis professor and senior author
La Conversación del Hearth Otra perspectiva de la historia
So a fish learned to bite rocks and suddenly there were thousands of species. That seems like a big leap.
It's not quite that sudden, but yes—the ability to feed from hard surfaces opened a door that had been closed. Before that, if you were a fish on a reef, you were competing for the same floating food as everyone else. Once some fish figured out how to scrape and bite the reef itself, they could eat things no one else could reach.
And that's enough to create thousands of new species?
Over 50 million years, yes. Because once you can eat from the reef, you can specialize. One lineage might become expert at scraping algae. Another might focus on snails. Another on coral polyps. Each specialization is a new way of living, and new ways of living eventually become new species.
But why didn't this happen in the open ocean? Fish live there too.
Open water is featureless. There's nothing to bite, nothing to scrape, nowhere to hide and develop a unique niche. A fish in open water has to compete in the same three-dimensional space as every other fish. A reef is a maze—thousands of crevices, surfaces, textures. Complexity breeds diversity.
The timing is interesting—right after that warming event.
The warming killed off a lot of what was there before. It was disruptive, chaotic. But for fish, it created an opportunity. The old order collapsed. Fish that could exploit new resources—like hard surfaces—suddenly had room to spread and specialize without competing against established dominants.
So the reef didn't change. The fish did.
Exactly. The corals didn't diversify after the warming. The fish did. That tells you the story is about innovation, not about the reef becoming more complex. A fish with a new way of feeding is a fish that can become many different fish.