Create a list of articles to read later. You will be able to access your list from any article in Discover.
You don't have any saved articles.
Date of first coral and algae symbiosis pushed back by 170 million years
The partnership between algae and coral is hundreds of millions of years old.
Analysis of fossilised reefs has revealed the chemical signature of ancient algae, suggesting their symbiotic relationship dates back to the Devonian Period.
One of the most important relationships in nature started at least 385 million years ago.
Shallow coral reefs are some of the most diverse habitats on Earth, supporting around a quarter of all life in the seas. The success of this ecosystem relies on the symbiosis between corals and algae, in which the coral provides shelter in return for nutrients from the algae.
New research, published in the journal Nature, shows this partnership is so old that it predates the evolution of modern corals. Fossils found in northern Africa and Germany suggest that many of their relatives were already living with symbiotic algae in the Devonian Period, which lasted from 419 to 359 million years ago.
The study, led by PhD student Jonathan Jung, pushes back fossil evidence of the first known coral symbiosis by over 170 million years.
“The fossil record of corals has many notable gaps, making it hard to understand when these animals first had symbiotes,” he says. “We turned to fossils from the Devonian to try and answer this question, as they formed part of the largest reefs the world has ever seen.”
“Our discovery of symbiosis in these corals will be a central part in elucidating the evolutionary success of these ecosystems, and their subsequent demise.”
The fossils studied by the team came from the remnants of ancient reefs found in Germany and northern Africa. © Simon F. Zoppe
Coral reefs in the Devonian
Today, coral reefs take up just 0.1% of the Earth’s oceans. But 385 million years ago, it was a very different story.
Elevated levels of calcium in the oceans provided ideal conditions for reefs to form, allowing them to grow along thousands of kilometres of coastline all around the world. But as well as being much bigger than modern reefs, these ecosystems had a very different structure.
Modern corals, known as the scleractinians, were yet to evolve, so a variety of other more ancient species took their place. Devonian reefs were dominated by tabulate corals, made up of tightly packed hexagonal chambers, and rugose corals, which are generally horn shaped.
Living together with sponges and a variety of other animals, these corals provided a complex habitat for all manner of newly evolved fishes.
“The Devonian period is often referred to as the age of fishes, and for good reason,” Jonathan says. “Many branches of marine life evolved and diversified during this time, often finding habitats and hunting grounds within and around coral reefs.”
“The record of these reefs is laid down in substantial limestone deposits around the world, which are studied to understand past marine environments.”
One crucial part of the corals that doesn’t survive, however, are their algal symbiotes. Their soft bodies don’t lend themselves to fossilisation, meaning it’s hard to know whether corals had evolved to live with them at this time.
Discovering whether Devonian reefs already had algae could help explain why they were so successful.
While many rugose corals are solitary, others formed colonies made up of many different individuals. © Nancy Bauer /Shutterstock
When did coral symbiotes first appear?
While the symbiotes don’t leave any fossils behind, they do leave their mark in other ways. One approach is to look for changes in the different forms of nitrogen in the corals.
Over 99.6% of all nitrogen is nitrogen-14, with a small amount of nitrogen-15 making up the rest. In corals with symbiotes, however, levels of nitrogen-15 are slightly lower than usual due to chemical processes carried out by the algae.
This signature is preserved in the coral’s skeleton, allowing it to be detected hundreds of millions of years later. As a result, it can be used to work out whether an ancient coral was symbiotic.
The researchers found that every tabulate coral in the study was symbiotic, whereas it was a more mixed picture for rugose species.
“We found that some rugose corals were symbiotic and others were not, which means we can’t be certain what their lifestyle was,” Jonathan explains. “It could mean they were flexible, and able to live with symbiotes some of the time and without them at others.”
However they lived, these 385-million-year-old fossils represent corals at the peak of their influence on Earth. The reefs they built would soon be devastated by the Devonian mass extinction, which caused around three quarters of all species to go extinct.
While the tabulate and rugose corals would survive for another 100 million years or so, they were replaced as the planet’s main reef builders by bacteria and algae. They would eventually be replaced by modern corals after finally going extinct at the end of the Permian 252 million years ago.
