Corals are one of the most ancient and primitive invertebrate creatures on Earth.
Modern corals were contemporaries of dinosaurs, their ancestors and the symbionts appeared in the Jurassic period and survived several mass extinctions of animals. To such conclusion genetics, published an article in the journal Current Biology.
“Previously, we assumed that modern corals appeared about 50 million years ago. Our analysis indicates that it happened much earlier, the ancestors of the polyps and algae began to live together at the dawn of the age of dinosaurs, about 160 million years ago. They’ve been through some of the most powerful cataclysms, almost completely erasing them from the face of the Earth,” said Todd Lagunes (Todd LaJeunesse) from the University of Pennsylvania (USA).
Corals are one of the most ancient and primitive invertebrate creatures, whose ancestors appeared during the Cambrian explosion. In the distant past they were predators, but today the majority of polyps provides itself with food, “growing” symbiotic single-celled algae.
Colonies of these polyps is one of the most sensitive indicators of climate change and environmental changes in ocean waters. The slightest changes in average water temperature, movement of currents and acidity affect the ability of corals to build calcareous shells. In particular, warming water and acidification prevent the polyps to capture calcium ions and carbon dioxide that destroys their shells and have a negative impact on the strength of reefs.
For this reason, scientists believed that modern corals and their colonies appeared relatively recently, as any serious climate change, and especially mass extinction of animals, was more likely to affect polyps and their companions.
Lagunes found that this idea was incorrect, having studied DNA of various species of algae of the genus Symbiodinium that live inside the polyps. Scientists have suggested that they, like all of today’s existing corals, are descended from a common ancestor.
If so, then small differences in the sets of mutations in the DNA of modern algae “will give” scientists the time when the corals and algae for the first time established relations. In parallel, biologists have studied the fossils of corals and shells so called dinoflagellates, single-celled algae, the alleged ancestors of the symbionts polyps.
It turned out that the algae is indeed descended from a common ancestor, and it was incredibly ancient. According to scientists, the symbionts of corals first appeared about 165 million years ago in the early Jurassic period, when their ancestors survived a kind of evolutionary explosion.
Over the years of separate evolution, according to Lagunas, the differences between these algae become so serious that they should be separated not only on individual species but also the genus. In total, the researchers propose 15 new categories in the family Symbiodiniaceae, which included seaweed “kind of” Symbiodinium.
“The fossilized remains of corals suggests that modern varieties of reef polyps appeared around the same time as their symbionts. It’s a coincidence, we believe, was no accident. It suggests that the formation of such a Union between algae and polyps allowed them to rapidly capture all the oceans of the world and to survive until today,” — concludes the scientist.