The web of waterways draining out of tropical regions of South America is home to dozens of species of freshwater stingrays. The fish evolved from seagoing ancestors, but exactly how they got inland has always been unclear. Now it seems they were carried by the Caribbean Sea reaching deep into the continent millions of years ago.
Central and South America are home to about 20 per cent of the world’s total fish species, says João Pedro Fontenelle at the University of Toronto in Canada.
Intrigued by what could be responsible for this biodiversity, Fontenelle and his colleagues looked to the evolutionary history of river stingrays, which are only found in South America. There are 38 species, most with spots or marble patterns, and they range in size from 25 centimetres to more than a metre across. The stingrays are also the only exclusively freshwater lineage of shark or ray alive today.
Advertisement
The researchers analysed DNA from 350 individual stingrays across 35 different species. By comparing genetic differences between species, they determined how the stingrays split into many species over time, giving insight into where and when they first left the sea.
Fontenelle and his team estimate that the river stingrays diverged from their ocean relatives around 26 million years ago, between the Oligocene and Miocene epochs, in the upper Amazon basin. Back then, this area looked quite different from the dense rainforest present today.
“The sea level was higher than it is today and the Andes were not that high, and we had a nice lowland formation on the western portion of the Amazon,” says Fontenelle. “That allowed for the sea to come into the continent.”
Fossil and chemical evidence in rock suggests the northwestern corner of South America was dominated by a vast, swampy sea for millions of years. Known as the Pebas wetlands, they may have stretched as far south as Argentina.
The Pebas’s mix of fresh water and saltwater probably varied between locations, says Fontenelle, adding that these complex salt gradients may have allowed the stingrays that were washed in by the sea to gradually adapt to fresh water.
Researchers had wondered whether the stingrays accessed the rivers via an ocean inundation, but such swamping had happened multiple times over tens of millions of years from different ocean basins, so it wasn’t clear which iteration of this inland sea marked the stingrays’ freshwater debut. The new research helps confirm the stingrays’ Pebas origins and pinpoints the timing.
As the Pebas receded, the river stingrays got another evolutionary boost: connections between drainages were created or broken and environments changed, leading to more opportunities to diversify.
“It’s kind of like building a jigsaw puzzle,” says Fontenelle. “We’re getting clues from our organisms, from geology and computer simulations, and [with] all of that together, we try to tell a story.”
Andrea Thomaz at Del Rosario University in Bogotá, Colombia, says she is surprised at how well the genetic data aligns with landscape changes in the Amazon basin. “South America is such a diverse continent in terms of biodiversity and geological processes that studies like this are of great importance to understand how the fauna responded similarly or not to geological events.”
The Amazon river is home to other animals with ocean origins, like manatees, dolphins, sponges and other types of fish. But these creatures may have developed a freshwater lifestyle independently of the Pebas flooding that birthed the river stingrays, says Fontenelle.
In the future, Fontenelle wants to explore how the stingrays evolve at much finer scales within river systems.
Journal of Biogeography
Sign up to Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth’s other weird and wonderful inhabitants
