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Tracing Cichlids Through the Seas
17 September 2013 7:15 pm
More than 1600 species of cichlids swim in fresh water around the world, spanning a rainbow of colors and a myriad of shapes. They’re popular with evolutionary biologists, who study the diverse, rapidly evolving fish in the rift lakes of East Africa and elsewhere. A new study casts doubt on an old hypothesis, that cichlids reached multiple continents by swimming in place while an ancient supercontinent split up. Instead, the researchers say the freshwater fish must have undertaken death-defying dispersals by paddling across the salty seas.
Cichlids live in South America, Africa, Madagascar, and India—all former components of the supercontinent Gondwana that broke up about 135 million years ago. Several decades ago, scientists hypothesized that cichlids dispersed by going along for the ride as the continents spread.
But this theory came with a big assumption: Cichlids must have evolved before Gondwana broke up. The oldest cichlid fossils are only about 45 million years old. That leaves about 90 million years of cichlid history unaccounted for. The fossil record is spotty, so it seemed possible that older cichlid fossils just hadn’t been discovered yet.
Matt Friedman, a paleobiologist at the University of Oxford in the United Kingdom, thought he could estimate just how likely it was that there are more ancient cichlid fossils out there. He and his colleagues assembled a database of known fossils of cichlids. They compared that to a list of sedimentary rocks that might plausibly contain cichlid fossils. The rocks were from the former Gondwana, had formed in fresh water, and contained other fish fossils. The researchers concluded it wasn’t likely that so many cichlid fossils remain undiscovered: The cichlids’ fossil records would have to be 10 to 30 times worse in the older time period than in more recent times—an unlikely scenario, Friedman says. The work is published today in the Proceedings of the Royal Society B.
The researchers also counted mutations in genes shared between cichlids and their close relatives. Mutations accumulate gradually and can be used to measure the time that has passed since two groups of species diverged. They sequenced 10 nuclear genes from 89 modern species of cichlids and 69 other fish to come up with dates when cichlids diverged from their fishy relatives.
All of these tests pointed toward a cichlid origin of 65 million to 57 million years ago, long after Gondwana split apart. “It seems much too late for these guys have been sitting and rafting with the continents as they broke up,” Friedman says.
That leaves a weirder alternative: that the fish dispersed after the continents started splitting up. Dispersal across salt water “seems highly unlikely” for these smallish freshwater fish, Friedman says, but not impossible. Strong east-west currents, a possible island chain, and even the Congo River’s enormous flow of fresh water might have helped cichlids across the South Atlantic, which was of course much narrower then. And some cichlids can handle salt water.
George Turner, an evolutionary biologist and self-described “cichlid nerd” at Bangor University in the United Kingdom, says the study is well done. Drawing conclusions from fossils is difficult, he adds. Not everything gets preserved; not everything that’s fossilized gets found; and after species diverge, millions of years can go by before any differences appear in the hard body parts that get fossilized. So cichlid ancestors could be in the fossil record, but looking so much like their closest relatives that nobody has recognized them. “Saying that, it’s a very interesting stab and it makes me feel a lot more skeptical of the Gondwanan idea than I was before,” he says.
“I think it’s definitely a move forward in our understanding of cichlid evolution,” says Leo Smith, an evolutionary biologist at the University of Kansas, Lawrence, who studies cichlid evolution. “We’re closing in on something that we haven’t really had good answers for yet,” Smith says.