The islands of the Gulf of Guinea are home to an abundance of species not found anywhere else on Earth. But for more than 100 years, scientists have questioned whether a population of limbless burrowing amphibians – known as caecilians – found on one of the islands is one or more species. Today, a team of researchers from the California Academy of Sciences and the Smithsonian National Museum of Natural History provided the strongest evidence to date that there is not one, but two different species of Cecilians. on the island of São Tomé. Their findings, published today in Molecular ecology, also suggest that volcanic activity may have led to the species’ divergence.
“To judge whether a species is in fact made up of multiple lineages, scientists have to construct a case,” says senior author and Academy of Herpetology curator Rayna Bell. “By conducting a population-level genomic study of these amphibians across the entire island, we add a crucial line of evidence that the São Tomé Caecilian is in fact two unique species.”
Originally described by Portuguese scientists in colonial times, São Tomé Caecilians were later divided into two distinct species based on their color variation and location on the island – solid lemon yellow in the north and yellow with brown spots in the south. Since then, subsequent research has rebounded, grouping the species together and then separating them again, based on the best available evidence.
Then, in 2014, a study by former Academy of Herpetology curator Robert Drewes and graduate student Ricka Stoelting using mitochondrial DNA indicated that not only were there likely two unique species, but that they could be crosses. Bell and his colleagues build on these previous findings by sampling 85 Cecilians from 21 locations across the island for genome-wide genetic markers that more specifically confirm the presence – and crossover – of the two species.
“This earlier study was the first clue to unravel the mystery of the Caecilians of São Tomé,” says Bell. “Our study provides further evidence for the presence of two distinct and mixed species and quantifies the degree of overlap – or hybridization – that occurs between them.”
Once the research team confirmed the existence of two different but crossed species, they began working backward in time to try to determine how the species diverged.
“It’s quite remarkable that there are two unique species on such a small island,” says Lauren Scheinberg, Academy collections manager and study co-author. “You really wonder how natural selection works to stimulate speciation.”
Through their analysis, the researchers found that the two species diverged around 300,000 years ago, a period that coincides with an explosion of volcanic activity on the island. Researchers suggest that lava flows during this period may have led to Caecilian speciation by dividing the island into a patchwork of smaller habitats with unique environmental pressures. As the lava flows eroded, creating suitable habitat for Cecilians, the two species returned to contact and began to hybridize, obscuring evidence of their separation.
“These results are an important reminder that islands are not static,” says Bell. “Although they may be small and isolated, they are dynamic systems that actively accumulate new species. It is also an important consideration for the conservation of São Tomé Caecilians to know that we have two genetically and morphologically unique species. “
Although the picture of their past is becoming clearer, there is still a lot to learn about these enigmatic amphibians. For example, while most Cecilians spend the majority of their time underground, São Tomé Caecilians can be easily found on the forest floor, raising questions about how bright yellow amphibians avoid predation. .
As a century-old mystery is about to be solved, it looks like others are taking its place. But Bell is eager to take on the challenge. “They are perhaps the most studied Cecilians on Earth due to their accessibility and the time they have been described to science. Yet there is still a lot to be learned about them, from their mating behavior to the way they deter predators, ”Bell says. “For a biologist, what could be more exciting than that?”
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