Volcanic activity during the Late Pleistocene may have led to the divergence of the São Tomé caecilian (Schistometopum thomense), an amphibian endemic to the oceanic islands of São Tomé and Ilhéu das Rolas in the Gulf of Guinea archipelago, into two separate species, according to new research from the California Academy of Sciences and the Smithsonian National Museum of Natural History.
The São Tomé caecilian (Schistometopum thomense). Image credit: Andrew Stanbridge.
The São Tomé caecilian is an elongate, limbless amphibian in the family Dermophiidae.
It has striking yellow color and uniform contour, and measures between 13 and 35 cm in length; the largest individual recorded at 37.5 cm. At birth, young are miniature replicas of the adult form.
Initially described by Portuguese scientists during colonial times, the São Tomé caecilian was later split into two distinct species based on their variation in color and location on the island — solid lemon yellow in the north and yellow with brown splotches to the south.
Since then, subsequent research has bounced back and forth, grouping the species together then separating them out again, based on the best available evidence.
Then, a 2014 study using mitochondrial DNA indicated that not only were there likely two unique species, but they might be interbreeding.
“That earlier study was the first clue towards unraveling the mystery of the São Tomé caecilians,” said Dr. Rayna Bell, curator of herpetology at the California Academy of Sciences.
“Our study provides further proof of the presence of two separate, interbreeding species and quantifies how much overlap — or hybridization — is occurring between them.”
In their new study, Dr. Bell and colleagues sampled 85 caecilians from 21 locations across the island for genome-wide genetic markers that more accurately confirm the presence of the two species.
“It’s pretty remarkable that there are two unique species on such a small island,” said Dr. Lauren Scheinberg, collections manager at the California Academy of Sciences.
“It really makes you wonder how natural selection is acting to drive speciation.”
The researchers also found that the two species diverged around 300,000 years ago, a time period that coincides with a burst of volcanic activity on the island.
They suggest that lava flows during this period may have led to the speciation of the caecilians by dividing the island into a patchwork of smaller habitats with unique environmental pressures.
As the lava flows eroded, resulting in suitable habitat for caecilians, the two species came back into contact and started to hybridize, obscuring the evidence of their separation.
“These findings are an important reminder that islands are not static,” Dr. Bell said.
“Even though they can be small and isolated, they are dynamic systems that are actively accumulating new species.”
“It’s also an important consideration for the conservation of São Tomé caecilians to know that we have two, genetically and morphologically unique species.”
The findings were published in the journal Molecular Ecology.
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Kyle A. O’Connell et al. Speciation and secondary contact in a fossorial island endemic, the São Tomé caecilian. Molecular Ecology, published online May 10, 2021; doi: 10.1111/mec.15928
