The Turtle That Never Was
A species of freshwater turtle deemed to be extinct may never have existed in the first place.
July 1, 2013|
In the late 19th century, German zoologist August Brauer toured the mountains of Mahé island in the Seychelles archipelago off the east coast of Africa, collecting specimens as he went. In 1901, three of his finds—freshwater turtles that seemed to belong to a unique, endemic species—made their way to the Zoological Museum in Hamburg, Germany. There, Austrian herpetologist Friedrich Siebenrock inspected the specimens, placing them in a new taxon, Sternotherus nigricans seychellensis (later changed to Pelusios seychellensis). The species was never again observed, however, leading researchers to assume that it had gone extinct. But new molecular evidence suggests the species never existed in the first place.
Taxonomic confusion accompanied P. seychellensis from its very introduction to science. Its validity as a separate species was first questioned by Siebenrock himself. As he made his original observations of Brauer’s specimens, he commented on their striking morphological similarity to a known species found on the other side of Africa, called P. castaneus. In the years that followed, researchers confused P. seychellensis with another freshwater turtle that also lives in the Seychelles, P. castanoides. Then, in the 1980s, Roger Bour of the Muséum National d’Histoire Naturelle in Paris reanalyzed Seychelles turtle diversity and declared P. seychellensis to indeed be distinct from the other Seychelles turtles. “In 1983, when I was hesitating to recognize it, I outlined about sixteen morphological characters in common with P. castaneus, but also found several unshared features,” Bour, now retired, writes in an e-mail to The Scientist.
After Bour published his report, researchers scoured Mahé for more evidence of P. seychellensis, but when they failed to locate any animals, the species was presumed extinct.
To put the matter to bed, Richard Gemel, a curator at the Natural History Museum in Vienna, Austria, where one of Brauer’s P. seychellensis specimens is now kept, invited herpetologist Uwe Fritz to study its genome. A few years earlier, Fritz had cataloged all the extant species of the Pelusios genus, and he remained “curious about the extinct species,” he says, so he jumped at the opportunity. He traveled to Vienna from the Museum of Zoology in Dresden, Germany, and extracted dried muscle tissue from the turtle’s thigh, then purified its DNA and compared three mitochondrial genes with the known sequences of other Pelusios species. The results suggest that P. seychellensis was not a different species from P. castaneus after all.
Fritz suspects that the specimens were simply mislabeled with regard to where they were collected. In addition to the genetic evidence that the species are one and the same, two of the specimens supposedly collected by Brauer had holes drilled in the sides of their shells, a common technique used to tie turtles together when bringing them to market. “This suggests that these terrapins were not collected by Brauer, but rather bought somewhere,” Fritz says.
But not everyone is convinced. “It is not the end of the story,” says Justin Gerlach, the scientific coordinator of the Nature Protection Trust of Seychelles, in an e-mail to The Scientist. “Those three specimens are similar to Pelusios castaneus, but are still morphologically distinct.”
Sara Rocha, an evolutionary biologist at the Portugal research center CIBIO (Centro de Investigação em Biodiversidade e Recursos Genéticos) who does field work in the Seychelles, agrees that there are still some questions surrounding the specimens. It’s possible, she says, that the turtles were introduced to the Seychelles by the early settlers of the region, and had not diverged significantly from the populations in West Africa at the time they were collected from the archipelago. The genes the researchers investigated may not be variable enough to detect small frequency changes in the island dwellers’ genomes, Rocha says. To truly know if the species are distinct, “you would really need to have . . . very variable molecular markers and population data,” she says. Unfortunately, with only three known specimens of P. seychellensis, that’s probably not possible, she adds. “I think it will remain a mystery.”