Does it Have to EXIST to be EXTINCT? The Story of Pelusios seychellensis

Pelusios seychellensis is thought to be a freshwater turtle species endemic to the island of Mahé, Seychelles. There are only three museum specimens from the late 19^th century known. The species has been never found again, despite intensive searches on Mahé. Therefore, P. seychellensis has been declared as “Extinct” by the IUCN and is the sole persumably extinct freshwater turtle species.

African Mud Turtles first appeared some 120 million years ago making them one of the most primitive turtle species on earth today.

Using DNA sequences of three mitochondrial genes of the historical type specimen and phylogenetic analyses including all other species of the genus, recent evidence states that the description of P. seychellensis was erroneously based on a widely distributed West African species, P. castaneus.

Consequently, the two species were synonymized and P. seychellensis was deleted from the list of extinct chelonian species and from the faunal list of the Seychelles.

In 1983, a scientist named Bour resurrected P. seychellensis from Siebenrock’s description in 1906 as a species endemic to Mahé.

In addition, Bour described the Seychellois populations of P. castanoides and P. subniger as the new subspecies P. c. intergularis and P. s. parietalis, endemic to the Seychelles.

However, a recent molecular genetic investigation provided firm evidence that at least P. subniger was introduced to the Seychelles, and this possibility could also not be ruled out for P. castanoides, fuelling the hypothesis of niche competition between an endemic species and naturalized freshwater turtles.

Scientists at the Senckenberg Research Institute in Dresden discovered, based on genetic evidence, that the turtle species Pelusios seychellensis regarded thus far as extinct never existed. The relevant study was published in the journal PLOS ONE.

Scientists at the Senckenberg Research Institute in Dresden discovered, based on genetic evidence, that the turtle species Pelusios seychellensis regarded hitherto as extinct never existed. The relevant study was published in the journal PLOS ONE.

Turtles are the vertebrates under the greatest threat. Among the approximately 320 turtle species, the species confined to islands have been especially hard hit. One of them is thought to be the Seychelles mud turtle Pelusios seychellensis.

The three specimens collected at the end of the 19^th century are still kept at the Natural History Museum in Vienna and the Zoological Museum in Hamburg.

Despite an intensive search for this species, which was declared as “extinct” by the International Union for Conservation of Nature (IUCN), no further specimens have been found since those in the 19^th century.

“Consequently, it was assumed the species had been exterminated,” says Professor Uwe Fritz, director of the Museum of Zoology at the Senckenberg Natural History Collections in Dresden. The Dresden biologist states quite clearly that this is not true.

“We have examined the DNA of the original specimen from the museum in Vienna and discovered that these turtles are not a separate species.”

The genetic analyses have shown that this supposed Seychellois species is, in reality, another species, Pelusios castaneus, that is widespread in West Africa. “The species Pelusios seychellensis has therefore never existed,” adds Fritz.

The Seychelles are 1466.436 miles from the African coast.

“In fact, for a long time, researchers were amazed that the supposed Seychelles turtles looked so deceptively similar to the West African turtles. But due to the great geographic distance, it was thought this had to be a different species, which is why the assumed Seychelles turtles were also described as a new species in 1906.”

Another species classified as native therefore disappears from the list of Seychelles species. Fritz and his team had also proven that another mud turtle species, Pelusios subniger, was not endemic to the Seychelles but had been introduced by man.

“In the Seychelles there is therefore at most one mud turtle species that could be native. And even with this species we are still uncertain whether it really is endemic,” says Fritz. So far, the biologists from Dresden have not been able to explore this possibility due to the incomplete sampling available, however.

“But what is certain even now is that the protection programmes for turtles in the Seychelles will have to be revised, so that truly endemic animal species are protected and the scarce funds available for species protection are put to good use,” says Fritz in conclusion.

Currently, the genus Pelusios comprises of about 20 recognized species. Yet, some recently identified, clearly divergent genetic lineages seem to represent further distinct species.

Pelusios species are small- to medium-sized freshwater turtles that occur in sub-Saharan Africa, Madagascar and on the Seychelles. Their most conspicuous morphological character is the movable plastral forelobe, allowing the closure of the anterior shell opening. Their common name, hinged terrapins, refers to this peculiarity.

The Seychelles are a mid-oceanic tropical archipelago of 115 islands, about halfway between Madagascar and India. Some of these islands, the granitic Seychelles, are remnants of the supercontinent Gondwana and were ‘lost in the sea’ during the north-eastward rafting of India after its detachment from Africa, approximately 63.4 million years ago.

Mahé, from where the thought to be extinct P. seychellensis was described, is the main island of the granitic Seychelles.

As a legacy of the breakup of Gondwana, the fauna of the Seychelles comprises many paleo-endemic species.

There is also a number of younger genetically clearly distinct endemics that arrived later by transoceanic dispersal. Considering their small surface and the paucity of suitable freshwater habitats, the Seychelles harbour an extraordinarily rich fauna of freshwater turtles.

There are three Pelusios species known from the Seychelles islands, P. castanoides, P. subniger, and the extinct P. seychellensis. While P. castanoides and P. subniger occur on several islands, P. seychellensis was recorded only from Mahé.

It has been speculated P. seychellensis is an ancient component of the Seychellois fauna and that P. castanoides and P. subniger, representing younger colonizers of the Seychelles, out competed P. seychellensis with increasing human pressure.

Starting with its original description by Siebenrock in 1906, several authors having studied P. seychellensis were explicitly puzzled by its morphological similarity to a West African Pelusios species, P. castaneus.

However, related to the complicated taxonomic and nomenclatural history of the genus Pelusios, other authors confused P. seychellensis with another superficially similar species occurring on the Seychelles (P. castanoides).

As a consequence, P. seychellensis was soon synonymized with one or the other species and not recognized as distinct until Bour resurrected it in 1983 in his revision of Seychellois hinged terrapins .

Yet, in the face of the morphological similarity of P. seychellensis and P. castaneus, the possibility that P. seychellensis was founded on a mislabelled museum specimen of P. castaneus requires examination. There is quite a number of vertebrate species that were erroneously described using museum specimens of well-known species bearing incorrect locality data, suggesting that this could be also the case in P. seychellensis.

Using phylogenetic and haplotype network analyses of 2036 bp of mitochondrial DNA, samples of the two hinged terrapin species were compared. Pelusios castanoides and P. subniger from continental Africa, Madagascar and the Seychelles to infer their biogeography.

Due to the long independent history of Madagascar and the Seychelles, the populations from those islands should be deeply divergent from their African conspecifics. Seychellois populations of the two species are currently recognized as Critically Endangered endemic subspecies.

However, even within P. subniger evidence was found for a cryptic species from the Democratic Republic of the Congo, all other samples assigned to this species were undifferentiated. This suggests that Malagasy and Seychellois populations of P. subniger were introduced by humans and that the Seychellois subspecies P. s. parietalis is invalid.

This has implications for current conservation strategies for the Critically Endangered Seychellois populations and suggests that measures should rather focus on endemic species. The situation of P. castanoides could be different. Samples from Madagascar and the Seychelles are weakly, but consistently, differentiated from continental African samples, and Malagasy and Seychellois samples are reciprocally monophyletic in maximum likelihood analyses. However, due to a lack of samples from central and northern Mozambique and Tanzania, it cannot exclude that identical continental haplotypes exist there.