Islands are the laboratories of the world. This statement can certainly be confirmed by the Seychelles mud terrapins. Science now indicates the two species of mud terrapin were introduced by humans and have naturalised to fill freshwater niches within the ecologically sensitive Seychelles.
Naturalisation occurs when a non-native species maintains a population outside its native range. “Mud turtles are a top-end freshwater invertebrate predator, feeding on a wide variety of freshwater invertebrates, small fish and aquatic plants,” explains Dr David Rowat, scientist at the Marine Conservation Seychelles Society (MCSS).
The Seychelles mud terrapin
Until recently, Seychelles was thought to be home to three species of mud terrapins, but it turns out there were only actually two. The so-called endemic Seychelles terrapin (Pelusios seychellensis) has been believed to be extinct for over a century, but it never existed. The confusion originated on an 1895 scientific expedition from Mahé: a West African specimen, Pelusios castaneus, was included with island terrapins bound for Europe and classified incorrectly as the Seychelles terrapin. “While the original field report clearly indicated three different types were found, no further example of this species has been captured,” explains Rowat.
Naturalised mud terrapins
The naturalised black mud terrapin (Pelusios subniger parietalis) still occurs on Mahé, Cerf, Praslin, La Digue and Fregate islands. While the black mud terrapin prefers marshes and swamps, the larger yellow mud terrapin from Madagascar is more adapted to life in water, preferring streams and larger water bodies. Speciation – the formation of a new species over time through evolution – has resulted from humans bringing terrapins to Seychelles.
black mud terrapin genetics
“Genetically, the black terrapin is almost certainly introduced from East Africa or Madagascar,” says Rowat. However, it is two mutational steps away from its original population, indicating it might have been introduced 1 000 to 2 300 years ago.
Yellow mud terrapin genetics
There are distinct genetic differences between yellow terrapin populations on the various islands, indicating this species was introduced much earlier due to the increase in mutations. “One of the primary routes for genetic differentiation is isolation and the mutation of genes over time to make a species better suited to the local isolated conditions,” explains Rowat. It is clear that both species are adapting to conditions in Seychelles.
The process of speciation is often difficult to establish in freshwater terrapins, as there has been significant human redistribution of the species in many areas.
As soon as the Seychelles terrapins established themselves they became threatened. “The freshwater lowland wetlands in Seychelles are now the most endangered habitat system, with only 5% of native freshwater wetlands remaining,” says Rowat. Seychelles wetlands are threatened by pollution, draining for agriculture or development, feral animals and invasive plants. Wetlands are an important filtration system and prevent sediments from land from entering the sea and damaging coral reefs.
Written by Georgina Lockwood
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