Terrapins in the Algarve. A North American Field Guide Would Have Been Useful

I was hoping we would see Spanish Terrapins (Mauremys leprosa) and European Pond Terrapins (Emys orbicularis) when we were in Portugal in early April. We did but in the Algarve we also saw very large numbers of North American sliders (Trachemys scripta) in a freshwater lake behind the saline lagoon and beach at Quinta do Lago (part of the Ria Formosa National Park). I spent ages looking through the photographs in order to try and identify what we had been seeing basking in the spring sunshine. I then found a paper from 2018 which describes an extensive survey of the National Park, which extends along the coast of the Algarve, done between 2011 and 2013. Over that period the most common species was M. leprosa (79% of all captures). Then came E. orbicularis (12%) with introduced North American species on 9%. That paper clearly demonstrated for the first time that the introduced terrapins were breeding. I also discovered that the lake, surrounded by a golf course and holiday apartments, is used as a place to release rescued native terrapins. Whether that lake is typical of other freshwater lakes and ponds of the Ria Formosa National Park I do not know but it was clear to us that introduced sliders vastly outnumbered M. leprosa while only a few E. orbicularis were evident. 

The authors of the 2018 paper raised the alarm in view of evidence that North American sliders compete for food and basking sites with E. orbicularis. Throughout its range E. orbicularis is decreasing in numbers and is now classified as Near Threatened in the IUCN Red List. The question is if and will anything be done to control the number of introduced terrapins in Portugal.

I should point out that I am following British terminology for freshwater chelonians in using ‘terrapin’ for all these species, rather than the North American ‘turtle’. However, to complicate matters, Emys orbicularis was often called the European Pond Tortoise. ‘Spanish Terrapin’ is also misleading since the species also occurs in North Africa. In Portugal and elsewhere, Mediterranean Pond Terrapin is also used as a name for M. leprosa. 

We also travelled inland and on one reservoir and a slow-moving river saw only the two native species.

I am puzzled by the presence in the same locations of E. orbicularis and M. leprosa, as I was at Butrint in Albania, where E. orbicularis and the Balkan Terrapin (Mauremys rivulata) live side by side. Are those pairs of species not in competition? And if not why not?

Sliders in Europe represent either the declining remnants of the once enormous trans-Atlantic trade in ‘baby terrapins’ in the north or a thriving feral  population in the south. As the baby terrapins grew, they were released by owners unable to house them. However, before that stage was reached only when word spread on how to rear them successfully. Until then virtually all died from being kept too cold and from being given unsuitable food deficient in vitamins and calcium. However, when kept properly the cute baby grows very rapidly.

The sliders we saw represent the two phases of the trade. Initially—and apart from ones captured in the wild for the dealers selling to amateur herpetologists—the ones which came in were Red-eared Sliders (Red-eared terrapins, Trachemys scripta elegans). The late 1950s saw the start of the mass trade in Britain. What we didn’t know was that these terrapins were being bred on ‘turtle farms’ in the southern USA and that they were fed the waste from chicken carcasses. It did not take all that long for Salmonella from the chicken and contaminated water to make its presence felt, both in the USA and in Europe. Eventually that trade was banned. But then a second wave, I think also coinciding with the mutant ninja turtle craze, this time of another subspecies of slider, brought the Yellow-bellied Slider (Trachemys scripta scripta) to pet shops. It was these two forms that were present at Quinta do Lago. In contrast to 2011-2013 data when Red-ears predominated (98% of the sliders), Yellow-bellied were as least as common, if not slightly more so, than  the Red-ears, as would be expected. 

Of the sliders I could see clearly, I did not spot any obvious hybrids between the two subspecies. This could mean that many of the large terrapins are of the original stock or it could indicate assortative mating.

The EU banned the import, sale, breeding or exchange of all forms of T. scripta in 2015, a position maintained in Britain after Brexit.

Martins BH, Azevedo F, Teixeira J. 2018. First reproduction report of Trachemys scripta in Portugal Ria Formosa Natural Park, Algarve. Limnetica, 37, 61-67. DOI: 10.23818/limn.37.06