Six new non-native turtles in Portugal: A growing threat apart from the Red-Eared slider and Chinese stripe-necked turtle

A new study published in Herpetozoa reports the first records of non-native freshwater turtles in Portugal beyond the well-known red-eared slider and Chinese stripe-necked turtle (Mauremys sinensis). Between 2024 and 2025, a research team surveyed 39 sites across the country and confirmed the presence of seven non-native species at seven locations, capturing 137 individuals in total. Portugal hosts two native freshwater turtle species: the European pond turtle (Emys orbicularis) and the Mediterranean pond turtle (Mauremys leprosa).

Both play important ecological roles in aquatic ecosystems but are particularly vulnerable to habitat degradation, pollution and competition with introduced turtles. The red-eared slider, meanwhile, is the most introduced and widespread turtle species in the world, earning its place among the world's 100 worst invasive species due to its ecological generalism, high mobility, brumation capacity, early sexual maturation and high fecundity. The negative impacts associated with red-eared sliders on other wildlife are well documented, including predation, competition and disease transmission.

Interactions with native European pond turtles have been shown to cause declines in body condition and even increased mortality. Additionally, pet red-eared sliders are known to be a source of zoonotic diseases such as Salmonella spp. To stop the invasion, the European Union adopted Regulation (EU) 1143/2014 and subsequently the Commission Implementing Regulation (EU) 2016/1141, which blacklists several invasive species of particular concern.

This blacklist prohibits the trade, breeding and release of red-eared sliders. However, following this blacklisting, several other turtle species began to be traded more widely. Portugal enacted Decreto-Lei no. 92/2019, the Portuguese blacklist published in 2019, which bans the trade of six taxa: red-eared sliders, Graptemys spp., Pseudemys spp., Chrysemys picta, Macrochelys temminckii and Chelydra serpentina.

Nevertheless, the free movement of people and goods within the Schengen Area hinders the enforcement of these blacklists. Research has demonstrated that several species of freshwater turtles possess attributes similar to red-eared sliders and that the risk of their establishment in Europe ranges from high to extreme. This is particularly concerning because many of these species are already present in Europe.

Other species are considered to have a moderate invasion risk, such as the Chinese stripe-necked turtle, as well as species with a high invasion risk, such as the false map turtle (Graptemys pseudogeographica) and the African helmeted turtle (Pelomedusa subrufa). In the new study, researchers captured non-native turtles at seven out of 39 sampled sites: three sites in public gardens in Lisbon, one site in Sintra's Natural Park and four sites in two protected areas in the Algarve. In total, seven non-native turtle species were identified: Graptemys pseudogeographica, Mauremys reevesii, Mauremys sinensis, Pseudemys concinna, Pseudemys nelsoni, Pseudemys peninsularis and Trachemys scripta.

Overall, the red-eared slider remained the most frequently recorded species (n = 68), followed by the Chinese stripe-necked turtle (n = 22) and the false map turtle (n = 17). Remarkably, a high number of Chinese stripe-necked turtle juveniles (n = 12) were captured in Sintra, which could be explained by reproduction in nature. This may constitute the first evidence of this species breeding in the wild in Portugal.

Notably, the researchers also documented the first evidence of nesting behaviour by the false map turtle in Sintra, indicating the potential for this species' establishment in the wild. The false map turtle is a species prohibited from being sold in Portugal, which indicates its potential for establishment and suggests it may be reaching the third stage of the invasion process. The diversity of non-native turtle communities differed significantly amongst regions.

The diversity of species recorded was higher in Lisbon than in the Algarve, a pattern likely related to the greater availability of species in the pet trade and the higher human population density in the capital. The only two species recorded in all areas, the false map turtle and the red-eared slider, showed similar size, weight and sex ratios, demonstrating similar invasion success across the country. However, the body condition index differed significantly between red-eared slider populations in Lisbon and the Algarve, possibly related to the type of ecosystem and food availability at each site.

In Lisbon's semi-natural ponds located in public gardens, turtles were regularly fed by people, whereas in the Algarve, turtles fed on naturally available resources. The time lag of three years between the establishment of the Union's list of invasive alien species and its incorporation into Portuguese legislation allowed for the continued trade of species that would later be prohibited. Some banned taxa are still available for purchase, particularly through social media platforms.

Although these turtles are often not released immediately after purchase, typically being kept until they become too large for private care, the findings suggest that all reported species have entered the second stage of the invasion process. Furthermore, several species currently available for legal sale, such as Mauremys sinensis and Mauremys reevesii, also pose invasion risks, leading to additional impacts not previously associated with red-eared sliders, such as potential hybridisation with native turtles in the case of Mauremys spp. and the introduction of new diseases. In Spain, the genus Mauremys has recently been included in the Spanish Catalogue of Invasive Alien Species due to the risk of hybridisation with the native Mediterranean pond turtle.

Considering both the documented invasion risks and future climate suitability, Europe, particularly Portugal, may face significant challenges related to non-native turtle introductions, even if currently established species are banned from trade. Prohibiting trade in specific species often leads to their replacement by other taxa, potentially making a white-list approach a more effective regulatory framework. However, the development of such positive lists must be preceded by rigorous risk assessments to ensure that only species with minimal risks of invasion or welfare are permitted.

The effectiveness of any listing system ultimately depends on its proper implementation and enforcement, comprising clear regulatory mechanisms, monitoring and sanctions for non-fulfilment. This study emphasises the urgent need to control not only well-established invasive species like the red-eared slider, but also emerging invaders such as Graptemys spp. and Mauremys spp. before they become further entrenched. In some cases, trade has already been prohibited, but natural reproduction may still increase their populations, complicating control efforts and raising potential economic costs.

In other cases, no legal measures currently prevent their trade despite their ecological risks. Closer cooperation between the scientific community and policy-makers would be invaluable in enhancing coordinated efforts to prevent and control non-native turtle invasions before the next wave of introductions takes hold.

Subject of Research: Invasive Species

Article Title: Six new non-native turtles in Portugal: A growing threat apart from the Red-Eared slider and Chinese stripe-necked turtle

Article References: Rato, J., Silva, M., Catarrinho, T., Filipe, R., Matthys, S., Brandão, P., Anastácio, P., & Banha, F. (2026). Six new non-native turtles in Portugal: A growing threat apart from the Red-Eared slider and Chinese stripe-necked turtle. Herpetozoa, 39, 1-5. https://doi.org/10.3897/herpetozoa.39.e167762

Image Credits: AI Generated

DOI: 10.3897/herpetozoa.39.e167762

Keywords: invasive species, freshwater turtles, Portugal, red-eared slider, Trachemys scripta, Mauremys sinensis, Graptemys pseudogeographica, biodiversity conservation, Herpetozoa, non-native, turtles, growing