Gozo’s alien frogs came from Turkey, genetic research shows

Alien frogs from three fresh water locations in Gozo trace their origins to southern Anatolia in Turkey, researchers have discovered in the wake of genetic tests.

The study focussed on water frogs of the genus Pelophylax that is not native to Malta and was first noted in Gozo at the turn of the millennium.

Researchers genetically examined 17 frogs from three separate populations found in the Gozitan localities of Mġarr ix-Xini, Għajn il-Papri and Ramla Valley.

The tests showed the three distinct frog communities had low genetic variability that suggests they came from only one source population. Given that the three areas have no interconnecting water courses and the alien water frogs are unable to cross arid land, researchers believe the alien species was introduced by humans in the three localities.

The localities in Gozo where the frog samples were taken: A. Mgarr ix-Xini; B. Ghajn il-Papri; C. Ramla Valley (Photos: Arnold Sciberras)

Genetic markers also suggest the frogs are similar to those found in southern Anatolia in Turkey, Cyprus and the Aegean Greek island of Ikaria. However, Ikaria was ruled out as a source population since genetic sequencing of frogs on the Greek island revealed certain differences from the Maltese samples.

The Maltese frogs are genetically similar to those in Cyprus and Turkey with researchers noting the possibility that the Cypriot frogs were also introduced to the island unintentionally by humans.

The study was carried out by Petr Papežík and Peter Mikulíček from the Department of Zoology at the Comenius University in Bratislava, Slovakia, Michal Benovics from the Masaryk University in Brno, Czechia, Alan Deidun from the University of Malta and renowned pest controller Arnold Sciberras and Jeffrey Sciberras.

The findings were presented in a paper titled ‘Far From Home: Tracing The Origin Of Non-native Water Frogs (genus Pelophylax) In Malta By Molecular Markers’, published in the January issue of Biological Invasions, a peer-reviewed journal by Springer.

The researchers said that according to published data, water frogs were introduced to Ta’ Sarraflu in Gozo in the early 1990s and first recorded in April 2000.

“The exact mode of their introduction, as well as the individual abundance within the same populations and their survival strategies remained unclear at the time. However, several possible hypotheses were proposed, including the introduction by local farmers or through deliberate releases from purchases made at Maltese street markets,” the paper reads.

No official importation data for Pelophylax species to the Maltese islands could be retrieved from archives held by Maltese environmental authorities. However, researchers said there are anecdotal reports from farmers based within the Ta’ Sarraflu area in Gozo suggesting a deliberate introduction of tadpoles of the species imported from Turkey.

“If confirmed, this deliberate introduction is similar to the one observed for a number of non-indigenous crustaceans (such as crayfish) within freshwater sites around the Maltese islands,” the paper says.

Sampling and localities

The 17 frogs were collected in the summer and autumn of 2022 and a toe clip was collected from each individual for genetic analysis. Tissue samples were preserved in ethanol and stored at -25°C.

Genomic DNA was extracted from the toe clips and mitochondrial and nuclear markers were used to find out the origins of the Gozitan water frogs.

Mġarr ix-Xini is a gorge located along the south-western coast of Gozo. Although it does not feature a permanent watercourse, its steep flanking rocky walls provide substantial shade and shelter leaving small bodies of freshwater and mud throughout most of the year along the valley floor.

Għajn il-Papri is a sizeable freshwater pond excavated in clay in the 1990s as a water reservoir, along the southern coast of Gozo.

The pond is also a haunt for the indigenous painted frog.

Ramla Valley runs from the village of Nadur to the sand dune remnants at Ramla l-Ħamra, along the north coast of Gozo. During the rainy season, the watercourse within the valley reaches the sea by meandering through sand dunes and the beach.