Distribution and evolution of the western European water frogs (genus <i>Pelophylax</i>) from Catalonia, northeastern Spain

European water frogs from the genus <i>Pelophylax</i> are particular among amphibians as they can produce hybrids (named kleptons) reproducing by hybridogenesis. Four klepton species have been described in Europe: <i>Pelophylax</i> kl. <i>esculentus</i>, <i>P.</i> kl. <i>hispanicus</i>, <i>P</i>. kl. <i>grafi,</i> and the putative klepton PK. While most of these kleptons originated naturally in areas where the parental species occur in sympatry, human-mediated translocations of water frogs across Europe have altered this dynamic. As a result, several <i>Pelophylax</i> species and kleptons are now found outside their natural ranges, posing a threat to autochthonous water frogs. Additionally, the subtle morphological differences between <i>Pelophylax</i> species make hybrid identification and, thus, conservation difficult. In the present study, we analyzed 423 specimens from 54 populations sampled across Catalonia and implemented a two-step molecular method to identify all species of water frogs present in Catalonia. We also examined the mitochondrial genome of the hybrid <i>Pelophylax</i> kl. <i>grafi</i> to obtain new insights into their reproductive system and spatial structure. Despite the large number of samples analyzed, only the native <i>P. perezi</i> and its klepton <i>P</i>. kl. <i>grafi</i> were found, with the proportion of the latter being unexpectedly high. Results showed a high misidentification rate based on morphology compared to molecular methods, indicating that identification of <i>P.</i> kl. <i>grafi</i> through morphological characters is unreliable. Furthermore, the mitochondrial DNA of hybrid specimens entirely belonged to <i>P. perezi</i> and showed high intra-specific variability. This suggests either a single hybridization event involving a male <i>P. ridibundus</i> or <i>P.</i> kl. <i>esculentus</i> and a female <i>P. perezi,</i> or that <i>P. ridibundus</i> mitochondrial DNA has been eliminated from the <i>P</i>. kl. <i>grafi</i> germline by adaptive or non-adaptive processes. This study offers new insights into the distribution and composition of the North Iberian <i>Pelophylax</i> hybridogenetic complex, providing comprehensive sampling across one of the main entry points of the complex into the Iberian Peninsula.