Unnoticed introductions, a silent threat for biodiversity: island endemic Ladigesocypris ghigii (Actinopterygii, Leuciscidae) translocated to continental Greece.

Morphological similarity between an intruder and a native species can hinder the detection of the intruder. The undetected non-native species will thus have more time to affect the local ecosystem and spread. This paper describes an instance of unnoticed introduction of a fish species morphologically resembling a native endemic species. We report on the first established population of the Rhodes minnow, Ladigesocypris ghigii (Actinopterygii: Leuciscidae), endemic to Rhodes Island, which has been recently introduced to a river near Athens, mainland Greece. When it was first noticed, in 2017, the species was misidentified due to its morphological similarity to Pelasgus marathonicus, the only native leuciscid of the streams and rivers around Athens, until it was subject to genetic and detailed morphological examination. Genetic analyses based on both mitochondrial (cytochrome b) and nuclear (recombination activating gene, RAG1) markers unambiguously identified the collected individuals and provided information on the relationships of Ladigesocypris at both the species and generic level. Our investigation supports that L. ghigii was recently established near Athens, and that the population probably originated from Rhodes Island, an eastern Aegean island. In 2021, a high-density thriving mainland population coexisted solely with the native European eel (Anguilla anguilla) along at least 9.7 km of river habitat. It was assessed as being of high invasive risk for the region where it was established by the Aquatic Species Invasiveness Screening Kit (AS-ISK) screening tool. We propose a feasibility study be undertaken to investigate the application of measures that may include actions to exterminate or contain the population. Special attention is required since the species is considered vulnerable in its native range, while the translocated population coexists with native biota within a stream ecosystem of outstanding local conservation value. [ABSTRACT FROM AUTHOR]