A subtle threat: behavioral and phenotypic consequences of invasive mosquitofish on a native paedomorphic newt

Aquatic invaders often cause severe declines of native amphibian populations, either through competition, predation and/or alterations of the habitat. Such situation has raised additional concerns for the persistence of endemic species exhibiting rare and alternative phenotypes. Here, we experimentally assessed the impact of the invasive mosquitofish (Gambusia holbrooki) on adult newts (Lissotriton graecus) exhibiting paedomorphosis, the retention of larval traits such as gills, making them fully aquatic. Mosquitofish had a negative impact on paedomorphic newts by inducing both behavioral and phenotypic changes. Paedomorphic newts exhibited avoidance behavior and higher metamorphosis rates in the presence of fish. Both female and male newts responded by decreasing mobility and foraging activity. Females stopped investing in egg-laying in presence of fish and males metamorphosed earlier than females. Hence, our results show that mosquitofish introductions, particularly in areas with populations exhibiting paedomorphosis, might have detrimental consequences on the preservation of alternative developmental pathways. Both behavioral and phenotypic effects should be assessed to understand the impacts of introduced species.