Competitive and predatory interactions between invasive mosquitofish and native larval newts

The invasive twospot livebearer's biology, and its current and potential global distribution

Poeciliids are widely recognized as successful invaders, possessing traits associated with invasion success. Native to Central America and south-eastern Mexico, the twospot livebearer (Pseudoxiphophorus bimaculatus) is a species recently recognized as invasive in both Central and northern Mexico. Despite its invasive status, limited research exists on its invasion process and the potential threats it poses to native species. In this study, we conducted a comprehensive review of the current knowledge on the twospot livebearer and mapped its current and potential distribution worldwide. The twospot livebearer shares similar traits with other successful invaders within the same family. Notably, it exhibits high fecundity throughout the year and demonstrates resilience to highly polluted and oxygen-deprived water conditions. This fish serves as a host for several parasites, including generalists, and has been extensively translocated for commercial purposes. Recently, it has also been used for biocontrol within its native range. Apart from existing outside its native range, the twospot livebearer, under current climate conditions and if transported there, could readily colonize biodiversity hotspots in tropical zones worldwide, including the Caribbean Islands, the Horn of Africa, North of Madagascar Island, south-eastern Brazil, and others located in southern and eastern Asia. Given that this fish is highly plastic and our Species Distribution Model, we consider that all areas with a habitat suitability >0.2 should prevent its arrival and establishment. Our findings underscore the urgent need to recognize this species as a threat to freshwater native topminnows and prevent its introduction and spread.

Relative strength of top-down effects of an invasive fish and bottom-up effects of nutrient addition in a simple aquatic food web

Introduction of exotic predators or runoff of fertilizers can alter aquatic food webs, in particular zooplankton communities, through top-down and bottom-up effects. In a mesocosm experiment, we manipulated the density of Western Mosquitofish (Gambusia affinis) and nutrient levels (nitrate and phosphate independently) and observed effects on zooplankton and phytoplankton in a fall, temperate zone system. If top-down regulation were important, we expected mosquitofish predation to reduce zooplankton abundance, which would indirectly benefit phytoplankton. If bottom-up regulation were important, we expected nutrient addition to increase both primary producers and zooplankton. Western Mosquitofish predation significantly decreased the abundance of several zooplankton taxa, resulting in a trophic cascade with increased chlorophyll a (i.e., primary productivity). This effect did not differ between mesocosms with 5 or 10 fish. Nutrient addition had no significant effects on zooplankton; however, chlorophyll a was positively affected by both nitrogen addition and phosphorus addition. Our results suggest weak bottom-up regulation in our experimental community, but strong top-down regulation, emphasizing the potential consequences of introducing non-native Western Mosquitofish to native aquatic ecosystems.

Reproductive fitness consequences of progenesis: Sex-specific pay-offs in safe and risky environments

Progenesis is considered to have an important role in evolution because it allows the retention of both a larval body size and shape in an adult morphology. However, the cost caused by the adoption of a progenetic process in both males and females remains to be explored to explain the success of progenesis and particularly its biased prevalence across the sexes and environments. Here, through an experimental approach, we used a facultative progenetic species, the palmate newt (Lissotriton helveticus) that can either mature at a small size and retain gills or mature after metamorphosis, to test three hypotheses for sex-specific pay-offs of progenesis in safe versus risky habitats. Goldfish were used because they caused a higher decline in progenetic than metamorphic newts. We determined that progenetic newts have a lower reproductive fitness than metamorphic newts. We also found that, when compared to metamorphs, progenetic males have lower reproductive activity than progenetic females and that predatory risk affects more progenetic than metamorphic newts. By identifying ultimate causes of the female-biased sex ratios found in nature, these results support the male escape hypothesis, that is the higher metamorphosis rate of progenetic males. They also highlight that although progenesis is advantageous in advancing the age at first reproduction, it also brings an immediate fitness cost and this, particularly, in hostile predatory environments. This means that whereas some environmental constraints could favour facultative progenesis, some others, such as predation, can ultimately counter-select progenesis. Altogether, these results improve our understanding of how developmental processes can affect the sexes differently and how species invasions can impair the success of alternative developmental phenotypes.

Invasive mosquitofish (Gambusia holbrooki) affect egg-laying and behaviour of Spanish pygmy newts (Triturus pygmaeus)

Invasive species are one of the main causes of amphibian declines worldwide, often through direct predation. Even species or life stages that may not be prone to predation by invasive animals can be affected through alterations of their reproductive behaviour and/or performance. This aspect is less commonly investigated, and may be important for understanding the full impact of invasive species on local amphibian populations. We used laboratory experiments to measure effects of the invasive Eastern mosquitofish (Gambusia holbrooki) on gravid pygmy newts (Triturus pygmaeus) from southern Spain. Gravid newts altered their position in the water column by moving from the bottom of the aquaria to the surface when in the presence of free-swimming mosquitofish, presumably to reduce physical contact with them. Newts also detected and consumed less prey in presence of free mosquitofish. Newts exposed to caged or free-swimming mosquitofish laid fewer eggs than newts not exposed to the invasive species, suggesting that chemical or visual cues alone were sufficient to alter the behaviour of gravid newts. Our results suggest that mosquitofish can reduce the reproductive success of native pygmy newts in the wild, highlighting the need for management efforts to mitigate this impact.