The widespread distribution of invasive species inevitably leads to the emergence of a new category of biotic relationships: interspecific predatory interactions between invasive species. We assessed the vulnerability of different life stages of the clawed frog, Xenopus laevis (eggs, hatchlings, tadpoles, newly-metamorphosed froglets, and adults), to predation by the Eastern mosquitofish, Gambusia holbrooki, and evaluated whether adult clawed frogs prey upon juvenile and adult mosquitofish. The results confirm differences in the palatability of different ontogenetic stages of the clawed frog by mosquitofish, as well as the low palatability of mosquitofish for adult frogs, with the exception of fish juveniles, which are relatively protected in the light but highly vulnerable under low-light conditions. Therefore, these fish-amphibian interactions are complex and can be defined as unequal bidirectional predation: the mosquitofish readily eliminates its early-stage opponent, but may become prey for its adult individuals. The revealed existence of a vulnerable mosquitofish stage makes it difficult to predict the unconditional suppression of clawed frog populations by this fish species when these two global aquatic invaders become syntopic in new regions. In any case, mosquitofish are likely to dominate in urban water bodies in locations with constant artificial lighting. We emphasise the importance of studying interactions at all ontogenetic stages, as well as taking into account the lifestyle of the organisms studied when analysing the mechanisms of predatory interactions between any pairs of invasive species in new areas of the planet. Graphical abstract

Biological control of undesirable organisms and pathogens often involves the introduction of alien species into new regions. However, alien species themselves pose a potential threat to local ecosystems and economies. The Eastern mosquitofish Gambusia holbrooki is recognised as a dangerous invasive species, but despite this, it is still used for biological control of mosquito larvae, a potential vector of malaria plasmodium transmission to humans, on the Black Sea coast of the Caucasus. We focused on the species composition of helminths in this fish species. We detected adult nematodes Pseudocapillaria (Pseudocapillaria) tomentosa and cestodes Schyzocotyle acheilognathi (formely Bothriocephalus acheilognathi). The above nematode was observed for the first time in fish of the genus Gambusia. Importantly, the cestode S. acheilognathi, which we found in G. holbrooki, is the most successful invasive alien parasite species in freshwaters of the planet and is extremely hazardous to natural ecosystems and aquaculture as it can cause mass mortality of young fish. Thus, the current practice of transferring mosquitofish between water bodies on the Black Sea coast of the Caucasus to control mosquito larvae may contribute to the undesirable spread of a harmful quarantine fish parasite.

The common frog (Rana temporaria) and the moor frog (R. arvalis) lived in all green areas of Moscow until the end of the 20th century. The reduction in the area of woody vegetation, the destruction of breeding sites and introduction of the invasive fish Perccottus glenii contributed to the extinction of these amphibians in most of the city. The paper provides information on the distribution of brown frogs in Moscow based on the results of monitoring in 2020. R. temporaria was found in 37 localities in the Northwestern (6 points), Northern (3 points), Northeastern (1 point), Eastern (9 points), Southeastern (1 point), Southern (5 points), Southwestern (5 points) and Western (7 points) administrative districts. Eleven finds (29.7%) are located outside of specially protected natural areas. Most of the species’ habitats (30 points, or 81.1%) are isolated from other populations. R. arvalis was found in 14 localities in the Northwestern (3 points), Northern (3 points), Eastern (4 points), Southwestern (1 point) and Western (3 points) administrative districts of Moscow. Of all the finds noted, three ones (21.4%) are located outside of specially protected natural areas. The vast majority of the habitats of the species (10 points, or 71.4%) are isolated.

Using the analysis of the diversity of the mitochondrial gene COI, it was revealed that the nine-spined stickleback invaded the Dnieper basin from the Baltic Sea basin and invaded the Volga basin from the same basin (the Baltic Sea) or, less likely, from the White Sea basin. Apparently, these migrations occurred recently via canals built in the late 18th–early 19th centuries. Analysis of the literature confirms that both the nine-spined stickleback and the closely related species, the southern nine-spined stickleback, which has spread from the south, are now widespread in the Volga and Dnieper basins. In both basins, the ranges of these two fish species now overlap and hybrid zones are likely to emerge in those areas.