The invasive alien freshwater flatworm Girardia tigrina (Girard, 1850) (Platyhelminthes, Tricladida) in Western Europe: new insights into its morphology, karyology and reproductive biology

Abstract

Invasions of alien species form one of the major threats to global biodiversity. Among planarian flatworms many species are known to be invasive, in several cases strongly affecting local ecosystems. Therefore, a detailed knowledge on the biology of an invasive species is of utmost importance for understanding the process of invasion, the cause of its success, and the subsequent ecological impact on native species. This paper provides new information on the biology of introduced populations of the freshwater flatworm Girardia tigrina (Girard, 1850) from Europe. This species is a native of the Nearctic Region that was accidentally introduced into Europe in the 1920s. Since then, numerous records across the European continent bear witness of the invasiveness of this species, although only a few studies focused on the biology of the introduced populations. We report on the morphology of sexualized individuals from a fissiparous Italian population, representing the second record of spontaneous sexualization of fissiparous individuals in this species. A detailed morphological account of the reproductive apparatus of these ex-fissiparous animals is presented. Our results increased the number of morphological groups previously recognized for European populations of G. tigrina, thus corroborating the hypothesis on multiple independent introductions to this continent. Karyological results obtained from our fissiparous Italian individuals revealed a constant diploid chromosome complement of sixteen chromosomes. Further, we document the marked intraspecific variation in several morphological features of this species.

Molecular characterization of a flatworm Girardia isolate from Guanajuato, Mexico

“…In these examples, sexualization was induced in response to a clear and definable change in temperature or in food, whereas our G. gua animals were maintained in a temperature controlled environment and fed a consistent diet of calf liver (see Materials and Methods). Spontaneous sexualization of G. tigrina laboratory animals has been reported on multiple occasions (17,40) and examples of these events share some similarities with our observation in G. gua, such as an immature copulatory apparatus and sterility. Nevertheless, these reports describe some features that we did not readily observe in our limited number of putatively sexual G.gua, such as hyperplastic ovaries and the presence of supernumerary copulatory apparatuses.…”

Molecular characterization of a flatworm Girardia isolate from Guanajuato, Mexico

“…(Guanajuato) animals that exhibited some evidence of spontaneous sexualization, including the development of ovaries and testes. Spontaneous sexualization of G. tigrina laboratory animals has also been reported on multiple occasions [17,41] and these specimens share some similarities with the rare sexualized G.sp . (Guanajuato) animals we studied, such as an immature copulatory apparatus and sterility.…”

Niche modelling and molecular phylogenetics unravel the invasion biology and worldwide colonization of three species of the freshwater planarian genus Girardia (Platyhelminthes, Tricladida)

“…Most likely, the latter were effectuated through the international trade in aquatic plants and the activity of aquarists or through the importation and subsequent culturing of North American strains of G. tigrina and G. dorotocephala as model organisms in education and scienti c studies all around the world. The fact that G. tigrina is very sticky, clearly enhances its attachment to all kinds of surfaces and thereby facilitates its passive dispersal (Stocchino et al 2019). Wright (1987) suggested that multiple occasions of disposal of the contents of aquaria and of small ornamental garden ponds into local rivers might underlie the pattern of distribution of G. tigrina in Great Britain.…”

“…The species Girardia tigrina (Girard, 1850) was rst reported in Germany in the 1920's, after which it was recorded also from other parts of mainland Western Europe, as well as the Balearic Islands and the Azores, its spread most likely effectuated through the international trade in aquatic plants and the activity of aquarists (Stocchino et al 2019 and references therein). Representatives of the genus have been reported also from Australia, Japan, and Hawaii (Kawakatsu et al 1984 For a long time, it has been assumed that only G. tigrina was introduced in Europe, although some studies have pointed to the possibility that other alien Girardia species may be present, in view of the diversity in morphology, karyology, and reproductive biology of the populations analysed from the Iberian Peninsula (Ribas, Riutort, and Baguñà 1989) and Italy (Benazzi 1993;Stocchino et al 2019). However, due to the fact that, generally, the introduced populations of Girardia show only asexual reproduction and do not possess a copulatory apparatus, their identi cation to species level is greatly hampered, since the main diagnostic characters reside in the reproductive structures (Sluys and Riutort 2018).…”