The Danubian cryptic invader Theodoxus fluviatilis (Gastropoda: Neritidae) in the River Rhine: a potential indicator for metal pollution?

Metal pollution poses a major threat to aquatic systems especially in anthropogenic influenced areas, in as much as metals are persistent in the environment. The freshwater snail Theodoxus fluviatilis has often been used as an indicator species for the ecological status in river monitoring. In the River Rhine, the native Northern-European form of T. fluviatilis is nowadays extinct, whilst the Danubian form is spreading along the river. The aim of our study was to investigate if the cryptic invader is affected by metal exposure present in the River Rhine and to discuss its potential as an indicator for metal pollution. Several environmental abiotic (14 water environmental variables plus five common metal concentrations in water and biofilm) and biotic parameters (biofilm mass) were measured across 23 sites along the River Rhine. Five population and six histopathological parameters were evaluated on snails collected at all 23 sites. Aqueous chromium concentration was positively correlated to the damage of male reproductive organs of T. fluviatilis, and higher ammonium concentration was correlated to a decrease in snail size and an increase in the proportion of juveniles. None of the analysed snail parameters was negatively correlated to concentrations of other metals measured, like copper and zinc. Therefore, based on the parameters evaluated, our results indicate that the Danubian form of T. fluviatilis is only restrictedly suitable as an indicator for metal pollution in the River Rhine system. Further field and laboratory investigations including other stressors are necessary to evaluate the indicator potential of the cryptic invader holistically.

The Ponto-Caspian parasite Plagioporus cf. skrjabini reaches the River Rhine system in Central Europe: higher infestation in the native than in the introduced Danubian form of the gastropod Theodoxus fluviatilis

The introduction of non-indigenous organisms in new areas in the context of host-parasite interactions is still poorly understood. This study aimed at a parasitological and histopathological comparison of two phylogenetically distinct forms of the freshwater snail Theodoxus fluviatilis in the River Rhine system: the native Northern-European form, which showed a decline for unknown reasons and is nowadays extinct in the River Rhine, and the non-indigenous Danubian form, which was introduced via the Main–Danube canal. We histopathologically examined populations of Northern-European T. fluviatilis from three smaller rivers of the Rhine system and of Danubian T. fluviatilis from the River Rhine, after confirming the phylogenetic background of the respective population genetically. Results showed differences in the prevalence of trematodes and histopathologic organic alterations between the two snail forms. Both were infected with an opecoelid trematode Plagioporus cf. skrjabini, whereby its prevalence was significantly higher in the Northern-European than in the Danubian form. The parasitic trematode is, to our knowledge, a new trematode species in the River Rhine system, presumably co-introduced through the invasion of its second intermediate and final hosts, i.e. Ponto-Caspian amphipods and gobies. Its impact on native populations of Northern-European T. fluviatilis needs to be subject of future studies.

Die Land- und Süßwassermollusken im Inselsee bei Güstrow und seiner Umgebung (Deutschland: Landkreis Rostock)

Im Rahmen des 38. Kartierungstreffens der Arbeitsgruppe Malakologie Mecklenburg-Vorpommern wurde die Land- und Süßwassermolluskenfauna des Inselsees bei Güstrow und seiner Umgebung erfasst. Dadurch konnte ein wichtiger Beitrag zur Molluskenkartierung in unserem Bundesland geleistet werden, denn es wurden in dem in den letzten zwei Jahrzehnten nur sporadisch bearbeiteten Gebiet 103 Molluskenarten nachgewiesen. Die Ergebnisse wurden mit den Daten früherer Erhebungen verglichen. Für den Inselsee konnten mit Ferrissia fragilis, Marstoniopsis scholtzi und Pseudanodonta complanata drei Neunachweise verzeichnet werden, während die Vorkommen von sechs früher nachgewiesenen Arten nicht bestätigt wurden. Somit sind aus diesem See nunmehr insgesamt 51 Arten gemeldet. Der offensichtlich durch Fraßdruck des invasiven ponto-kaspischen Amphipoden Dikerogammarus villosus in den letzten Jahren ausgestorbene Theodoxus fluviatilis wurde nur noch als Leergehäuse gefunden. In Moorseen in der Umgebung des Inselsees gelang unter anderem der Nachweis von Anisus vorticulus, einer Art der Anhänge II und IV der FFH-Richtlinie. Unter den 52 nachgewiesenen Landschneckenarten ist insbesondere der Nachweis rezenter Leergehäuse von Vallonia enniensis erwähnenswert. During the 38th mapping meeting of the working group malacology in Mecklenburg-West Pomerania, the land and fresh water molluscs of Lake Inselsee and vicinity near Güstrow were assessed, to provide a substantial contribution to the knowledge of mollusc mapping in our federal state. In this previously only sporadically studied area, as much as 103 species were recorded. The results were compared with earlier studies. Within Lake Inselsee, three species were found for the first time: Ferrissia fragilis, Marstoniopsis scholtzi and Pseudanodonta complanata. Six of the formerly recorded species were not found again. Altogether 51 species of molluscan fauna are known from the lake. Theodoxus fluviatilis was observed as empty shells only and has probably disappeared within recent years owing to predation by the invasive Ponto–Caspian amphipod Dikerogammarus villosus. In fen lakes near Lake Inselsee we encountered Anisus vorticulus, a species listed in the EU Habitats Directive (annex II and IV). Among the recorded 52 land snail species, the record of fresh shells of Vallonia enniensis is worth mentioning in particular.

Inter- and intra-specific variability in δ13C and δ18O values of freshwater gastropod shells from Lake Lednica, western Poland

This study focuses on the inter- and intra-specific variability in δ13C and δ18O values of shells and opercula of gastropods sampled live from the littoral zone of Lake Lednica, western Poland. The δ13C and δ18O values were measured in individual opercula of Bithynia tentaculata and in shells of Bithynia tentaculata, Gyraulus albus, Gyraulus crista, Lymnaea sp., Physa fontinalis, Radix auricularia, Theodoxus fluviatilis and Valvata cristata. The gastropods selected for the study are among the species most commonly found in European Quaternary lacustrine sediments. The carbon isotope composition of the gastropod shells was species-specific and the same order of species from the most to the least13C-depleted was observed at all sites sampled. Differences in shell δ13C values between species were similar at all sampling sites, thus the factors influencing shell isotopic composition were interpreted as species-specific. The δ18O values of shells were similar in all the species investigated. Significant intra-specific variability in shell δ13C and δ18O values was observed not only within the populations of Lake Lednica, which can be explained by heterogeneity of δ13C DIC, δ18O water and water temperature between the sites where macrophytes with snails attached were sampled, but also between individuals sampled from restricted areas of the lake’s bottom. The latter points to the importance of factors related to the ontogeny of individual gastropods.

Theodoxus fluviatilis (Gastropoda, Neritidae) as an environmental marker

The phenomenon of Theodoxus fluviatilis presence in freshwater and marine habitats for a possible use of the species as a bioindicator is considered. The information about finding of T. fluviatilis in the basins of the Black, Baltic and North Seas is analyzed, and the possibility of the use of laboratory experiments, connected with the adaptive ability of the species to different salinity, is critically examined. Materials of the drilling in Sevastopol Bay are examined, and the results are compared with the published data on chorology and ecology of the species. The presence of T. fluviatilis in Holocene marine sediments of the Bay and in the contemporary brackish-water environment is explained by the mollusk localization near the sources of the submarine groundwater discharge. The following conclusion is drawn: T. fluviatilis can be used as an indicator species in freshwater or in low brackish (up to 5 ‰) water environment.