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The various uses of copper or copper compounds in industrial, biocidal or pesticidal products lead to inputs of the metal into aquatic environments. To assess effects of copper ions on non-target organisms, the freshwater snail Theodoxus fluviatilis was used as test organism for a three-week laboratory experiment. Snails were exposed to four copper concentrations ranging from 4 to 39 µg Cu2+/L, and besides mortality, several sublethal parameters were evaluated. Concerning survival, an aqueous copper concentration of 6 µg/L was determined as NOEC, and 16 (± 0.1) µg/L as LC50. Negative sublethal effects on reproduction, activity and pathological modifications in the snails were detected in the treatment with an aqueous copper concentration of 15 µg/L. Our results using T. fluviatilis as a mere grazer exclusively feeding on biofilms contribute to findings that field relevant copper concentrations have a significant effect on non-target organisms in aquatic environments.

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.

2. Theodoxus fluviatilis (Linnaeus, 1758) Findings in Serbia. Theodoxus fluviatilis: Matoničkin et al. (1975); Karaman (2001, 2012); Jakovčev-Todorović et al. (2005); Paunović et al. (2007a, 2008, 2012 a); Marković et al. (2012, 2013, 2014); Martinović-Vitanović et al. (2013); Tomović et al. (2014). Common name. River nerite. Morphology. Medium-sized species. Oval shaped shell has dimensions up to 13 mm. Colouration pattern is extremely variable, ranging from almost black to various bright variants, often with dark zigzag lines, and triangular irregular light flecks. Operculum is reddish, pseudo-apophysis is absent, while apophysis has a wide platelet part. For more details see Glöer (2019: 41, figs 26, 27). Distribution and ecology. The most widespread among Theodoxus species, this snail is fairly common throughout Europe, and recorded in Asia (Near East) and northern Africa (Bunje 2005; Glöer 2019). It prefers hard substrate of larger rivers in the region (Marković et al. 2014). The most abundant populations in Serbia are found in the large rivers such as the Danube, Sava, and Tisza Rivers. Other remarks. Although being considered of Ponto-Caspian origin (Bunje 2005) nowadays it is regarded as an introduced, or even an invasive taxon, in the Danube basin by some authors (Kebapçý & Van Damme 2012). It has LC status on the IUCN Red List (Seddon & Killeen 2010)., Published as part of Marković, Vanja, Gojšina, Vukašin, Novaković, Boris, Božanić, Milenka, Stojanović, Katarina, Karan-Žnidaršič, Tamara & Živić, Ivana, 2021, The freshwater molluscs of Serbia: Annotated checklist with remarks on distribution and protection status, pp. 1-64 in Zootaxa 5003 (1) on page 9, DOI: 10.11646/zootaxa.5003.1.1, http://zenodo.org/record/5120059, {"references":["Linnaeus, C. (1758) Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Vol. 1. Apud J. B. Delamolliere, Detroit, Lugduni, 220 pp. https: // doi. org / 10.5962 / bhl. title. 542","Matonickin, I., Pavletic, Z., Habdija, I. & Stilinovic, B. (1975) Prilog valorizacije voda ekosistema rijeke Save. Sveucilisna naklada Liber, Zagreb, 95 pp.","Jakovcev-Todorovic, D., Paunovic, M. M., Stojanovic, B., Simic, V. M., Dikanovic, V. & Veljkovic, A. (2005) Observation of the quality of Danube water in the Belgrade region based on benthic animals during periods of high and low water conditions in 2002. Archives of Biological Sciences, 57 (3), 237 - 242. https: // doi. org / 10.2298 / ABS 0503237 J","Paunovic, M., Csanyi, B., Knezevic, S., Simic, V., Nenadic, D., Jakovcev-Todorovic, D., Stojanovic, B. & Cakic, P. (2007 a) Distribution of Asian clams Corbicula fluminea (Muller, 1774) and C. fluminalis (Muller, 1774) in Serbia. Aquatic Invasions, 2 (2), 105 - 112. http: // doi. org / 10.3391 / ai. 2007.2.2.3","Markovic, V., Atanackovic, A., Tubic, B., Vasiljevic, B., Kracun, M., Tomovic, J., Nikolic, V. & Paunovic, M. (2012) Indicative status assessment of the Danube River (Iron Gate sector 849 - 1,077 rkm) based on the aquatic macroinvertebrates. Water Research and Management, 2 (2), 41 - 46.","Martinovic-Vitanovic, V. M., Rakovic, M. J., Popovic, N. Z. & Kalafatic, V. I. (2013) Qualitative study of Mollusca communities in the Serbian Danube stretch (river km 1260 - 863.4). Biologia, 68 (1), 112 - 130. https: // doi. org / 10.2478 / s 11756 - 012 - 0137 - 3","Tomovic, J., Paunovic, M., Atanackovic, A., Markovic, V., Gacic, Z., Csanyi, B. & Simic, V. (2014) Biotic typology of the Dan- ube River based on distribution of mollusc fauna as revealed by the Second Joint Danube Survey (2007). Acta zoologica bulgarica, 66 (4), 527 - 537.","Gloer, P. (2019) The freshwater gastropods of the West-Palaearctis. Biodiversity Research Laboratory, Hetlingen, 399 pp.","Bunje, P. M. (2005) Pan-European phylogeography of the aquatic snail Theodoxus fluviatilis (Gastropoda: Neritidae). Molecular Ecology, 14 (14), 4323 - 4340. https: // doi. org / 10.1111 / j. 1365 - 294 X. 2005.02703. x","Markovic, V., Tomovic, J., Ilic, M., Kracun-Kolarevic, M., Novakovic, B., Paunovic, M. & Nikolic, V. (2014) Distribution of the species of Theodoxus Montfort, 1810 (Gastropoda: Neritidae) in Serbia: an Overview. Acta Zoologica Bulgarica, 66 (4), 477 - 484.","Kebapcy, U. & Van Damme, D. (2012) Theodoxus fluviatilis (errata version published in 2017). The IUCN Red List of Threatened Species, 2012, e. T 165352 A 113400624. [downloaded 24 March 2021] https: // doi. org / 10.2305 / IUCN. UK. 2012 - 1. RLTS. T 165352 A 1081028. en","Seddon, M. B. & Killeen, I. (2010) Theodoxus fluviatilis. The IUCN Red List of Threatened Species, 2010, e. T 165352 A 6005432. [downloaded 3 May 2021]"]}

A population of Theodoxus fluviatilis L in the littoral zone of Lake Esrom was investigated from November 1977 to February 1979. The population was sampled every month in the winter period and twice during the rest of the year. Biomass was estimated as ash-free dry weight (AFDW) of the organic matter both of the soft parts of the animal and the shell itself. The relation between AFDW ( c ) and shell length ( l ) was log c =2.9509×log ( l )−1.7120. The population comprised more than 1 year-class, which could be separated by shell length, by a narrow band on the shells and the growth of algae on the shell. The life cycle lasted 2 1 2 - 3 years. The oldest animals had a shell length of 7.0–7.5 mm. A few individuals who were estimated to be 3 1 2 years had a shell length up to 8.6 mm. Population density varied between 575 and 2115 individuals m −2 on the stony substratum. The average was 1160 individuals m −2 . Mortality was low during the summer period. In winter many animals died due to the effect of ice and stormy weather on the stony substratum. Growth of the animals was estimated from the shell length. Maximum growth was observed from May to August with no growth during the winter. Egg capsules were found on the stones all year round. New capsules were found from late May to the middle of November. Most freshly laid capsules were observed in May–June and August–September. Capsules from the late summer hatched in spring and capsules laid in the spring hatched in August–September. The average annual net production for the whole population was estimated by three methods. The Allen curve method gave 1.895 AFDW m −2 , the growth-increment method gave 1.784 mg AFDW m −2 and the Hynes method 2.284 mg AFDW m −2 . Corresponding estimated P/B ratios were 1.29, 1.30 and 1.57. Annual net-production of the four investigated year-classes was 16 mg AFDW m −2 year −1 for 1975, 224 mg AFDW m −2 year −1 for 1976, 1.258 mg AFDW m −2 year −1 for 1977 and 287 mg AFDW m −2 year −1 for 1978. P/B ratios for the three oldest year-classes were, respectively, 0.32, 0.50 and 1.67. A comparison with other investigations on gastropod life cycles, reproduction and P/B ratios is made and differences discussed. Variations are correlated to temperature, and food quality and quantity.

The species compositions of fish trematodes in the neighboring Volga and Don basins are similar in many ways, however, there are some differences. One difference is that trematodes of the genera Nicolla and Plagioporus were not previously recorded in the Volga basin. Their absence was accounted for by the fact that their initial hosts, snails of the genera Lithoglyphus (Gastropoda: Lithoglyphidae) and Theodoxus (Gastropoda: Neritidae), did not occur in the Volga until recently. At present, these snails, trematodes, and some other parasites have successfully naturalized in the Volga and actively expand their ranges in its basin (Zhokhov and Pugacheva, 2001). Here, we present new data on the dispersal of these invaders in the Volga basin. The distribution of the trematode Nicolla skrjabini Iwanitzki, 1928 is limited to the rivers of the Azov‐ Black Sea and Baltic basins. That of the trematode Plagioporus skrjabini Kowal, 1951 is confined to rivers of the Azov‐Black Sea basin alone. The life cycles of these parasites are known. For N. skrjabini , the first intermediate host is the snail Lithoglyphus natocoides Pfeiffer, and the second intermediate host (according to experimental data) is Gammarus balcanicus Schaferna (Sten’ko, 1976). This trematode has a wide range of definitive hosts that comprises 27 fish species. In P. skrjabini , the snail Theodoxus fluviatilis and six gammarid species are involved in the life cycle (Chernogorenko et al., 1978). The definitive hosts of P. skrjabini are Cobitis taenia L., Neogobius fluviatilis Pall., and Mesogobius batrachocephalus Pall. In 1952, the Volga‐Don connected the two river basins, and the snails with associated trematodes found an opportunity to invade the Volga. The snails of the genus Lithoglyphus appeared in the lower Volga in the late 1960s (Belyavslaya and Vyushkova, 1971). In 1971, they were already numerous in the Volga delta (Pirogov, 1972). At present, these snails occur almost all over the Volga delta and form very dense aggregations (up to 11 180 ind./m 2 ) in the upper part of the delta and in arms of its western part (Biserova, 1990, 1996

Nutrient enrichment in coastal marine systems can have profound impacts on trophic networks. In the Baltic Sea, the population of Great Cormorant (Phalacrocorax carbo sinensis) has increased nearly exponentially since the mid-1990 s, and colonies of these seabirds can be important sources of nitrogen enrichment for nearby benthic communities due to guano runoff. In this study we used stable isotope analyses and diet mixing models to determine the extent of nitrogen enrichment from cormorant colonies, as well as to examine any possible changes in herbivore diet preferences due to enrichment. We found significantly higher levels of δ(15)N in samples from colony islands than control islands for producers (the dominant macroalga Fucus vesiculosus, filamentous algae, and periphyton) and herbivores, as well as a positive correlation between enrichment and nest density in colony sites. We also found that enrichment increased over the breeding season of the cormorants, with higher enrichment in late summer than early summer. While the amount of total nitrogen did not differ between colony and control sites, the amount of guano-based nitrogen in algae was >50% in most sites, indicating high nitrogen enrichment from colonies. Herbivores (the isopod Idotea balthica and the gastropod Theodoxus fluviatilis) preferred feeding upon the dominant macroalga Fucus vesiculosus rather than on filamentous algae or periphyton in both control and colony, and there was a significant increase in periphyton consumption near colony sites. Overall, guano from cormorant colonies seems to have effects on both producers and herbivores, as well as the potential to modify algae-herbivore interactions.

The euryhaline gastropod Theodoxus fluviatilis is found in northern Germany in freshwater or in brackish water habitats in the Baltic Sea. Previous studies have revealed that individuals from both habitats are not distinguishable by morphological characters or by sequence comparison of DNA encoding 16S RNA or cytochrome C. As reported in this study, animals collected in the two habitats differ substantially in their physiological ability to adapt to different salinities. Comparison of accumulation rates of ninhydrin-positive substances (NPS) in foot muscle upon transfer of animals to higher medium salinities revealed that brackish water animals were perfectly able to mobilize NPS, while freshwater animals had only limited ability to do so. In an attempt to explore whether this difference in physiology may be caused by genetic differentiation, we compared protein expression patterns of soluble foot muscle proteins using 2D gel electrophoresis and silver staining. Of the 40 consistently detected protein spots, 27 showed similar levels in protein expression in animals collected from freshwater or brackish water habitats, respectively. In 12 spots, however, protein concentration was higher in brackish water than in freshwater animals. In four of these spots, expression levels followed increases or decreases in medium salinities. In a different set of 4 of these 12 spots, protein levels were always higher in brackish water as compared to freshwater animals, regardless of their physiological situation (14 days in artificial pond water or in medium with a salinity of 16 per thousand). The remaining 4 of the 12 spots had complex expression patterns. Protein levels of the remaining single spot were generally higher in freshwater animals than in brackish water animals. These expression patterns may indicate that freshwater and brackish water animals of T. fluviatilis belong to different locally adapted populations with subtle genetic differentiation.

Horizontal starch gel electrophoresis was employed to reveal possible species specific patterns of esterases and peroxidases in four species of Lymnaea, Myxas glutinosa, Bithynia tentaculata, Theodoxus fluviatilis and the genus Hydrobia. From four of the species specimens from both fresh and brackish water were used. Esterase patterns were species specific in the anodic portion of the zymograms in all species. The remainder of the patterns displayed marked individual variation, the nature of which was not investigated. The peroxidases also revealed clearly species specific patterns. The amount of variation exhibited in fresh and brackish water populations of L. peregra and T. fluviatilis, the most thoroughly investigated species, was found to be roughly of the same magnitude, indicating a similar amount of intra-specific variation in fresh and brackish water. In the four Lymnaea species it appears from the peroxidase patterns that alternative alleles from an ancestral polymorphic locus have become established in different species. The esterase patterns of L. peregra and L. auricularia revealed distinct differences between the two taxa, which is also supported by the appearance of the spermathecal duct, whereas shell morphology and immunological investigations fail to show inter-taxon differences.

1. The marine form Nerita fulgurans is unable to osmoregulate, and regulates ions only to a very minor degree. The kidney plays no part in controlling the ionic or osmotic composition of the haemolymph. 2. The freshwater neritid Neritina latissima produces a dilute urine. Salts are removed from the urine by the glandular part of the kidney. The composition of haemolymph in the freshwater Theodoxus fluviatilis has been analysed. It contains surprisingly little calcium. 3. Observations on terrestrial species of Eutrochatella and Alcadia showed that these snails all produce a dilute urine. The site of salt reabsorption is not clear, but the results indicate that it may be reabsorbed by the glandular part of the kidney, the bladder, and also by the mantle epithelium. The haemolymph of the terrestrial species is similar in composition to that of Theodoxus . 4. The renal systems of marine, freshwater and terrestrial neritaceans are compared. Without much gross structural reorganization from the marine species, the Kidney of the freshwater forms have been developed into organs for reabsorbing salts from the urine. The terrestrial species have probably evolved from these freshwater ones, and have retained the production of a dilute urine; this is certainly appropriate because they live in very damp environments.

The mature typical spermatozoon of Theodoxus fluviatilis (L.) (Gastropoda: Prosobranchia) is 66 μm in length and has a unique mode of locomotion, with the tail first and trailing back over the head. This previously undescribed type of movement seems to derive from its particular morphology (Giusti and Selmi, 1982) (Plate 1).

This paper describes the morphological plasticity of Theodoxus fluviatilis (Linnaeus, 1758) (Mollusca: Gastropoda: Neritidae).