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Although the New Zealand mudsnail Potamopyrgus antipodarum has been observed in Italy since 1961, no investigations have gone beyond recording its presence. In this study, populations from three Italian watercourses with different environmental features were comparatively analysed with a view to shedding light on the relationships between habitat type and reproduction. At the study sites, populations were exclusively composed of females. Ultra-structural details showed that egg growth takes place through the accumulation of stored materials, which are essentially constituted by translucent vesicles (lipids) and by electron-dense granules with a darker rim (proteins), the latter deriving from the confluence of smaller granules of the same type. The egg surface displays numerous microvilli, which interdigitate with those emerging from both adjacent eggs and bordering cells. Our comparative analysis shows that the populations differ slightly in fecundity, whereas the number of brooded embryos/female markedly decreases, in parallel with the mean density, in running-water habitats lacking aquatic vegetation. Statistical analysis confirms that P. antipodarum positively selects biological substrates, such as beds of Vaucheria sp. and even, on occasion, dead leaves and wood floating on the water surface. The results suggest that biological substrates constitute one of the key factors favouring the massive spread of P. antipodarum in watercourses.

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Monthly monitoring over one year (June 2003-May 2004 allowed us to evaluate the distribution of the Eurasian invader Dreissena polymorpha along the shores of Lake Trasimeno (Central Italy) and to shed light on the reproduction of these zebra mussels. Sampling was carried out on 12 study sites, 10 with hard substrata and 2 with soft sediments. The mollusk colonized different substrata, a feature that affected its density: from 40 on wooden boat keels to 200,000 individuals/m 2 on concrete. In all months, gonad and gamete development was investigated by means of histological examination. The gonad of both sexes consisted of a single Y-shaped organ, which increased in volume during maturation until it occupied the whole visceral mass in ripe individuals. Only one period of successful reproduction seemed to exist, and the gonadal development of males and females coincided fairly closely. Gamete development seemed to depend on a rapid increase in water temperature (from 4.3uC to 13.4uC). Inactive gonads were observed in a small percentage of individuals (18%). Females and males constituted 38% and 44% of the population, respectively (sex ratio of 1:1.16). No hermaphrodites were found. This investigation provides additional information on the invasive zebra mussel, a non-native organism that has established a self-sustaining population in Lake Trasimeno, and may be useful for management and control of the establishment and spreading of invasive species.