Valvata

Abstract: Analysis of predation by carp (Cyprinus carpio L.) on mollusc species in Skadar Lake, Yugoslavia, indicated both generalized and specific features of a predator-prey interaction. Vertical distribution of molluscs in sediments seemed unrelated to predation rates by carp. Although size selective predation may occur, electivity indices appeared most related to shell shape and thickness. Of the three most abundant species, electivity was high for Valvata piscinalis but low for Pyrgula annulata and Dreissencia polymorpha. Valvata has a more readily crushed shell, a higher proportion of occupied shells and a greater percentage of organic content than Pyrgula. Although a potentially preferred prey item, Dreissencia probably escaped intensive predation by being attached to larger objects. Carp appeared to utilize a sequence of tactile and gustatory cues in the prey selection process.

Abstract: Using published records and original data from recent field work and revision of Iranian material of certain species deposited in the collections of the Natural History Museum Basel, the Zoological Museum Berlin, and Natural History Museum Vienna, a checklist of the freshwater gastropod fauna of Iran was compiled. This checklist contains 73 species from 34 genera and 14 families of freshwater snails; 27 of these species (37%) are endemic to Iran. Two new genera, Kaskakia and Sarkhia, and eight species, i.e., Bithynia forcarti, Bithynia starmuehlneri, Bithynia mazandaranensis, Pseudamnicola georgievi, Kaskakia khorrasanensis, Sarkhia sarabensis, Valvata nowsharensis and Acroloxus pseudolacustris are described as new to science; Ecrobia grimmi (Clessin & Dybowski, 1888), Heleobia dalmatica (Radoman, 1974) and Hippeutis complanatus (Linnaeus, 1758) are reported for the first time from Iran. Additional field work is highly desirable for a more appropriate evaluation of the extant freshwater snail biodiversity in Iran.

Abstract: Population statistics were calculated for natural populations of Physa gyrina, Gyraulus deflectus, Valvata humeralis, and Pisidium compressum. Total net production calculated for model populations was within 10% of the value calculated graphically. Data showed a generally good fit to the model curve. The average of the two methods of determining total annual net production of the gastropods was 5.150 g/m2. Assuming an efficiency of 0.25, total gastropod assimilation was approximately 10% of the annual net primary production. Growth rates apparently were not limited by food supply. Mortality rates were highest in spring and summer and lowest in winter with the principal causes predation and loss during high water. Reproductive periods of Physa, Gyraulus, and Pisidium extended from spring to fall; Valvata reproduction was largely limited to August.

Abstract: The interaction of population stability and habitat permanence has a major influence on the microdistribution of freshwater snails. For two years (February 1980-January 1982), we monitored the abundance of macrophytes and the abundance and size structure of four species of macrophyte-associated freshwater snails in an English pond. Previous work (Lodge, in press) showed that two species, the pulmonate Lymnaea peregra (Mull.) and the prosobranch Valvata piscinalis (Mull.), were associated with cubmersed macrophytes, while two other species, the pulnonate Planorbis vortex (Linn.), and the prosobranch Bithynia tentaculata (Linn.), were associated with emergent macrophytes. A dramatic decline of submersed macrophytes provided a test of the hypotheses that the population stability of Lymnaea and Valvata was 1) high, and 2) an important cause of the association of those two species with submersed macrophytes.When the submersed macrophytes declined in August 1980, >99% of the Lymnaea and about 35% of the Valvata population died. The populations of Planorbis and Bithynia were not reduced. In 1980, Lymnaea and Valvata had simple annual life cycles, but with the regrowth of submersed macrophytes in spring 1981, the Lymnaea and Valvata populations responded with early, high, and repeated reproduction with some overlap of generations. In both years, Planorbis had an annual semelparous life cycle, while Bithynia lived up to 3 years and bred iteroparously.Following the terminology of Connell and Sousa (1983), Lymnaea exhibited low resistance to habitat disturbance but high adjustment following the disturbance. Valvata showed higher resistance than Lymnaea, and also high adjustment. Although the population stability of Planorbis and Bithynia could not be rigorously evaluated, published accounts of those species' life cycles suggest that stability, specially the adjustment component, was low. We suggest that the population stability of the four species is a major determinant of the association of Lymnaea and Valvata with the impermanent macrophyte habitat and that of Planorbis and Bithynia with the permanent macrophyte habitat.