Laboratory studies in 1991Laboratory studies in -2001 with observations in natural habitat, made it possible to establish the following characteristics of the life cycle of Valvata cristata O. F. Müll.: female maturity at shell diameter of 2.05-2.95 mm, number of whorls 2.375-3, 55-533 days from hatching, depending mainly on food conditions. Mean life span in good food conditions 377 days, in bad food conditions 896 days (maximum 1,170 days). Snails kept in pairs deposited a mean number of 51 cocoons per lifetime (maximum 105), with a total of 134 eggs (maximum 353); the number of eggs per cocoon ranged from 0 to 10, rarely more than 16 (mean 2.6). Depending on life span there were 1-3 reproductive seasons. Of 20 adult snails kept singly, only one laid eggs which hatched. The mean maximum shell size for snails kept in pairs: 3.28 mm, 3.125 whorls, for snails kept singly: 3.93 mm, 3.375 whorls (maximum 4.70 mm, 3.750 whorls). Snails with shells of strongly descending whorls were almost always infected with trematode larvae located in their gonads. Snails infected when young never reached female maturity.
Laboratory culture in 1994-1999 provided the following data on the life cycle of Valvata macrostoma Mörch: in favourable food conditions female maturity is attained in 60-250 days (mean: 108) at shell diameter 2.40-3.30 mm (rarely more: up to 4.10) and 2.50-3.00 whorls (rarely up to 3.37); in unfavourable conditions the snails mature later, at a somewhat smaller shell size. Snails kept in pairs/groups produce a maximum of 166 cocoons during their lifetime; the cocoons contain a total of up to 1,436 eggs, the number of eggs per cocoon ranging from 0 to 25 (rarely up to 34). Snails kept singly deposit few cocoons and eggs; and the eggs fail to develop. The average life span in favourable food conditions is 383 days, in unfavourable conditions 706 days (maximum 1,192 days). The mean shell size attained in favourable conditions is: diameter 4.46 mm, 3.46 whorls; in unfavourable conditions: diameter 3.65 mm, 3.09 whorls. Mortality of adult snails in laboratory shows a clear seasonal variation, with maximum in June and July.
Laboratory and field observations in 1994-2004 made it possible to ascertain the following parameters of life cycle of Valvata piscinalis (O. F. Müller, 1774). In favourable food conditions (filamentous diatoms) female maturity was attained in 42-85 days from hatching (mean 59 days) at the body whorl diameter of 3.45-4.65 mm (mean 4.10 mm) and the number of whorls 3.25-3.75 (mean 3.53). In slightly less favourable food conditions (periphyton) the maturity was attained later at a somewhat lower mean shell size. The number of cocoons and eggs produced during lifetime by snails kept in pairs was 18-113 and 387-2,862, respectively; the number of eggs per cocoon ranged from 0 to 76. Among singly kept snails only half laid cocoons (maximum 7 cocoons/6 eggs during lifetime), and the eggs failed to develop. The mean life span of snails fed mainly with periphyton was ca. 1 year (maximum 641 days); it was distinctly shorter in snails fed with filamentous diatoms (usually 5-7 months, rarely more than 11 months). In very unfavourable food conditions (small quantities of periphyton) female maturity was attained only after 1-2 years and the life span was not much longer: maximum 830 days. In natural conditions (lake Sosnowe near S¹polno) reproduction started in the second half of April and lasted till half of July; the cocoons contained 10-37 eggs.
The predatory snail Aegopinella nitidula (Drap.) (Zonitidae) can perforate a rather thick shell and extract from it even a strongly contracted snail body. Numerous empty shells of Perforatella bidentata (Gmel.) with a characteristic hole, found in the wild, indicate that this species often falls a prey to Ae. nitidula.
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