Thereafter, mortality increased and only five snails (5%) survived by the next spring (2010). Juveniles also suffered considerable mortality (14%) during the first month, but their survival rate remained high until the next summer (1 year after their release). 2A) and no snail survived to the next spring. Egg production per female was analysed for the two density levels using MANOVA, with the data in the peak spawning period within each year being used as the dependent variables.įor the snails released in July 2008, adult snails began to die soon after the start of the experiment (Fig. We estimated the number of eggs in each egg mass using the regression equation: ln y = 166.4 ln x – 404.5, where y is the number of eggs and x is the maximum length (mm) of the egg mass ( Yoshida et al., 2009). The difference in SH between the sexes was tested with MANOVA. The difference in lifespan between the treatments and between the sexes was examined with a generalized Wilcoxon test. Survival rates were analysed by the Kaplan–Meier method, which enabled calculation of the duration with 50% mortality (LDay 50). The life span of each released snail was estimated by assuming that the snail died in the middle of the period between the day when it was found dead and the preceding census day. Other small food material could enter the cages through the mesh. No food was provided in the cages, but sediment (approximately 1 kg mud) was added to them at the beginning of the experiment. Snails with SH of 25 mm or larger were considered as adults ( Estoy et al., 2002) and these snails were sexed according to the shapes of the operculum and the shell aperture ( Cazzaniga, 1990). The egg masses were removed from the cages to maintain the low-density conditions usually observed in canals ( Hara et al., 2015). At each survey, we checked the survival of the snails and measured the shell heights of the living snails and the maximum length of each egg mass found on the net. We conducted surveys once or twice a month from April to October and only twice from November to March. The snails were individually marked with paint before release. The juveniles had been born on May 20 and reared in a cage of the same size with a finer mesh. Juveniles with 8–14 mm SH were released at two snail densities to examine the effects of density on their traits: four juveniles per cage (five replicates) and 16 juveniles per cage (three replicates). A second experiment began on 18 June 2009. Adults had been collected in a nearby canal and juveniles were obtained as hatchlings from egg masses laid in the experimental canal.
Eight adults (four males and four females) with shell heights (SH) of 41–53 mm and 16 juveniles of 8–14 mm SH were released into each cage (six replicates). A first experiment was conducted on 20 July 2008. We released snails into the experimental cages on two occasions and monitored their life-history traits. canaliculata under seminatural conditions, we placed experimental cages (mesocosms) made of steel frames (1 × 1 × 1 m 3) and nylon netting (5 mm mesh covering all sides, including top and bottom) in the irrigation canal. To investigate the life-history traits of P. canaliculata, according to Seuffert & Martín (2013b) and Seuffert, Burela & Martín (2010). Abbreviation: TTD, approximate threshold temperature (15 ☌) for development and activity of P. Monthly mean temperature was drawn from the data of the Automated Meteorological Data Acquisition System (operated by the Japanese Meteorological Agency) point at Saga, 13 km from the experiment site. Fluctuations of temperatures (monthly mean temperature, ambient temperature and water temperature) measured at each survey during the experimental period.
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