17β-Oestradiol may prolong reproduction in seasonally breeding freshwater gastropod molluscs

Whilst the effects of oestrogenic contaminants in the aquatic environment are well documented in fish, effects in invertebrate species has been subject to debate, possibly due to differences in experimental conditions (temperature, timing and duration of exposure) between studies. It has been suggested that molluscs are only susceptible to oestrogens in periods either following the main spawning or leading up to the maturation of gametes. To investigate this possibility, two temperate, seasonally reproducing gastropods (Planorbarius corneus and Viviparus viviparus) were exposed to two concentrations of 17β-oestradiol (E2; 10 ng/l and 100 ng/l nominal) in an outdoor mesocosm (subject to natural seasons). In addition, P. corneus was also exposed to E2 (1, 10 and 100 ng/l) in the laboratory at temperatures and photoperiods to simulate summer and autumn.In the mesocosm, both snail species produced similar numbers of eggs/embryos as reference groups in the summer, but the groups exposed to 10 ng E2/l (nominal) had significantly higher productivities after the onset of autumn, when entering their quiescent phase, whilst the snails exposed to a higher concentration (100 ng/l, nominal) had an increased rate of mortality, and did not experience increased reproduction. In the laboratory, the rate of egg laying in P. corneus was unaffected in simulated summer (20 °C, 16 h photoperiod), but snails exposed to 10 and 100 ng/l (nominal) in simulated autumn (15 °C, 12 h photoperiod) showed a concentration-dependent inhibition of the natural decline in egg laying observed in the control snails. Overall, rather than an increase in reproductive rate, the response of this species was a perpetuation of summer reproductive rates into autumn. We conclude that exposure to E2 can affect reproduction in the freshwater gastropods studied, but in P. corneus at least, this is dependent on the seasonal conditions (temperature and photoperiod) at which exposures are made.