Salinity-induced heterokairy in an upper-estuarine population of the snail Radix balthica (Mollusca: Pulmonata)

Climate change is predicted to increase sea level and cause saline intrusion of coastal freshwaters. This will have consequences for freshwater organisms inhabiting such areas; developmental phenotypic plasticity may facilitate the persistence of freshwater species under such scenarios of increased salinity. Here we investigated developmental plasticity under different salinity treatments (S = 2, 5, 7 and 9, with artificial pond water [APW] and deionised water [S = 0] as controls) in embryos from an upper-estuarine population of the gastropod Radix balthica. We focused on plasticity in the timing of developmental events (heterokairy) at different salinities, including the time of onset (relative and absolute) of 10 developmental events and duration of 4 developmental stages; we also assessed whether salinity affected hatchling morphology and embryonic cardiac activity. There were significant differences in the absolute time of onset of several events with increased salinity, including a delayed first heartbeat in S5 and S7 compared with APW and in the relative time of onset of eye spot formation, first heartbeat and foot attachment between treatments. Stage duration also varied between salinity treatments: the hippo developmental stage lasted significantly longer in S7 compared with APW and development overall was prolonged in S7, with embryos in S9 not developing past the trochophore stage. Salinity also affected both shell size and shell shape of hatchlings. Hence salinity influences the time of onset (absolute and relative) of developmental events and duration of several developmental stages and the morphology of hatchling snails, although the mechanistic basis and fitness implications are, as yet, unknown.