Local adaptation and embryonic plasticity affect antipredator traits in hatchling pond snails

The biology of induced defences in very early life history stages is poorly understood in freshwater invertebrates, but may be equally, if not more, important than later stages in influencing population dynamics and survival. Here, we investigated how exposure of embryos of the pond snail Lymnaea stagnalis to predator kairomones altered traits associated with protection (crawl out behaviour and shell size) in hatchling snails. We also investigated whether levels of plasticity were influenced by habitat of origin by comparing reaction norms from sites with different levels of predation risk: three high risk (predatory fish present) and three low risk (predatory fish absent). F2 embryos from snails from each population were exposed to kairomones from a predatory fish (Tinca tinca) or control conditions for their entire development up to the point of hatching. Their avoidance behaviour and size (spire height) were then measured 1 day and 7 days after hatching. One day post-hatching, levels of predator avoidance, primarily time spent at the water line, were significantly greater in high-risk versus low-risk populations; however, avoidance was also increased in hatchling snails exposed to predator cues as embryos, compared with those developing in control conditions. Snails exposed to predator cues during development were smaller, and there was a strong negative relationship between size and avoidance behaviour, suggesting a trade-off or trait compensation between growth and behaviour. At 7 days post-hatching, a similar pattern in avoidance behaviour was found, with increased avoidance in snails from high-risk habitats and in those exposed to predator kairomones during their development. The primary avoidance response switched to crawling above the waterline at 7 days post-hatching. There was no significant difference in shell size between treatments, suggesting that predator-exposed snails had exhibited 'catch-up' growth. This enhanced investment in growth between 1 day and 7 days post-hatching was positively correlated with elevated avoidance behaviour across treatment groups, demonstrating that snails switched to trait co-specialisation of defence traits. During the first few days post-hatching, snails alter the type of avoidance behaviour and also the relationship between morphological and behavioural defence traits. The degree of expression of these defensive traits in hatchling snails is influenced by both their developmental environment and local adaptation to their predation habitat that may have large implications for survival.