We hypothesised that predation risk should impact the growth and energetic budget of Nucella offspring, over the period from emergence to 8 months old. In addition, our mesocosm based approach also allowed us to test the role of parental habitat in modifying offspring responses to threat. Our original hypotheses were partially supported. We showed clear negative impacts of exposure to risk on both shell length and tissue mass as well as the ratio of shell to tissue investment. The influence of predatory risk on energy budget was however much less clear, with some effects on protein and carbohydrate content, but not on energy allocation. In general effects strengthened with increasing proportion of time exposed to predators i.e., from no risk, to variable risk, to constant risk. This effect of predation threat on growth and energetic responses of offspring was modified by parental habitat although effects varied across different response variables. Parental habitat modified the response to predatory risk for growth (shell length, tissue mass) and the level of protein. We also observed main effects of parental habitat for lipid content and total energy, with higher levels in offspring from sheltered shores.
There was a clear effect of parental habitat on growth and energetics in Nucella, and abundant evidence that it modified the effect of individual experience of risk. However, across the range of response variables measured there was a diversity of outcomes (i.e., main effect of parental habitat, interactive effect with risk; no effect). The contrasting parental habitats chosen (exposed versus sheltered shores) have well evidenced differences in predatory pressure (Karythis et al. 2020) which is highly relevant in the context of our focus on impacts of predatory risk. This pattern led to our expectation for snails from sheltered shore parents, which experience high predation pressure, to show adaptive responses to risk, while those of exposed shores should not. However, it is important to note that differences in the two parental habitats extend beyond these biotic differences and the complex mix of physical and biotic factors may explain the variable responses across our experimental work.
Growth responses (shell length and tissue mass) demonstrated that parental habitat modifies the effect of predatory risk, though only tissue mass responded exactly as we hypothesised with no negative effect of risk on growth in sheltered shore offspring, but a clear negative effect in exposed shore offspring. Previous work has shown that the extent to which parental experience is able to modify risk effects is highly context dependent (Coslovsky and Richner 2011; Giesing et al. 2011; Stratmann and Taborsky 2014; Basso and Richner 2015; Donelan and Trussell 2018a). The influence of parental experience on the growth of their offspring can either be adaptive, resulting in relative increases in offspring growth under risky conditions, or it may be detrimental, with maternal stress resulting in lower offspring growth rates. In the current study, tissue mass in offspring of parents from exposed shores where predators are at a low density, was negatively impacted by risk as predicted, with greater risk resulting in lower mass. By contrast, offspring of parents from sheltered environments where predation threat is high was the same irrespective of the risk they experienced throughout development. Similar adaptive parental effects to predation risk have been identified in birds (Coslovsky and Richner 2011), fresh water snails (Beaty et al. 2016) and fish (Stratmann and Taborsky 2014). Preparing offspring to more effectively cope with the risks they are likely to encounter, will translate into higher growth rates, decreasing prey susceptibility to the predictably high predation rates of their parental habitat, thus increasing their lifetime fitness.
The ability of an organism to scale the investment in defences in response to the actual predation risk they experience enables prey to optimise the cost-benefit balance of inducible defences (Teplitsky et al. 2005). Interestingly, our results clearly show that Nucella allocated resources to tissue growth and shell deposition in accordance with the developmental risk they experienced but independent of parental habitat. In line with this, the ratio of shell mass to tissue mass of Nucella was lowest in the no risk treatment, intermediate under a variable risk regime and highest under constant risk. The development of neck teeth in Daphnia pulex and the tail length of tadpoles show an equivalent threat-sensitive investment in defences (Tollrian 1993; Van Buskirk and Arioli 2002; Teplitsky et al. 2005), with increased perception of risk resulting in adaptable increases in both. In the case of Nucella, the deposition of shell to increase its robustness to predatory attacks is their primary form of defence against predators and as such the drivers of this deposition are important in determining individual fitness (Crothers, 1983). Debate remains on whether increases in shell deposition in gastropods are a direct result of the chemical cues released by crabs or as a result of reductions in foraging under elevated levels of predation risk (Trussell and Nicklin 2002; Trussell et al. 2003; Brookes and Rochette 2007; Bourdeau 2010; Pascoal et al. 2012b). Although our results are unable to shed more light on this issue, they do show the importance of incorporating different temporal patterns of risk in studies investigating the antipredator responses of prey.
For some response variables, parental effects alone were detected, with no effect of individual experience of risk. Total lipids and total energy were significantly greater in offspring from parents originating from sheltered shores, but did not respond to predatory threat, potentially demonstrating a selective dominance of wave-action over predation in this prey species (Pascoal et al. 2012a, b). In their study into the heritable nature of shell morphology in Nucella, Pascoal et al (2012) argue that wave action may vary monotonically between shores, in contrast to the more spatially variable patterns of predation risk. Similarly, the ability of parentally driven adaptations to environmental stressors to supersede those in response to consumers have been previously identified in other taxa. For example, González, et al, (2017) demonstrated that offspring of the white clover (Trifolium repens) showed drought resistance rather than resistance to herbivory despite parents being exposed to both types of stresses. As with drought resistance in white clover, the dominant role of parental ecotypes over predation risk in the current study, possibly represents the higher predictability or selective pressure posed by wave-action, over the less predictable and more variable risk posed by predation (Trussell et al. 1993; Stein et al. 2018). These results agree with the findings of a recent meta-analysis by Moore and collegues (2019), revealing that maternal effects may be most influential on the morphology and phenology of organisms, especially during the juvenile stage, as morphological adaptations impose architectural constraints which can be harder to reverse. The importance of these parental effects indicates that predators are only a part of the risk environment faced by parents and that transgenerational effects may supersede individual experience when one stressor (such as wave-action) plays a dominant role in the lifetime fitness of prey.
Differences in the amount of energy stored as lipids between wave-sheltered and wave-exposed populations, may be explained by the role played by lipids in egg production in Nucella (Crothers, 1974). Notably, Nucella adopt different reproductive strategies in order to best cope with local wave-action (Etter 1989; Adams and Freeman 2017), with exposed shore populations producing greater numbers of smaller egg capsules than their sheltered shore counterparts. This modification of reproductive strategies across different abiotic conditions is a feature common to a number of taxa (Giesel 1976; Etter 1989; Haag and Warren 1998; Morrongiello et al. 2012). For example the freshwater southern pygmy perch (Nannoperca australis), for instance, inhabits a range of environments, ranging from transitory creeks to large river, and populations in each environment have different reproductive strategies to maximise offspring fitness (Morrongiello et al. 2012). In more ephemeral streams, mothers produce many small eggs to maximise the chances of their offspring dispersing to more permanent waterways. As is the case with parentally informed antipredator morphologies, it also seems adaptable that in response to long-term and less variable stressors such as wave exposure, parental experience may inform the reproduction and ultimately the fitness of their offspring.
In line with previous findings (Van Dievel et al. 2019), we predicted that predation would act as a long term stressor, resulting in reductions in CEA through impacts on both energy storage and usage. However, our CEA analysis revealed no difference in this ratio, despite sheltered shore offspring having overall higher energy reserves owing to higher protein and lipid content. When Van Dievel et al., (2019) exposed damselfly larvae (Enallagma cyathigerum) to a predatory threat, they showed an increase in metabolic rate, which resulted in a reduction in CEA of risk exposed individuals. Differences between these findings and those of the current study may be accounted for by the differences in the duration of the two experiments (days vs months). Nucella are known to respond metabolically to the detection of a predator (Karythis et al. 2020), but metabolic changes may subside with prolonged exposure, as is the case with many other species (Holopainen et al. 1997; Steiner and Van Buskirk 2009). For instance, tadpoles (Rana temporaria) show anticipated rises in metabolic rate when exposure to predation risk is brief, but this pattern is reversed when exposure is prolonged (Steiner and Van Buskirk 2009). Finally, growth differences and the lack of any difference in energy usage between risk treatments would seem to indicate that differences in the energetic budget of prey must come from increases in resource acquisition and utilisations efficiency.
The results of this study highlight the intricate nature of the indirect interactions between predators and prey. With each generation facing multiple stressors and as these may affect long- and short-term fitness, insights into the trade-offs between parentally acquired information and individual experience will be crucial in understanding predator-prey interactions comprehensively. Parental effects that enable offspring to grow more efficiently under the probable risks they may face, and threat-sensitive antipredator morphologies based solely on an individual experience of risk, indicate the complex nature of these interactions. Our results suggest that the importance of individual and parental experience of risk on the development of traits in prey may depend on the predictability and fitness costs of each stressor.