Escargots cooked just right: telling apart the direct and indirect effects of heat waves in freashwater snails
Direct and transgenerational effects of an experimental heat wave on early life stages in a freshwater snail
Amongst the many challenges and forms of environmental change that organisms face in our era of global change, climate change is perhaps one of the most straightforward and amenable to investigation. First, measurements of day-to-day temperatures are relatively feasible and accessible, and predictions regarding the expected trends in Earth surface temperature are probably some of the most reliable we have. It appears quite clear, in particular, that beyond the overall increase in average temperature, the heat waves locally experienced by organisms in their natural habitats are bound to become more frequent, more intense, and more long-lasting . Second, it is well appreciated that temperature is a major environmental factor with strong impacts on different facets of organismal development and life-history [2-4]. These impacts have reasonably clear mechanistic underpinnings, with definite connections to biochemistry, physiology, and considerations on energetics. Third, since variation in temperature is a challenge already experienced by natural populations across their current and historical ranges, it is not a completely alien form of environmental change. Therefore, we already learnt quite a lot about it in several species, and so did the species, as they may be expected to have evolved dedicated adaptive mechanisms to respond to elevated temperatures. Last, but not least, temperature is quite amenable to being manipulated as an experimental factor.
For all these reasons, experimental studies of the consequences of increased temperature hit some of a sweetspot and are a source of very nice research, in many different organisms. The work by Leicht and Seppala  complements a sequence of earlier studies by this group, using the freshwater snail Lymnaea stagnalis as their model system [6-7].
In the present study, the authors investigate how a heat wave (a period of abnormally elevated temperature, here 25°C versus a normal 15°C) may have indirect effects on the next generation, through maternal effects. They question whether such indirect effects exist, and if they exist, how they compare, in terms of effect size, with the (more straightforward) direct effects observed in individuals that directly experience a heat wave. Transgenerational effects are well-known to occur following periods of physiological stress, and might thus have non negligible contributions to the overall effect of warming.
In this freshwater snail, heat has very strong direct effects: mortality increases at high temperature, but survivors grow much bigger, with a greater propensity to lay eggs and a (spectacular) three-fold increase in the number of eggs laid . Considering that, it is easy to consider that transgenerational effects should be small game. And indeed, the present study also observes the big and obvious direct effects of elevated temperature: higher mortality, but greater propensity to oviposit. However, it was also found that the eggs were smaller if from mothers exposed to high temperature, with a correspondingly smaller size of hatchlings. This suggests that a heat wave causes the snails to lay more eggs, but smaller ones, reminiscent of a size-number trade-off. Unfortunately, clutch size could not be measured in this experiment, so this cannot be investigated any further. For this trait, the indirect effect may indeed be regarded as small game : eggs and hatchlings were about 15 % smaller, an effect size pretty small compared to the mammoth direct positive effect of temperature on shell length (see Figure 4 ; and also ). The same is true for developmental time (Figure 3).
However, for some traits the story was different. In particular, it was found that the (smaller) eggs produced from heated mothers were more likely to hatch by almost 10% (Figure 2). Here the indirect effect not only goes against the direct effect (hatching rate is lower at high temperature), but it also has similar effect size. As a consequence, taking into account both the indirect and direct effects, hatching success is essentially the same at 15°C and 25°C (Figure 2). Survival also had comparable effect sizes for direct and indirect effects. Indeed, survival was reduced by about 20% regardless of whom endured the heat stress (the focal individual or her mother; Figure 4). Interestingly, the direct and indirect effects were not quite cumulative: if a mother experienced a heat wave, heating up the offspring did not do much more damage, as though the offspring were ‘adapted’ to the warmer conditions (but keep in mind that, surprisingly, the authors’ stats did not find a significant interaction; Table 2).
At the end of the day, even though at first heat seems a relatively simple and understandable component of environmental change, this study shows how varied its effects can be effects on different components of individual fitness. The overall impact most likely is a mix of direct and indirect effects, of shifts along allocation trade-offs, and of maladaptive and adaptive responses, whose overall ecological significance is not so easy to grasp. That said, this study shows that direct and indirect (maternal) effects can sometimes go against one another and have similar intensities. Indirect effects should therefore not be overlooked in this kind of studies. It also gives a hint of what an interesting challenge it is to understand the adaptive or maladaptive nature of organism responses to elevated temperatures, and to evaluate their ultimate fitness consequences.
 Meehl, G. A., & Tebaldi, C. (2004). More intense, more frequent, and longer lasting heat waves in the 21st century. Science (New York, N.Y.), 305(5686), 994–997. doi: 10.1126/science.1098704
 Adamo, S. A., & Lovett, M. M. E. (2011). Some like it hot: the effects of climate change on reproduction, immune function and disease resistance in the cricket Gryllus texensis. The Journal of Experimental Biology, 214(Pt 12), 1997–2004. doi: 10.1242/jeb.056531
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 Sentis, A., Hemptinne, J.-L., & Brodeur, J. (2013). Effects of simulated heat waves on an experimental plant–herbivore–predator food chain. Global Change Biology, 19(3), 833–842. doi: 10.1111/gcb.12094
 Leicht, K., & Seppälä, O. (2019). Direct and transgenerational effects of an experimental heat wave on early life stages in a freshwater snail. BioRxiv, 449777, ver. 4 peer-reviewed and recommended by PCI Ecology. doi: 10.1101/449777
 Leicht, K., Seppälä, K., & Seppälä, O. (2017). Potential for adaptation to climate change: family-level variation in fitness-related traits and their responses to heat waves in a snail population. BMC Evolutionary Biology, 17(1), 140. doi: 10.1186/s12862-017-0988-x
 Leicht, K., Jokela, J., & Seppälä, O. (2013). An experimental heat wave changes immune defense and life history traits in a freshwater snail. Ecology and Evolution, 3(15), 4861–4871. doi: 10.1002/ece3.874
vincent calcagno (2019) Escargots cooked just right: telling apart the direct and indirect effects of heat waves in freashwater snails. Peer Community in Ecology, 100015. 10.24072/pci.ecology.100015
Revision round #22019-03-19
Decision round #2
Dear authors, Thank you for th eeffort you put into revising your preprint. The reviewers were satisfied and so am I. I am willing to recommend this preprint and I've almost finished writing up a recommendation text. In the process I have picked a few typos. I list them below and perhaps you could fix them in the final preprint. All the best
-- Line 84: the sentence starting with "Especially..." is strange. Perhaps you want to start it with "In particular, trasngenerational..."
-- Line 109: Remove "of" after "despite"
-- In legend of Figure 1 :"Hatchlings" is mispelled
-- In the references : journal name is BMC Evolutionary Biology, with BMC capitalized
-- The paper by Wadgymar et al in Am Nat is no longer in press: please update the reference
Reviewed by arnaud sentis, 2019-02-26 17:53
Reviewed by Amanda Lynn Caskenette, 2019-02-13 16:07
Revision round #12018-12-04
Decision round #1
I have read the preprint and had it evaluated by three expert reviewers. All reviewers expressed concerns related to the presentation of the results and some aspects of experimental or statistical methods that could not easily be parsed from the manuscript. Importantly, all reviewers pointed that egg number/size is not shown and very little analyzed, and that it would be important to do so because egg size and egg number might show correlated responses in the context of a size-fertility tradeoff. This may relate to a, possibly adaptive, shift in the egg-laying strategy of females as a function of temperature, and deserves consideration.
There were also several concerns regarding the statistical procedures and interpretations. These should be clarified (possibly, as suggested, with the help of an additional Figure). Similarly, the comparison of effect sizes between direct and indirect effect should be moderated somehow, since not all traits were not equally affected.
I agree with the reviewers. Personally,I recommend the authors to homogenize the different result figures: whilst several follow the exact same pattern, some are presented as barplots, some as simple dots, some show mean+-SE, some show median/quartiles. This is confusing. I even think that figures are pretty small and contain one panel only, and that they would benefit from being lumped (at least, Figs 1&2 and 3&4 constitute pairs of companion panels). This way, they might get some extra space for the reporting of the additional results and figures that reviewers have asked for.
Last, the manuscript needs some restructuring, and the reviewers made several suggestions to this end. For instance, some parts are found in the Material & Methods section while they really are Results material (I am thinking for instance of survival and fertility values, provided on page 7 in the Methods, and that one reviewer found missing in the Results). An additional figure highlighting the general experimental setup would certainly help.
Considering all these elements, I cannot recommend the preprint as it is, even though it addresses an interesting topic and reports some nice results. If the authors can, and are willing to, address all the points in the present letter and in the review documents, the preprint may certainly become recommendable. In any case, I hope these reviews will be of some help to the authors.