Facultative sexuality combines clonal propagation with sexual reproduction within a single life cycle. Clonal propagation enables quick population growth and the occupancy of favorable habitats. Sex, on the contrary, results in the production of offspring that are more likely to survive adverse conditions (such as the resting eggs of many freshwater invertebrates). In seasonal environments, the timing of sex is often triggered by environmental cues signaling the onset of winter (e.g., temperature drop or changes in photoperiod). Organisms switching to sex to produce resting eggs under these conditions face a trade‐off: Responding too early to an environmental cue increases the chances of missing out in clonal propagation, while having a delayed response to deteriorating conditions entails the risk of parental mortality before sexual reproduction could be completed. To mitigate these risks, increased sensitivity toward environmental cues with the onset of the winter might be an adaptive strategy. To test this hypothesis, we investigated sexual propensity and time to gonadogenesis in clonal strains derived from spring‐ and autumn‐collected polyps of Hydra oligactis, a facultatively sexual freshwater cnidarian where sex only occurs prior to the onset of winter. We show that autumn‐collected individuals and their asexual offspring have a higher propensity for sex and require less time for gonad development compared with strains established from spring‐collected individuals that were kept under similar conditions in the laboratory. To see whether the above results can be explained by phenotypic plasticity in sexual readiness, we exposed cold‐adapted laboratory strains to different lengths of warm periods. We found that sexual propensity increases with warm exposure. Our results suggest that reciprocal cold and warm periods are required for sex induction in H. oligactis, which would ensure proper timing of sex in this species. Increased sensitivity to environmental deterioration might help maximize fitness in environments that have both a predictable (seasonal) and an unpredictable component.

Within-species variation in animal body size predicts major differences in life history, for example, in reproductive development, fecundity, and even longevity. Purely from an energetic perspective, large size could entail larger energy reserves, fuelling different life functions, such as reproduction and survival (the "energy reserve" hypothesis).Conversely, larger body size could demand more energy for maintenance, and larger individuals might do worse in reproduction and survival under resource shortage (the "energy demand" hypothesis). Disentangling these alternative hypotheses is difficult because large size often correlates with better resource availability during growth, which could mask direct effects of body size on fitness traits. Here, we used experimental body size manipulation in the freshwater cnidarian Hydra oligactis, coupled with manipulation of resource (food) availability to separate direct effects of body size from resource availability on fitness traits (sexual development time, fecundity, and survival). We found significant interaction between body size and food availability in sexual development time in both males and females, such that large individuals responded less strongly to variation in resource availability. These results are consistent with an energy reserve effect of large size in Hydra. Surprisingly, the response was different in males and females: small and starved females delayed their reproduction, while small and starved males developed reproductive organs faster. In case of fecundity and survival, both size and food availability had significant effects, but we detected no interaction between them. Our observations suggest that in Hydra, small individuals are sensitive to fluctuations in resource availability, but these small individuals are able to adjust their reproductive development to maintain fitness.

14Facultative sexuality combines clonal propagation with sexual reproduction within a single life cycle. 15 Clonal propagation enables quick population growth and the occupancy of favorable habitats. Sex, on 16 the other hand, results in the production of offspring that are more likely to survive adverse conditions 17 (such as the resting eggs of many freshwater invertebrates). In seasonal environments, the timing of sex 18 is often triggered by environmental cues signaling the onset of winter (e.g. temperature drop or changes 19 in photoperiod). Organisms switching to sex to produce resting eggs under these conditions face a 20 trade-off: responding too early to an environmental cue increases the chances of missing out in clonal 21 propagation, while having a delayed response to deteriorating conditions entails the risk of parental 22 3 36