Warming increases survival and asexual fitness in a facultatively sexual freshwater cnidarian with winter diapause

Abstract

Temperature is a key abiotic factor controlling population dynamics. In facultatively sexual animals inhabiting the temperate zone, temperature can regulate the switch between asexual and sexual modes of reproduction, initiates growth or dormancy, and acts together with photoperiod to mediate seasonal physiological transitions. Increasing temperature due to recent global warming is likely to disrupt population dynamics of facultatively sexual animals because of the strong temperature dependence of multiple fitness components. However, the fitness consequences of warming in these animals are still poorly understood. This is unfortunate since facultatively sexual animals—through their ability for asexual reproduction resulting in quick population growth and sexual reproduction enabling long‐term persistence—are key components of freshwater ecosystems. Here, I studied the fitness effects of warming in Hydra oligactis , a freshwater cnidarian that reproduces asexually throughout most of the year but switches to sexual reproduction under decreasing temperatures. I exposed hydra polyps to a simulated short summer heatwave or long‐term elevated winter temperature. Since sexual development in this species is dependent on low temperature, I predicted reduced sexual investment (gonad production) and elevated asexual fitness (budding) in polyps exposed to higher temperatures. The results show a complex effect of warming on sexual fitness: While gonad number decreased in response to warming, both male and female polyps exposed to high winter temperature were capable of multiple rounds of gamete production. Asexual reproduction and survival rate, on the contrary, clearly increased in response to higher temperature, especially in males. These results predict increased population growth of H. oligactis in temperate freshwater habitats, which will likely affect the population dynamics of its main prey (freshwater zooplankton), and through that, the whole aquatic ecosystem.

Investment in regeneration versus asexual reproduction is resource‐dependent in a freshwater annelid

“…Hydra vulgaris ), provide a basis for making predictions in naids. Seasonal triggers are believed to contribute similarly to a switch in sexual mode in hydra (Miklós et al., 2021; Tökölyi, 2023); as temperatures fall and primary production, and thus food availability, declines, engaging in sexual reproduction generates hardy embryos that can endure food scarcity while introducing genetic diversity into the population, which can improve resiliency against such environmental changes (Miklós et al., 2021; Sebestyén et al., 2018). Facultative reproduction thus offers a distinct advantage in heterogeneous environments, but the consequences of injury can modulate the fitness benefit of mode switching if the frequency of damage is also variable.…”

Temperature‐dependent scaling of fitness traits with body size in hydra

“…The day after size manipulation, animals were randomly assigned to one of the 12°C or 8°C temperature treatment groups (temperatures below 12°C are required in this species for sex induction; Tökölyi, 2023). The animals were housed individually in six‐well plates, fed two times a week with freshly hatched Artemia and checked four times a week for the presence and number of gonads.…”