Diurnal variation around an optimum and near‐critically high temperature does not alter the performance of an ectothermic aquatic grazer

Abstract

The growing threat of global climate change has led to a profusion of studies examining the effects of warming on biota. Despite the potential importance of natural variability such as diurnal temperature fluctuations, most experimental studies on warming are conducted under stable temperatures. Here, we investigated whether the responses of an aquatic invertebrate grazer (Lymnaea stagnalis) to an increased average temperature differ when the thermal regime is either constant or fluctuates diurnally. Using thermal response curves for several life‐history and immune defense traits, we first identified the optimum and near‐critically high temperatures that Lymnaea potentially experience during summer heat waves. We then exposed individuals that originated from three different populations to these two temperatures under constant or fluctuating thermal conditions. After 7 days, we assessed growth, reproduction, and two immune parameters (phenoloxidase‐like activity and antibacterial activity of hemolymph) from each individual. Exposure to the near‐critically high temperature led to increased growth rates and decreased antibacterial activity of hemolymph compared to the optimum temperature, whilst temperature fluctuations had no effect on these traits. The results indicate that the temperature level per se, rather than the variability in temperature was the main driver altering trait responses in our study species. Forecasting responses in temperature‐related responses remains challenging, due to system‐specific properties that can include intraspecific variation. However, our study indicates that experiments examining the effects of warming using constant temperatures can give similar predictions as studies with fluctuating thermal dynamics, and may thus be useful indicators of responses in nature.

Transcriptome profiling of Lymnaea stagnalis (Gastropoda) for ecoimmunological research

“…The obtained detailed information regarding the molecular immunology of L. stagnalis can now be integrated into future ecoimmunological investigations. Studies on L. stagnalis typically measure two phenotypic immune defence traits from snail haemolymph, namely antibacterial activity and PO-like activity (e.g., [ 8 , 52 – 56 , 60 ]). Antibacterial activity is quantified as a reduction in optical density of a solution in which lyophilised E. coli cells are mixed with snail haemolymph (see [ 52 , 59 ]).…”

“…This finding suggests that the high-temperature treatment may not have been highly stressful to snails. Although 25 °C is above the thermal optimum of adult L. stagnalis snails (21–23 °C depending on the trait), it was still not close to the critically high temperature of this species [ 56 ]. Additionally, the exposure time was short compared to earlier studies (see the previous paragraph) and to a typical 8.4-day summer heatwave the snails are exposed to in nature [ 73 ].…”

“…In addition to immune challenges, snails’ responses to environmental changes were tested in two treatments. Snails in treatment eight (elevated temperature; N = 4) were placed in water baths where the water temperature was raised to 25 °C during several hours and maintained at an elevated level for 2 days (see [ 55 , 56 ]). After the transfer, additional snails were added to all water baths to keep the experimental snails continuously at a density of twelve snails per water bath (data were not collected from these extra snails).…”

“…Here, we applied short-read based RNAseq to characterise transcriptome profiles of L. stagnalis exposed to various immune elicitors (injury, bacterial and trematode pathogens) and changes in environmental conditions (temperature, food availability) using a genetically diverse laboratory population of snails. Lymnaea stagnalis is a model organism in multiple biological disciplines (reviewed in [ 51 ]), including ecological immunology (e.g., [ 8 , 52 – 56 ]). Earlier ecoimmunological work, however, employs only a few phenotypic immune assays.…”

Long‐term warming affects ecosystem functioning through species turnover and intraspecific trait variation

“…the lack of interspecific interactions or temperature fluctuations, and can therefore under-or overestimate community grazing rate or metabolism. But since quantifying these traits in situ would be very challenging, laboratory measurements give a good indication of responses in more realistic scenarios (Salo et al 2019a). The water temperature for grazing rate and metabolism assays always corresponded to the ambient water temperature experienced by the individuals during the sampling (17.4 ± 0.2°C and 21.7 ± 0.2°C for the control and heated populations, respectively).…”