Physiological responses to short-term thermal stress in mayfly (Neocloeon triangulifer) larvae in relation to upper thermal limits

Abstract

Understanding species' thermal limits and their physiological determinants is critical in light of climate change and other human activities that warm freshwater ecosystems. Here, we ask whether oxygen limitation determines the chronic upper thermal limits in larvae of the mayfly Neocloeon triangulifer, an emerging model for ecological and physiological studies. Our experiments are based on a robust understanding of the upper acute (∼40°C) and chronic thermal limits of this species (>28°C, ≤30°C) derived from full life cycle rearing experiments across temperatures. We tested two related predictions derived from the hypothesis that oxygen limitation sets the chronic upper thermal limits: (1) aerobic scope declines in mayfly larvae as they approach and exceed temperatures that are chronically lethal to larvae; and (2) genes indicative of hypoxia challenge are also responsive in larvae exposed to ecologically relevant thermal limits. Neither prediction held true. We estimated aerobic scope by subtracting measurements of standard oxygen consumption rates from measurements of maximum oxygen consumption rates, the latter of which was obtained by treating with the metabolic uncoupling agent carbonyl cyanide-4-(trifluoromethoxy) pheylhydrazone (FCCP). Aerobic scope was similar in larvae held below and above chronic thermal limits. Genes indicative of oxygen limitation (LDH, EGL-9) were only upregulated under hypoxia or during exposure to temperatures beyond the chronic (and more ecologically relevant) thermal limits of this species (LDH). Our results suggest that the chronic thermal limits of this species are likely not driven by oxygen limitation, but rather are determined by other factors, e.g. bioenergetics costs. We caution against the use of short-term thermal ramping approaches to estimate critical thermal limits (CT max ) in aquatic insects because those temperatures are typically higher than those that occur in nature.

Why adult mayflies ofCloeon dipterum(Ephemeroptera:Baetidae) become smaller as temperature warms

“…For example, we did not manipulate O 2 levels and did not include important ancillary data regarding metabolomics, the expression of genes related to oxygen stress, and other physiological parameters. However, we can report that measurements of metabolism in our study and recent results of aerobic-scope studies for other mayflies (Kim et al 2017) suggest that mayfly larvae seem to have sufficient capacity to extract O 2 at CT max . A recent test of the OCLTT (Verberk et al 2016) showed that thermal responses exacerbated the sensitivity of test organisms to hypoxia in the field and the laboratory and often at O 2 levels well above those considered stressful.…”

“…4). This pattern also was observed in the mayfly Neocloeon triangulifer, where aerobic scope was similar at tolerated and chronically lethal temperatures (Kim et al 2017). This result suggests that larvae still have substantial aerobic capacity at a temperature that results in 0 reproductive output.…”

“…One step toward unraveling the question may be to include metabolomic parameters in the experimental design as an independent measure of what is happening inside these animals as developmental decisions are being made. For example, recent studies of the mayfly N. triangulifer strongly suggest that genes indicative of physiological hypoxia did not respond in larvae exposed to temperatures approaching their chronic thermal limit (i.e., 307C for N. triangulifer; Kim et al 2017). Similar metabolomic data gathered on animals exposed to various temperature and O 2 conditions during life-cycle testing would greatly clarify these issues.…”

Acute and Chronic Toxicity of Nickel and Zinc to a Laboratory Cultured Mayfly (Neocloeon triangulifer) in Aqueous but Fed Exposures

“…Finally, for the present study, the acute and chronic toxicity tests were conducted at 25 °C like some previously published studies (Soucek and Dickinson 2015; Struewing et al 2015; Weaver et al 2015; Raby et al 2018; Soucek et al 2018). The clone in our culture (WCC‐2; Stroud Water Research Center) was originally collected in White Clay Creek, Pennsylvania, USA, which reaches a maximum temperature of approximately 23 °C (Kim et al 2017). The Illinois Natural History Survey (2019) insect collection database reports collections of Centroptilum (the former generic name for Neocloeon ) throughout southern Illinois, including the Kaskaskia River, which has an average temperature >25 °C over the summer months (US Geological Survey 2020).…”

Metabolomics reveal physiological changes in mayfly larvae (Neocloeon triangulifer) at ecological upper thermal limits

“…In this study, we ask the question whether pathways related to energy or anaerobic metabolism are affected at ecologically relevant thermal limit. We design the experiment to ramp mayfly Neocloeon triangulifer (Ephemeroptera: Baetidae) larvae at a rate of 1°C/hour from 22°C (a temperature associated with excellent survival and reproductive output) to 30 °C (a temperature that is chronically lethal to larvae based on life history survivorship data (Kim et al, accepted). We use both NMR and GC-TOF-MS techniques to observe and compare metabolite changes from samples taken at 22 °C and 30 °C…”