Comparative ecophysiological responses of supratidal rockpools Ochthebius species (Coleoptera: Hydraenidae) to multiple stressors

Abstract

The ecophysiologists face one of today's greatest challenges: understanding species' adaptation to environmental changes in an ever-transforming world. They use ecosystems with extreme environmental conditions as models, which present significant physiological challenges to the organisms inhabiting them. One interesting study system are the Mediterranean supratidal rockpools that suffer long periods of desiccation, resulting in significant environmental fluctuations with extreme conditions of high temperature and salinity. This thesis focuses on the ecophysiological comparative responses to multiple stressors of three aquatic beetle species from the genus Ochthebius: O. quadricollis, O. lejolisii, and O. subinteger, exclusive of rockpools. The main goal was to seek their common or different response patterns. Chapter 1. The temperature at which heat coma occurs was determined and compared among populations of the three Ochthebius species. The effect of acclimation salinity (at non-stressful and sublethal levels) on thermal tolerance in adults and larvae of O. quadricollis and O. lejolisii was studied. Furthermore, thresholds of temperature for escape responses (water emergence and flight) were identified. Significant differences were found in thermal tolerance between species and populations. Ochthebius quadricollis exhibited greater thermal tolerance without the effect of saline acclimation. In contrast, O. lejolisii showed effects on thermal tolerance at a sublethal salinity, increasing tolerance in larvae and decreasing in adults. Thresholds of temperature for escape responses varied between species in accordance with their physiological tolerance. Salinity affected the thresholds of temperature for water emergence in both species. An additive effect of temperature and salinity on the frequency of emergence and flight was observed in both species. These findings provide valuable information for developing survival models against climate change. Chapter 2. The realized and fundamental saline niches of adults, larvae, and eggs of O. quadricollis and O. lejolisii were identified and compared. The realized niche was determined using field data on the abundance of adults and larvae, while the fundamental niche was analysed in laboratory experiments exposing adults, larvae, and eggs to different salinity levels. A discrepancy was found between the realised and fundamental niches. Both species proved to be euryhaline, tolerating extreme salinities in their natural habitat, especially O. quadricollis. In the laboratory, O. lejolisii showed greater physiological tolerance than O. quadricollis in all life stages. Comparing life stages, both larvae and eggs were more tolerant than adults. These species exhibited a high physiological capacity to withstand extreme salinities, a factor that could be exacerbated by climate change. Chapter 3. The cross-tolerance of O. quadricollis and O. lejolisii to salinity and desiccation was evaluated. Adults, larvae, and eggs of both species were acclimated to non-stressful and sublethal salinity levels, followed by exposure to extreme desiccation. Both species showed similar responses to desiccation throughout ontogeny, with larvae and eggs being more resistant than adults. Ochthebius lejolisii larvae were more desiccation-tolerant than O. quadricollis larvae, while O. quadricollis eggs had greater success following acclimation to non-stressful salinity. Ochthebius lejolisii eggs maintained similar hatching rates at both salinity levels. Chapter 4. Two hypotheses were tested in populations of O. lejolisii from areas with different thermal variability: the Metabolic Cold Adaptation (MCA) and Climatic Variability Hypotheses (CVH). Reciprocal acclimation experiments under different fluctuating temperature regimes were conducted to evaluate phenotypic plasticity and local adaptation in metabolic rates and thermal limits. The population at higher latitude, featuring a colder climate, showed higher metabolic rates at low temperatures, confirming the MCA. The lower latitude population, with greater climatic variability, demonstrated higher thermal tolerance. Only the higher latitude population exhibited plasticity in its upper thermal limit. These results suggest trade-offs between tolerance and plasticity in the thermal adaptation, increasing the vulnerability of Mediterranean coastal populations against higher latitude Atlantic populations to extreme temperature increases by climate change. All these findings contribute to understanding the effect of stress factor interactions on the studied species and their responses throughout their life cycle. The differences between species and life stages may be related to the different microenvironmental niches each species occupies.