Plasticity of thermal limits in the aquatic saline beetle Enochrus politus (Küster 1849) (Coleoptera: Hydrophilidae) under changing environmental conditions

espanolLa informacion sobre la tolerancia fisiologica de las especies es esencial para preveer sus respuestas al cambio climatico, especialmente en especies de distribucion restringida o que habitan ambientes extremos como las aguas salinas continentales. En estos ecosistemas se preve un aumento de temperatura y salinidad como consecuencia del cambio climatico, y ambos factores influyen en la tolerancia termica de insectos acuaticos. El objetivo de este trabajo fue determinar bajo una aproximacion experimental los limites termicos y su sensibilidad frente a distintas condiciones ambientales en una especie de coleoptero acuatico, Enochrus politus, comun en rios salinos del sureste de la Peninsula Iberica. Como limite termico inferior se determino el Supercooling, o temperatura previa a la congelacion, y como limite superior se considero el Heat coma, o punto de paralisis anterior a la muerte, combinando un metodo dinamico y termografia infrarroja, previa aclimatacion de los ejemplares a diferentes condiciones de temperatura y salinidad. Los resultados mostraron que E. politus tiene un amplio rango de tolerancia termica (–10.38 ± 0.32 ◦C y 57.37 ± 0.19 ◦C), siendo mayor en individuos aclimatados a 20 ◦C y 12 g/L, condiciones similares a las de los habitats donde frecuentemente vive la especie. A bajas temperaturas y salinidad, su rango de tolerancia disminuyo. Tanto la temperatura como la salinidad tuvieron un efecto significativo en ambos limites termicos. Salinidades elevadas provocaron una disminucion de la tolerancia al calor e incrementaron la tolerancia al frio. El limite termico inferior de la especie mostro mayor plasticidad que el superior. Debido a su amplio rango termico en comparacion con otros insectos, esta especie seria potencialmente capaz de hacer frente a un incremento de temperatura, aunque la salinizacion de su habitat podria reducir su tolerancia a altas temperaturas EnglishInformation on the physiological tolerance of species is essential when forecasting their responses to climatic change, especially for those with restricted distributions, as well as those inhabiting extreme environments, such as inland saline waters. Temperature and salinity are expected to increase in these ecosystems under climate change, and both factors are known to affect the thermal tolerances of aquatic insects. The objective of this study was to determine, in the laboratory, the thermal limits and their plasticity to changes under the environmental conditions of the aquatic beetle Enochrus politus, a typical species of saline streams from the southeast of Spain. Supercooling temperature (point before the freezing temperature) and Heat coma (point of paralysis prior to death) were used as the lower and upper thermal limits, respectively. They were determined by a dynamic method combined with infrared thermography in groups of individuals previously acclimated at different conditions of temperature and salinity. E. politus showed a wide range of thermal tolerance (–10.38 ± 0.32 ◦C-57.37 ± 0.19 ◦C) and was higher in specimens acclimated at 20 ◦C and 12 g/L, coinciding with the mean environmental conditions of the habitats where the species often lives. At lower temperatures and salinities, its thermal tolerance range decreased. The temperature and salinity of acclimation both had significant effects on the thermal limits. Elevated salinity decreased the heat tolerance and increased the freezing tolerance. The lower thermal limit of the species showed more plasticity than the upper limit. The species would potentially be able to tolerate temperature increase according to its wide thermal range, when compared to other insects; however, the salinization of its habitat might reduce its heat tolerance.