Phenotypic plasticity is not a cline: Thermal physiology of an intertidal barnacle over 20° of latitude

Abstract

Our understanding of the plastic and evolutionary potential of ectothermic organisms and their populational impacts in the face of rapid global change remains limited. Studies attempting on the relationship between the magnitude of thermal variability across latitude and the degree of phenotypic plasticity exhibited by marine ectotherms are inconclusive. We state that the latter arises from the narrow range of thermal variability captured by the limited span of the latitudinal gradients studied to date. Using a mechanistic ecophysiological approach and a satellite‐based assessment of the relevant environmental variables (i.e. temperature and food availability), we studied individuals of the intertidal barnacle Jehlius cirratus from seven local populations widely spread along the Humboldt current system that spanning two biogeographic regions. At the same time, we synthesized published information on the local abundance of our study species across a total of 76 sites representing 20° of latitude, and spanning from 18 to 42°S. We examined the effects of latitude and environmental variability on metabolic rate plasticity, thermal tolerance (thermal breadth and thermal safety margins) and their impacts on the abundance of this widespread marine invertebrate. We demonstrate that the phenotypic plasticity of metabolic rate in J. cirratus populations is not related to latitude. In turn, thermal breadth is explained by the temperature variability each population experiences. Furthermore, we found clinal variation with a poleward decrease of the critical thermal minimum, suggesting that episodic extreme low temperatures represent a ubiquitous selective force on the lower thermal limit for ectotherms. Across our study gradient, plasticity patterns indicate that populations at the equatorial extreme are more vulnerable to a warming climate, while populations located in the biogeographic transitional zone (i.e. high environmental heterogeneity), on the centre of the gradient, display higher levels of phenotypic plasticity and may represent a genetic buffer for the effects of ocean warming. Together, our results suggest the existence of a fitness trade‐off involving the metabolic cost of plasticity and population density that is evident only across the vast latitudinal gradient examined.

Variable Environments in an Upwelling System Trigger Differential Thermal Sensitivity in a Low Intertidal Chiton

“…Additionally, our results show that individuals living in more variable environments (i.e., Talcaruca) not only have greater thermal amplitudes (T br ) and are capable of tolerating higher temperatures (CT max ) compared to individuals from more constant environments (i.e., Los Molles), but they also present greater variance in these traits (see Barria et al, 2018). These results are in agreement with the predictions of the CVH, which indicates that populations inhabiting environments with fluctuating temperatures require wider tolerance limits (Angilletta, 2009;Gaitán-Espitia et al, 2017;Broitman et al, 2021;Lardies et al, 2021). However this increase in thermal tolerance is not related with latitude in our chitons populations (see Bozinovic et al, 2011).…”

“…The pattern of energy use of an organism is reflected in its energy expenditure, and the metabolic rate is the most common way to measure it (Seibel and Drazen, 2007;Lardies et al, 2011Lardies et al, , 2021. In this context, our results show that individuals with larger body sizes have higher energy expenditures, which would impact the reproduction, feeding and growth, among others consequences (Lardies and Bozinovic, 2008;Lardies et al, 2011;Ramajo et al, 2020;Broitman et al, 2021). Furthermore, the increase in energy expenditure with body size is more pronounced in the Talcaruca population, which suggests that exposure to a fluctuating environment has important implications on the performance of individuals (Bozinovic et al, 2016;Barria et al, 2018;Lardies et al, 2021).…”

“…Together, our observational and experimental results suggest that environmental conditions around large upwelling centers (i.e., high environmental heterogeneity) display higher levels of phenotypic variability (i.e., plasticity) in local population performance of adult individuals, and may represent a genetic buffer for the effects of ocean warming. Therefore, it is necessary to understand the thermal performance of marine organisms, considering the consequences of climate change, where the increase in ocean temperature could cause an increase in the frequency and intensity of upwelling systems on the Pacific Coast (Sydeman et al, 2014;Broitman et al, 2021) generating repercussions on the distribution, performance and fitness of the coastal populations and communities.…”

“…This is particularly true for the broad daily (see Osores et al, 2018) and seasonal variations in temperature, which can be as extensive as their cold/heat tolerance limits (Helmuth et al, 2006;Somero, 2010). Furthermore, as the environmental temperature is the abiotic factor with major incidence in the physiology and ecology of marine ectotherms (Pörtner, 2001), the abrupt temperature changes in upwelling systems could generate important variations in these organisms' functional process (e.g., growth, reproduction, physiology) (Lardies et al, 2011;Barria et al, 2014Barria et al, , 2018Broitman et al, 2021).…”

“…The TPC has been used to describe mechanistically the thermal sensitivity among populations of ectothermic species (Kingsolver et al, 2004;Barria et al, 2018;Broitman et al, 2018). In addition, it is well known that the parameters of the TPC usually show geographic variation depending on the local environment and genetic background of populations (Gilchrist, 1996;Kingsolver et al, 2004;Gaitán-Espitia et al, 2014Barria et al, 2018;Broitman et al, 2021). This geographic variation usually is in accordance with the predictions of the climate variability hypothesis (CVH), which states that organisms inhabiting more variable environments (i.e., high altitudes or latitudes) should have broader ranges of environmental performance in order to cope with the fluctuating environmental conditions they experience (e.g., Gaitán-Espitia et al, 2017).…”