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

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.

Nuestro conocimiento del potencial plástico y evolutivo de organismos ectotérmicos y de los posibles impactos poblacionales a la luz del rápido cambio global sigue siendo limitado. Los estudios que relacionan la magnitud de la variabilidad térmica y el grado de plasticidad fenotípica a través de la latitud realizados en organismos ectotérmicos marinos no son concluyentes. Lo anterior creemos que es consecuencia del estrecho rango latitudinal y por consecuencia el menor rango de variabilidad térmica abarcado por los estudios previos. Utilizando un enfoque ecofisiológico mecanicista e información satelital de las variables ambientales relevantes (i.e., temperatura y disponibilidad de alimento), estudiamos individuos del cirripedio intermareal Jehlius cirratus a lo largo de siete poblaciones locales que se distribuyen ampliamente a lo largo del Sistema de la corriente de Humboldt abarcando dos regiones biogeográficas. Al mismo tiempo, sintetizamos la información publicada sobre la abundancia local de nuestro modelo de estudio en un total de 76 sitios que representan 20 grados de latitud y abarcan desde los 18° a los 42°S. Examinamos los efectos de la latitud y la variabilidad ambiental en la plasticidad de la tasa metabólica, la tolerancia térmica (i.e. amplitud térmica y márgenes de seguridad térmica) y los impactos en la abundancia de este invertebrado marino con amplia distribución geográfica. Demostramos que la plasticidad fenotípica de la tasa metabólica en poblaciones de J. cirratus no está relacionada con la latitud. A su vez, la amplitud térmica se explica por la variabilidad térmica que cada población experimenta. Además, encontramos un patrón de variación clinal con una disminución hacia los polos del crítico térmico mínimo, lo que sugiere que las temperaturas episódicas extremadamente bajas representan una fuerza selectiva ubicua en el límite térmico inferior para los ectotermos. A lo largo de nuestro gradiente estudiado, los patrones de plasticidad indican que las poblaciones en el extremo ecuatorial son más vulnerables al calentamiento, mientras que las poblaciones ubicadas en la zona de transición biogeográfica (i.e., alta heterogeneidad ambiental), en el centro del gradiente, muestran mayores niveles de plasticidad fenotípica, lo que puede representar un reservorio genético para los efectos del calentamiento de los océanos. Nuestros resultados sugieren la existencia de un compromiso en la adecuación biológica que involucra el costo metabólico de la plasticidad y la densidad de población que es sólo evidente dado el vasto gradiente latitudinal examinado.

Minimal impact at current environmental concentrations of microplastics on energy balance and physiological rates of the giant mussel Choromytilus chorus.

Microplastic particles (MP) uptake by marine organisms is a phenomenon of global concern. Nevertheless, there is scarce evidence about the impacts of MP on the energy balance of marine invertebrates. We evaluated the mid-term effect of the microplastic ingestion at the current higher environmental concentrations in the ocean on the energy balance of the giant mussel Choromytilus chorus. We exposed juvenile mussels to three concentrations of microplastics (0, 100, and 1000 particles L-1) and evaluated the effect on physiology after 40 days. The impacts of MP on the ecophysiological traits of the mussels were minimum at all the studied concentrations. At intermediate concentrations of MP, Scope for Growth (SFG) had little impact. Other relevant key life-history and physiological processes, such as size and metabolism, were not affected by microplastics. However, individuals treated with MP presented histopathological differences compared to control group, which could result in adverse health effects for mussels.

Discovering divergence in the thermal physiology of intertidal crabs along latitudinal gradients using an integrated approach with machine learning.

In intertidal marine crustaceans, phenotypic variation in physiological and life-history traits is pervasive along latitudinal clines. However, organisms have complex phenotypes, and their traits do not vary independently but rather interact differentially between them, effect that is caused by genetic and/or environmental forces. We evaluated the geographic variation in phenotypic integration of three marine crab species that inhabit different vertical thermal microhabitats of the intertidal zone. We studied seven populations of each species along a latitudinal gradient that spans more than 3000 km of the Chilean coast. Specifically we measured nine physiological traits that are highly related to thermal physiology. Of the nine traits, we selected four that contributed significantly to the observed geographical variation among populations; this variation was then evaluated using mixed linear models and an integrative approach employing machine learning. The results indicate that patterns of physiological variation depend on species vertical microhabitat, which may be subject to chronic or acute environmental variation. The species that inhabit the high- intertidal sites (i.e., exposed to chronic variation) better tolerated thermal stress compared with populations that inhabit the lower intertidal. While those in the low-intertidal only face conditions of acute thermal variation, using to a greater extent the plasticity to face these events. Our main results reflect that (1) species that inhabit the high-intertidal maintain a greater integration between their physiological traits and present lower plasticity than those that inhabit the low-intertidal. (2) Inverse relationship that exists between phenotypic plasticity and phenotypic integration of the physiological traits identified, which could help optimize energy resources. In general, the study of multiple physiological traits provides a more accurate picture of how the thermal traits of organisms vary along temperature gradients especially when exposed to conditions close to tolerance limits.

Exploring physiological plasticity and local thermal adaptation in an intertidal crab along a latitudinal cline.

Intertidal organisms have evolved physiological mechanisms that enable them to maintain performance and survive during periods of severe environmental stress with temperatures close to their tolerance limits. The level of these adaptive responses in thermal physiology can vary among populations of broadly distributed species depending on their particular environmental context and genetic backgrounds. Here we examined thermal performances and reaction norms for metabolic rate (MR) and heart rate (HR) of seven populations of the porcelanid crab Petrolisthes violaceus from markedly different thermal environments across the latitudinal gradient of ~3000km. Physiological responses of this intertidal crab under common-garden conditions suggest the absence of local thermal adaptation along the geographic gradient (i.e., lack of latitudinal compensation). Moreover, thermal physiological sensitivities and performances in response to increased temperatures evidenced the existence of some level of: i) metabolic rate control or depression during warm temperature exposures; and ii) homeostasis/canalization (i.e., absence or low levels of plasticity) in physiological traits that may reflect some sort of buffering mechanism in most of the populations. Nevertheless, our results indicate that elevated temperatures can reduce cardiac function but not metabolic rate in high latitude crabs. The lack of congruence between HR and MR supports the idea that energy metabolism in marine invertebrates cannot be inferred from HR and different conclusions regarding geographic differentiation in energy metabolism can be obtained from both physiological traits. Integrating thermal physiology and species range extent can contribute to a better understanding of the likely effects of climate change on natural populations of marine ectotherms.

Geographic variation in thermal physiological performance of the intertidal crab Petrolisthes violaceus along a latitudinal gradient.

Environmental temperature has profound effects on the biological performance and biogeographical distribution of ectothermic species. Variation of this abiotic factor across geographic gradients is expected to produce physiological differentiation and local adaptation of natural populations depending on their thermal tolerances and physiological sensitivities. Here, we studied geographic variation in whole-organism thermal physiology of seven populations of the porcelain crab Petrolisthes violaceus across a latitudinal gradient of 3000 km, characterized by a cline of thermal conditions. Our study found that populations of P. violaceus show no differences in the limits of their thermal performance curves and demonstrate a negative correlation of their optimal temperatures with latitude. Additionally, our findings show that high-latitude populations of P. violaceus exhibit broader thermal tolerances, which is consistent with the climatic variability hypothesis. Interestingly, under a future scenario of warming oceans, the thermal safety margins of P. violaceus indicate that lower latitude populations can physiologically tolerate the ocean-warming scenarios projected by the IPCC for the end of the twenty-first century.