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ABSTRACT Physiology defines individual responses to global climate change and species distributions across environments. Physiological responses are driven by temperature on three time scales: acute, acclimatory and evolutionary. Acutely, passive temperature effects often dictate an expected 2-fold increase in metabolic processes for every 10°C change in temperature (Q10). Yet, these acute responses often are mitigated through acclimation within an individual or evolutionary adaptation within populations over time. Natural selection can influence both responses and often reduces interindividual variation towards an optimum. However, this interindividual physiological variation is not well characterized. Here, we quantified responses to a 16°C temperature difference in six physiological traits across nine thermally distinct Fundulus heteroclitus populations. These traits included whole-animal metabolism (WAM), critical thermal maximum (CTmax) and substrate-specific cardiac metabolism measured in approximately 350 individuals. These traits exhibited high variation among both individuals and populations. Thermal sensitivity (Q10) was determined, specifically as the acclimated Q10, in which individuals were both acclimated and assayed at each temperature. The interindividual variation in Q10 was unexpectedly large: ranging from 0.6 to 5.4 for WAM. Thus, with a 16°C difference, metabolic rates were unchanged in some individuals, while in others they were 15-fold higher. Furthermore, a significant portion of variation was related to habitat temperature. Warmer populations had a significantly lower Q10 for WAM and CTmax after acclimation. These data suggest that individual variation in thermal sensitivity reflects different physiological strategies to respond to temperature variation, providing many different adaptive responses to changing environments. 

Abstract Genetic data from nonmodel species can inform ecology and physiology, giving insight into a species’ distribution and abundance as well as their responses to changing environments, all of which are important for species conservation and management. Moreover, reduced sequencing costs and improved long-read sequencing technology allows researchers to readily generate genomic resources for nonmodel species. Here, we apply Oxford Nanopore long-read sequencing and low-coverage (∼1x) whole genome short-read sequencing technology (Illumina) to assemble a genome and examine population genetics of an abundant tropical and subtropical fish, the hardhead silverside (Atherinomorus stipes). These fish are found in shallow coastal waters and are frequently included in ecological models because they serve as abundant prey for commercially and ecologically important species. Despite their importance in sub-tropical and tropical ecosystems, little is known about their population connectivity and genetic diversity. Our A. stipes genome assembly is about 1.2 Gb with comparable repetitive element content (∼47%), number of protein duplication events, and DNA methylation patterns to other teleost fish species. Among five sampled populations spanning 43 km of South Florida and the Florida Keys, we find little population structure suggesting high population connectivity. 

To better understand temperature's role in the interaction between local evolutionary adaptation and physiological plasticity, we investigated acclimation effects on metabolic performance and thermal tolerance among natural Fundulus heteroclitus (small estuarine fish) populations from different thermal environments. Fundulus heteroclitus populations experience large daily and seasonal temperature variations, as well as local mean temperature differences across their large geographical cline. In this study, we use three populations: one locally heated (32°C) by thermal effluence (TE) from the Oyster Creek Nuclear Generating Station, NJ, and two nearby reference populations that do not experience local heating (28°C). After acclimation to 12 or 28°C, we quantified whole-animal metabolic (WAM) rate, critical thermal maximum (CT max ) and substrate-specific cardiac metabolic rate (CaM, substrates: glucose, fatty acids, lactate plus ketones plus ethanol, and endogenous (i.e. no added substrates)) in approximately 160 individuals from these three populations. Populations showed few significant differences due to large interindividual variation within populations. In general, for WAM and CT max , the interindividual variation in acclimation response (log 2 ratio 28/12°C) was a function of performance at 12°C and order of acclimation (12–28°C versus 28–12°C). CT max and WAM were greater at 28°C than 12°C, although WAM had a small change (2.32-fold) compared with the expectation for a 16°C increase in temperature (expect 3- to 4.4-fold). By contrast, for CaM, the rates when acclimatized and assayed at 12 or 28°C were nearly identical. The small differences in CaM between 12 and 28°C temperature were partially explained by cardiac remodeling where individuals acclimatized to 12°C had larger hearts than individuals acclimatized to 28°C. Correlation among physiological traits was dependent on acclimation temperature. For example, WAM was negatively correlated with CT max at 12°C but positively correlated at 28°C. Additionally, glucose substrate supported higher CaM than fatty acid, and fatty acid supported higher CaM than lactate, ketones and alcohol (LKA) or endogenous. However, these responses were highly variable with some individuals using much more FA than glucose. These findings suggest interindividual variation in physiological responses to temperature acclimation and indicate that additional research investigating interindividual may be relevant for global climate change responses in many species.