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Short-term, acute warming events are increasing in frequency across the world’s oceans. For short-lived species like most copepods, these extreme events can occur over both within- and between-generational time scales. Yet, it is unclear whether exposure to acute warming during early life stages of copepods can cause lingering effects on metabolism through development, even after the event has ended. These lingering effects would reduce the amount of energy devoted to growth and affect copepod population dynamics. We exposed nauplii of an ecologically important coastal species, Acartia tonsa , to a 24-hour warming event (control: 18°C; treatment: 28°C), and then tracked individual respiration rate, body length, and stage duration through development. As expected, we observed a decrease in mass-specific respiration rates as individuals developed. However, exposure to acute warming had no effect on the ontogenetic patterns in per-capita or mass-specific respiration rates, body length, or development time. The lack of these carryover effects through ontogeny suggests within-generational resilience to acute warming in this copepod species. 

Organismal thermal limits affect a wide range of biogeographical and ecological processes. Copepods are some of the most abundant animals on the planet and play key roles in aquatic habitats. Despite their abundance and ecological importance, there is limited data on the factors that affect copepod thermal limits, impeding our ability to predict how aquatic ecosystems will be affected by anthropogenic climate change. In a warming ocean, one factor that may have particularly important effects on thermal limits is the availability of food. A recently proposed feedback loop known as “metabolic meltdown” suggests that starvation and exposure to high temperatures interact to drastically reduce organismal thermal limits, increasing vulnerability to warming. To investigate one component of this feedback loop, we examined how starvation affects thermal limits (critical thermal maxima: CT_max) ofAcartia tonsa, a widespread estuarine copepod. We found that there was no effect of short‐duration exposure to starvation (up to 2 days). However, after 3 days, there was a significant decrease in the CT_maxof starved copepods relative to the fed controls. Our results provide empirical evidence that extended periods of starvation reduce thermal limits, potentially initiating “metabolic meltdown” in this key species of coastal copepod. This suggests that changes in food availability may increase the vulnerability of copepods to increasing temperatures, amplifying the effects of climate change on coastal systems.

 

Abstract Copepods are key components of aquatic habitats across the globe. Understanding how they respond to warming is important for predicting the effects of climate change on aquatic communities. Lethal thermal limits may play an important role in determining responses to warming. Thermal tolerance can vary over several different spatial and temporal scales, but we still lack a fundamental understanding of what drives the evolution of these patterns in copepods. In this Horizons piece, we provide a synthesis of global patterns in copepod thermal tolerance and potential acclimatory capacities. Copepod thermal tolerance increases with maximum annual temperature. We also find that the effects of phenotypic plasticity on thermal tolerance are negatively related to the magnitude of thermal tolerance, suggesting a potential trade-off between these traits. Our ability to fully describe these patterns is limited, however, by a lack of spatial, temporal and phylogenetic coverage in copepod thermal tolerance data. We indicate several priority areas for future work on copepod thermal tolerance, and accompanying suggestions regarding experimental design and methodology. 

Many species face extinction risks owing to climate change, and there is an urgent need to identify which species' populations will be most vulnerable. Plasticity in heat tolerance, which includes acclimation or hardening, occurs when prior exposure to a warmer temperature changes an organism's upper thermal limit. The capacity for thermal acclimation could provide protection against warming, but prior work has found few generalizable patterns to explain variation in this trait. Here, we report the results of, to our knowledge, the first meta-analysis to examine within-species variation in thermal plasticity, using results from 20 studies (19 species) that quantified thermal acclimation capacities across 78 populations. We used meta-regression to evaluate two leading hypotheses. The climate variability hypothesis predicts that populations from more thermally variable habitats will have greater plasticity, while the trade-off hypothesis predicts that populations with the lowest heat tolerance will have the greatest plasticity. Our analysis indicates strong support for the trade-off hypothesis because populations with greater thermal tolerance had reduced plasticity. These results advance our understanding of variation in populations' susceptibility to climate change and imply that populations with the highest thermal tolerance may have limited phenotypic plasticity to adjust to ongoing climate warming. 

Organisms experience variation in the thermal environment on several different temporal scales, with seasonality being particularly prominent in temperate regions. For organisms with short generation times, seasonal variation is experienced across, rather than within, generations. How this affects the seasonal evolution of thermal tolerance and phenotypic plasticity is understudied, but has direct implications for the thermal ecology of these organisms. Here we document intra-annual patterns of thermal tolerance in two species of Acartia copepods (Crustacea) from a highly seasonal estuary, showing strong variation across the annual temperature cycle. Common garden, split-brood experiments indicate that this seasonal variation in thermal tolerance, along with seasonal variation in body size and phenotypic plasticity, is likely affected by genetic polymorphism. Our results show that adaptation to seasonal variation is important to consider when predicting how populations may respond to ongoing climate change. 

Whether populations can adapt to predicted climate change conditions, and how rapidly, are critical questions for the management of natural systems. Experimental evolution has become an important tool to answer these questions. In order to provide useful, realistic insights into the adaptive response of populations to climate change, there needs to be careful consideration of how genetic differentiation and phenotypic plasticity interact to generate observed phenotypic changes. We exposed three populations of the widespread copepodAcartia tonsa(Crustacea) to chronic, sublethal temperature selection for 15 generations. We generated thermal survivorship curves at regular intervals both during and after this period of selection to track the evolution of thermal tolerance. Using reciprocal transplants between ambient and warming conditions, we also tracked changes in the strength of phenotypic plasticity in thermal tolerance. We observed significant increases in thermal tolerance in the Warming lineages, while plasticity in thermal tolerance was strongly reduced. We suggest these changes are driven by a negative relationship between thermal tolerance and plasticity in thermal tolerance. Our results indicate that adaptation to warming through an increase in thermal tolerance might not reduce vulnerability to climate change if the increase comes at the expense of tolerance plasticity. These results illustrate the importance of considering changes in both a trait of interest and the trait plasticity during experimental evolution.

 

Organisms experience variation in the thermal environment on several different temporal scales, with seasonality being particularly prominent in temperate regions. For organisms with short generation times, seasonal variation is experienced across, rather than within, generations. How this affects the seasonal evolution of thermal tolerance and phenotypic plasticity is understudied, but has direct implications for the thermal ecology of these organisms. Here we document intra‐annual patterns of thermal tolerance in two species ofAcartiacopepods (Crustacea) from a highly seasonal estuary, showing strong variation across the annual temperature cycle. Common garden, split‐brood experiments indicate that this seasonal variation in thermal tolerance, along with seasonal variation in body size and phenotypic plasticity, is likely affected by genetic polymorphism. Our results show that adaptation to seasonal variation is important to consider when predicting how populations may respond to ongoing climate change.