Abstract
Latitudinal trends in cold tolerance have been observed in many terrestrial ectotherms, but few studies have investigated interpopulational variation in the cold physiology of marine invertebrates. Here, the intertidal copepod Tigriopus californicus was used as a model system to study how local adaptation influences the cold tolerance of a broadly distributed marine crustacean. Among five populations spanning 18° in latitude, the following three metrics were used to compare cold tolerance: the temperature of chill-coma onset, the chill-coma recovery time and post-freezing recovery. In comparison to copepods from warmer southern latitudes, animals from northern populations exhibited lower chill-coma onset temperatures, shorter chill-coma recovery times and faster post-freezing recovery rates. Importantly, all three metrics showed a consistent latitudinal trend, suggesting that any single metric could be used equivalently in future studies investigating latitudinal variation in cold tolerance. Our results agree with previous studies showing that populations within a single species can display strong local adaptation to spatially varying climatic conditions. Thus, accounting for local adaptation in bioclimate models will be useful for understanding how broadly distributed species like T. californicus will respond to anthropogenic climate change.
Highlights
Temperature influences the performance and fitness of ectothermic animals, affecting their physiology, ecology, behaviour and evolution (Somero, 1997)
In comparison to a >70% recovery rate in northern copepods, a significantly lower proportion of the southern individuals recovered within 24 h after having been in frozen water, indicating a reduced ability to respond adaptively to cold (Fig. 4A)
Each metric was significantly related to the average low winter temperature and annual number of freezing days of the collection site of each population, suggesting directional selection for the traits associated with cold tolerance (Castañeda et al, 2005)
Summary
Temperature influences the performance and fitness of ectothermic animals, affecting their physiology, ecology, behaviour and evolution (Somero, 1997). In species that have geographically heterogeneous thermal ranges, populations often evolve differences in thermal physiology that improve their fitness under local conditions (Huey and Kingsolver, 1989; Gaston et al, 2009; Sanford and Kelly, 2011). Local adaptation studies can provide insight into how species’ distributions may change in response to anthropogenic climate change, because separate populations may have different ranges for thermal tolerance from the species as a whole (Kuo and Sanford, 2009; Hill et al, 2011; Kelly et al, 2011; Sanford and Kelly, 2011)
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