Abstract
According to the Climatic Variability Hypothesis [CVH], thermal plasticity should be wider in organisms from temperate environments, but is unlikely to occur in tropical latitudes where temperature fluctuations are narrow. In copepods, food availability has been suggested as the main driver of phenotypic variability in adult size if the range of temperature change is less than 14°C. Leptodiaptomus garciai is a calanoid copepod inhabiting Lake Alchichica, a monomictic, tropical lake in Mexico that experiences regular, narrow temperature fluctuations but wide changes in phytoplankton availability. We investigated whether the seasonal fluctuations of temperature and food produce phenotypic variation in the life-history traits of this tropical species. We sampled L. garciai throughout a year and measured female size, egg size and number, and hatching success, along with temperature and phytoplankton biomass. The amplitude of the plastic responses was estimated with the Phenotypic Plasticity Index. This index was also computed for a published dataset of 84 copepod populations to look if there is a relationship between the amplitude of the phenotypic plasticity of adult size and seasonal change in temperature. The temperature annual range in Lake Alchichica was 3.2°C, whereas phytoplankton abundance varied 17-fold. A strong pattern of thermal plasticity in egg size and adult female size followed the inverse relationship with temperature commonly observed in temperate environments, although its adaptive value was not demonstrated. Egg number, relative reproductive effort and number of nauplii per female were clearly plastic to food availability, allowing organisms to increase their fitness. When comparing copepod species from different latitudes, we found that the magnitude of thermal plasticity of adult size is not related to the range of temperature variation; furthermore, thermal plasticity exists even in environments of limited temperature variation, where the response is more intense compared to temperate populations.
Highlights
Phenotypic plasticity, the ability of an organism to respond to environmental stimuli by changing its phenotype [1] has been recorded for diverse species, response traits and environmental variables
Phytoplankton biomass followed the typical dynamics of Lake Alchichica [26]: it was homogeneously distributed during the mixing of the water column (1.4 ± 0.1 mg L-1 in January 2008) but as the lake stratified and the deep chlorophyll maximum (DCM) developed it differed markedly between epilimnion and metalimnion (0.9 and 6.1 mg L-1, respectively, in November) (Fig 2A)
Regarding relationships among life-history traits, we found that female size affected positively the Reproductive effort (RE) (Fig 6A), i.e., the absolute biomass that females invested per reproductive event, but not the proportion of their own biomass invested in reproduction (RRE), which remained constant
Summary
Phenotypic plasticity, the ability of an organism to respond to environmental stimuli by changing its phenotype [1] has been recorded for diverse species, response traits and environmental variables. In many species the size of adults is inversely related to the temperature of the environment where they developed; this, the temperature–size rule (TSR), affects other life-history traits such as size and number of descendants [4] It is not known whether thermal plasticity of body size in ectotherms is adaptive [4], and its physiological mechanisms remain unclear [3,5,6,7]. According to the Climatic Variability Hypothesis, natural selection would favour adaptive traits allowing organisms to withstand the contrasting and challenging conditions of fluctuating temperate environments [12,13] Among those traits, the capacity to produce alternative phenotypes in response to the thermal variability (and the accompanying changes in biotic and abiotic conditions) would be advantageous. Tropical organisms, experiencing narrow temperature ranges, are not in need of differentiated phenotypes, and phenotypic plasticity would be weak or absent [14,15,16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
