Ecological implications of geographic and temporal variation in excretion and somatic stoichiometry in extremophile, Poecilia mexicana

Background/Question/Methods

Biologists now have evidence for evolution in ecological time. Such rapid evolutionary changes occur as populations locally adapt to new environments with novel conditions. Whether such evolutionary shifts are ecologically relevant is a central question. We studied potential ecological effects of local adaption in the extremophile livebearer, Poecilia mexicana, using the framework of ecological stoichiometry. Ecological stoichiometry uses the elemental constitution of organisms to make quantitative predictions on the ecological functions of species. Populations of P. mexicana exhibit replicated phenotypic divergence between extremophile (sulfide spring dwelling) and ancestral (nonsulfidic) ecotypes. We investigated how local adaptation has affected elemental constitution between ancestral and extremophile ecotypes as well as whether these differences predict parallel divergence in the excretion of two key nutrients (soluble reactive phosphorus and ammonia). We quantified carbon , nitrogen, sulfur, and phosphorus composition of 200 individual fish from 6 populations using automated elemental analyses or acid digestion and SRP and NH₃ excretion spectrophotometrically in a total of 96 individuals from 6 populations. We predicted replicated differences in elemental content and nutrient excretion across geographically isolated sulfidic and nonsulfidic populations. Furthermore, we predicted a strong correlation between somatic stoichiometry and nutrient excretion. 

Results/Conclusions

P. mexicana demonstrated replicated differences in tissue content of carbon, phosphorus and sulfur, although the magnitude of these differences varied across evolutionarily independent lineages. While nitrogen content also varied across lineages, the direction of these differences was idiosyncratic. Nutrient excretion rates also showed variation across geographically isolated populations, however; differences between ancestral and extremophile populations alone did not predict excretion rates. Current analysis of elemental constitution and elemental availability over three years, also including additional cavernicolous extremophile P. mexicana, is expected to aid in elucidating this relationship. Furthermore, comparison of common garden raised individuals with wild populations supports genetic and environmental components in the determination of tissue stoichiometry as the direction of differences in elemental content is maintained, although to a lesser magnitude. Variation in elemental content and excretion rates within a species is important for developing quantitative predictions of species’ interactions in natural systems.  Additionally, understanding the role of intraspecific variation in ecosystem processes will aid in a formal synthesis of ecology and evolution.