Investment in boney defensive traits alters organismal stoichiometry and excretion in fish

Abstract

Understanding how trait diversification alters ecosystem processes is an important goal for ecological and evolutionary studies. Ecological stoichiometry provides a framework for predicting how traits affect ecosystem function. The growth rate hypothesis of ecological stoichiometry links growth and phosphorus (P) body composition in taxa where nucleic acids are a significant pool of body P. In vertebrates, however, most of the P is bound within bone, and organisms with boney structures can vary in terms of the relative contributions of bones to body composition. Threespine stickleback populations have substantial variation in boney armour plating. Shaped by natural selection, this variation provides a model system to study the links between evolution of bone content, elemental body composition, and P excretion. We measure carbon:nitrogen:P body composition from stickleback populations that vary in armour phenotype. We develop a mechanistic mass-balance model to explore factors affecting P excretion, and measure P excretion from two populations with contrasting armour phenotypes. Completely armoured morphs have higher body %P but excrete more P per unit body mass than other morphs. The model suggests that such differences are driven by phenotypic differences in P intake as well as body %P composition. Our results show that while investment in boney traits alters the elemental composition of vertebrate bodies, excretion rates depend on how acquisition and assimilation traits covary with boney trait investment. These results also provide a stoichiometric hypothesis to explain the repeated loss of boney armour in threespine sticklebacks upon colonizing freshwater ecosystems.

Adaptation in temporally variable environments: stickleback armor in periodically breaching bar‐built estuaries

“…Previous studies have shown that investment in bony structures can increase phosphorus demand, which can potentially alter how fish forage and recycle nutrients (El‐Sabaawi et al ., ; Durston & El‐Sabaawi, ; Leal et al ., ). We find that, despite dramatic environmental fluctuations, the expected association between %P and armor remains strong.…”

“…Importantly, lateral plates and Eda vary dramatically within and among the estuaries, generating the high levels of variation in %P. This variation should have a major influence on whole fish elemental ratios and thus the observed variation in %P is likely to influence the balance between excretion rates and diet choice (El‐Sabaawi et al ., ; Durston & El‐Sabaawi, ). Given that allelic variation at Eda appears to be driving variation in elemental composition, and because Eda is generally subject to strong natural selection in the wild (Colosimo et al ., , ; Barrett et al ., ; Jones et al ., ), it is likely that elemental composition in %P can evolve just as rapidly as can lateral plates (see Durston & El‐Sabaawi, ).…”

“…For instance, variation in fish elemental composition can indicate specific changes in individual behaviour (e.g. foraging) that influence zooplankton community structure (El‐Sabaawi et al ., ; Durston & El‐Sabaawi, ). We structured our analysis around four key questions: Is gene flow sufficiently restricted to enable adaptive divergence among the estuary populations?…”

From Elements to Function: Toward Unifying Ecological Stoichiometry and Trait-Based Ecology

“…Linking TBE with ES therefore enables us to make predictions regarding the impact of human activities on both community structure and functioning. For example, it has been observed that communities dominated by species with higher optimal N:P ratios will substantially influence biogeochemical cycles by preferentially recycling N over P Knoll et al, 2009;El-Sabaawi et al, 2016).…”

Variability of functional traits and their syndromes in a freshwater fish species (Phoxinus phoxinus): The role of adaptive and nonadaptive processes

“…Nonetheless, we failed to detect the environmental pressures driving divergences in these two traits. The variability in excretion rate probably stands in trophic and stoichiometric factors, such as trophic niche, elemental composition of resources, or allochthonous nutrient inputs (El‐Sabaawi, Warbanski, Rudman, Hovel, & Matthews, ; Evangelista et al, ), which could be characteristic of each geographical site. Hence, measuring stoichiometric variability of individuals and populations would benefit to infer hypotheses regarding variability in excretion rate.…”