Linking Life History Strategies and Ontogeny in Crustacean Zooplankton: Implications for Homeostasis

Abstract

Work to date has established that consumers are constrained in their chemical content variability. Such a constraint generates many different kinds of ecological relationships ranging from aspects of animal mineral nutrition to factors affecting consumer‐driven nutrient recycling. Although previous studies have shown variation in zooplankton nutrient content within and between taxa, most stoichiometric studies assume that consumers are homeostatic in their elemental composition. In this paper, this assumption is reexamined as we describe the variability in the elemental content of the calanoid copepod Mixodiaptomus laciniatus Lilljeborg throughout its ontogeny (interstage variability) and for its specific developmental stages (intrastage variability). Mean copepod carbon content as dry mass increased significantly during ontogeny from 36.3% in nauplii to 51.5% in immature copepodites and 55.8% in adults. Mean phosphorus content decreased from 0.98% in nauplii to 0.87% in immature copepodites and 0.51% in adults. The copepod molar C:N:P ratio was 99:3:1 for nauplii, 165:13:1 for immature copepodites, and 234:25:1 for adults. These ontogenetic variations in stoichiometry within a single copepod species are comparable to the previously described range between taxa. Stoichiometric and physiological approaches are used as complementary tools to explain the two‐part life history strategy of this copepod. First, the nauplii and early copepodite stages (CI–CIV) with high carbon demands adopt a growth strategy during which carbon sources are mainly allocated to increases in body size. The susceptibility of these copepodite stages to carbon limitation is reflected in the decline in the carbon content for a given stage (e.g., from 66% to 38% C in CII) and in the 40‐fold decrease in neutral‐to‐polar lipid ratio toward the end of the ice‐free season, suggesting a mobilization of food reserves toward growth. In contrast, copepodite stage CV and adults with less carbon demands for growth followed a survival–reproductive strategy, where the invariability in carbon content as dry mass (near 60%) and in neutral‐to‐polar lipid ratio through time indicates no accumulation of storage lipids, but instead an allocation of resources toward maintenance or reproduction. The basic assumption of predictions of stoichiometric models that elemental composition remains constant with time are not supported by changes in the intrastage carbon and nitrogen contents of Mixodiaptomus laciniatus. Thus, we suggest that the idea of strict homeostasis may be replaced with the concept of rheostasis, which allows the homeostatic regulatory mechanisms of an organism to adjust to a changing environment over time.

Resource quality effects on life histories of Daphnia

“…We conclude that phosphorus limitation overrides biochemical limitations if the C : P ratio of the food is Ͼ350 and that only when phosphorus is present in ample supply will other factors such as the content of fatty acids become important (see also . Ontogenetic differences in food preferences are probably not very important in daphniids, and one would expect such changes in limiting factors to be more important in for example copepods, where food spectra and prey size can change considerable during life (e.g., Hart and Santer 1994;Villar-Argaiz et al 2002). Nevertheless, our results suggest that, even in daphniids such shifts in importance of different aspects of food quality could occur.…”

Growth rate, protein:RNA ratio and stoichiometric homeostasis of submerged macrophytes under eutrophication stress

“…Most of previous studies on stoichiometric homeostasis have made comparisons between autotrophs and heterotrophs (Persson et al, 2010), or among heterotrophs (Karimi and Folt, 2006;Villar-Argaiz et al, 2002), whereas comparisons across plant species are relatively lacking. Yu et al (2011) and Xing et al (2015) made comparisons across vascular plants species in the Inner Mongolia grassland and across submerged macrophytes species in Yunnan plateau lakes, respectively.…”

Reproduction as one of the main causes of temporal variability in the elemental composition of zooplankton

“…The only previous field study examining the ontogenetic changes in elemental composition of a calanoid copepod (Mixodiaptomus laciniatus) (Villar-Argaiz et al 2002) showed substantial variations during ontogeny. However, because of the overlapping cohorts in the population, they also recorded high seasonal variability in elemental composition at each stage, to which most of the ontogenetic variability may be ascribed, at least for copepodites.…”

“…Previous studies have shown that highly P-deficient food (DeMott et al 1998(DeMott et al , 2001Boersma and Kreutzer 2002;DeMott et al 2004) or interstage variability (Villar-Argaiz et al 2002) can be a relevant cause of elemental variability in some species. Our results indicate that reproduction may also be a cause of significant elemental variation in zooplankton, the extent probably depending on the reproductive mode of the species.…”

“…Other studies have shown that the homeostatic nature of the stoichiometric composition of crustacean zooplankton (e.g., N : P ratio) has an influence on the recycling ratios between distinct elements, and in consequence, on the entire dynamics of plankton (Sterner and Hessen 1994). More recently, some studies have found that certain zooplankton species exhibit a relevant temporal elemental variation (Main et al 1997;DeMott et al 1998;Villar-Argaiz et al 2002;Boersma and Kreutzer 2002;DeMott et al 2004). For these species, the direct application of nutrient recycling models (Olsen et al 1986) based on the assumption of strict elemental composition is inaccurate.…”