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 consumerdriven 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 neutralto-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.

Zooplankton eat what they need: copepod selective feeding and potential consequences for marine systems

“…In the case of copepod‐dominated marine zooplankton, P‐rich nauplii may be a key player in lowering N:P ratios in the water column. They selectively retain P in their biomass (Elser et al , Villar‐Argaiz et al ) and, as shown here, also preferentially remove P‐rich and reject N‐rich cells during feeding. Consistent with this, we observed that nauplii abundances fluctuate together with the dissolved N:P ratio but not with temperature or food abundance.…”

Stoichiometric Constraints Do Not Limit Successful Invaders: Zebra Mussels in Swedish Lakes

“…The positive relation between N∶P ratio and body size among invertebrates is due to the negative allometric scaling of growth rate to body size [11]. Increasing P content along with higher growth rate with decreasing body size is a pattern that has been observed in littoral mayflies [36], pelagic invertebrates [38], [39] and many benthic invertebrates [40]. Still, if food quality is low, ontogenetic shifts in stoichiometric constraints could result in a stoichiometric bottleneck in fast-growing juveniles [41], which have elevated P levels and demands.…”

Nucleic Acid Content in Crustacean Zooplankton: Bridging Metabolic and Stoichiometric Predictions

“…Immediately afterwards, nauplii become self-feeding and show an increase in RNA synthesis, which possibly results from the higher protein demand for growth, cellular proliferation, and differentiation before metamorphosis [57]. Although the relative NA content remained relatively low after metamorphosis, a further decrease was observed in adulthood, possibly attributable to the major increase in weight due to lipid storage [19]. This pattern is consistent with the ontogenetical decrease in RPII TNAs , which reflects a reduction in P allocation to NAs towards adulthood, which may be linked to an increase in other P-enriched biomolecules [11].…”