Ontogenetic variation in the body stoichiometry of two fish species

One of the central questions of ecological stoichiometry theory is to what extent animal species maintain constant elemental composition in their bodies. Although several recent studies demonstrate intraspecific variation in animal elemental composition, relatively little is known about ontogenetic changes in vertebrates, especially during early life stages. We studied the intraspecific and interspecific ontogenetic variation in the body stoichiometry of two fish species in two different orders; fathead minnow (Pimephales promelas) and sheepshead minnow (Cyprinodon variegatus), reared under controlled laboratory conditions. During ontogeny, we measured the chemical composition of fish bodies, including carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), and ribonucleic acid (RNA) contents. We found that N and RNA contents were relatively high in early life stages and declined substantially during development. In contrast, body C and C:N ratios were relatively low in embryos, post-embryos and larvae, and increased remarkably thereafter. Concentrations and ratios of some elements (e.g., Ca, P, Ca:P) did not exhibit consistent ontogenetic trends, but fluctuated dynamically between consecutive developmental stages in both species. Specific growth rates correlated significantly with RNA contents in both species. Analyses of the relative importance of different P pools at each developmental stage revealed that RNA was a considerable P pool in post-embryos, while bone-associated P was the dominant body P pool in later stages. Our results suggest that the elemental composition of fish bodies changes considerably during ontogeny. Each ontogenetic stage has its own stoichiometric signature, but the timing, magnitude and direction of ontogenetic changes can vary substantially between taxa.