Food dependence and energetics of freeliving nematodes

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Abstract
Some bioenergetic parameters of Caenorhabditis briggsae, a saprophagous nematode, were analysed under different conditions of food availability. Respiration (R) and production rates (P) of experimental animals grown on media of defined bacterial concentrations were measured throughout the life cycle of the species at 20°C. Energetics are expressed in the form of instantaneous rates and as cumulative budgets. Food dependence: The food threshold of the species is defined as A (assimilation)=R, P=zero. The respiratory level of the species is generally high compared to other nematode species and increases only weakly with food availability. Starvation (food densities below threshold) is expressed in a strong reduction in metabolism within 48 h. The food dependence of biosynthetic processes (body growth and egg production) follows a hyperbolic form, which can be described by the Michaelis-Menten function. The relationship P:R changes drastically with availability of food, e.g. the production efficiency for the period of maximal reproduction is 0% at 2·108 cells ml-1 (threshold) and 86% at 1010 cells ml-1. R and P follow different forms of size dependence in the course of the life cycle. The relationship between R and body weight (W) can be described by an allometric function, e.g. at high food density, R=2.8 W 0.75 (R in nl O2 ind-1 h-1; W in μg fresh weight). Weight-specific production rates vary considerably during the life cycle: ±constant in the early larval phase (exponential growth, “g”=1.44 day-1 at 1010 cells ml-1); decreasing in the latter larval phase; peak values shortly after onset of reproduction as a result of both body growth and egg production. Differences in resource allocation at varying food densities are also manifest in cumulative energy budgets, e.g. higher R cum is necessary to achieve the same body size at lower food densities. Size at maturation and egg size are reduced to a different degree at low food densities, indicating bioenergetical constraint and trade-off between metabolic processes.