Summary 1.Trophic position is a fundamental feature of food-web structure, knowledge of which is being improved by stable isotope approaches which assume a constant enrichment in heavier isotopes in consumers relative to their diet. 2. We argue that the typical enrichment reflects a dynamic equilibrium between fractionation vectors associated with assimilation and excretion. 3. We develop a linear model to characterize the relationship between the equilibrium isotopic signature and the feeding rate influenced by isotopic discrimination during assimilation and excretion. 4. We present new data for both diet switching and starvation experiments using a marine worm Nereis virens and use this, and previously published data for a fish, bird and mammal to calculate controlling parameters from observations of the isotopic signature following diet-switching and the onset of starvation. 5. We show that the observed variance in isotopic signatures at each trophic step carries substantially more information than has been used hitherto and is influenced by feeding rate in addition to the isotopic signature of the food source. 6. Using the linear model as a tool we predict that parasitic organisms may be depleted relative to the tissues of their host.

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The development of the oocytes of Nereis (Hediste) diversicolor and Nereis (Nereis) pelagica takes up to 18 months from their first appearance in the coelom to maturity, and in the natural conditions experienced by the population studied in north-east England the females are in their second year before the oocytes begin to appear. The duration of spermatogenesis is less than that of oogenesis, taking up to 6 months for completion. This information permits a more rigorous interpretation of the size frequency data which have been obtained from a biometrical study of the jaws. In the populations of both species the majority of individuals seem to reach maturity when 3 years old.