A food web analysis of the juvenile blue crab, Callinectes sapidus, using stable isotopes in whole animals and individual amino acids

The stable isotope compositions (C and N) of plants and animals of a marsh dominated by Spartina alterniflora in the Delaware Estuary were determined. The study focused on the juvenile stage of the Atlantic blue crab, Calline ctes sapidus, and the importance of marsh-derived diets in supporting growt h during this stage. Laboratory growth experiments and field data indicated that early juvenile blue crabs living in the Delaware Bay habitat fed prim arily on zooplankton, while marsh-dwelling crabs, which were enriched in C- 13 relative to bay juveniles, utilized marsh-derived carbon for growth. In laboratory experiments, the degree to which juvenile blue crabs isotopicall y fractionated dietary nitrogen, as well as the growth rate, depended on th e protein quality of the diet. The range of delta(13)C Of amino acids in la boratory-reared crabs and their diets was almost 20 parts per thousand, sim ilar to the isotopic range of amino acids of other organisms. In laboratory studies, the delta(13)C of nonessential and essential amino acids in the d iet were compared to those in juvenile crabs. Isotopic fractionation at the molecular level depended on diet quality and the crabs' physiological requ irements. Comparison of whole-animal isotope data with individual amino aci d C isotope measurements of wild juvenile blue crabs from the bay and marsh suggested a different source of total dietary carbon, yet a shared protein component, such as zooplankton.