Trophic plasticity among spring vs. cave populations of Gammarus minus: examining functional niches using stable isotopes and C/N ratios
Using natural-abundance 13C/12C ratios as tracers, carbon turnover rates were determined for postlarval brown shrimp, Penaeus aztecus, in five laboratory growth experiments. Although tissue turnover in adult animals generally occurs during maintenance metabolism and is a function of time, turnover for young postlarval shrimp was accelerated during growth, and was primarily a function of weight gained rather than time. Metabolic loss of tissue carbon during growth was usually approximated by the function, Fraction lost=1-(initial weight/final weight). For shrimp that switch diets in the sea, model calculations show that this high turnover rate coupled with a four-fold weight increase suffices for shrimp to achieve a close isotopic resemblance of 1‰ or less (δ13C units) to the new diet. In accordance with these predictive calculations, shrimp which had increased in weight by a factor of four or more in the culture experiments showed essentially constant isotopic values reflecting their new diets. For these larger animals, the average animal-diet difference varied across three diets from-0.9 to +11‰, and the δ13C range among individuals was ≦1.4‰ in each experiment.