Intrapopulation variation in carbon and nitrogen stable isotope ratios in the earthworm Aporrectodea longa

The natural abundance variations in carbon and nitrogen stable isotope rati os in a population of the earthworm Aporrectodea longa, a species known to feed on both soil and plant litter, is reported in this paper. Worms were c ollected from a small land area of an old white clover field and body tissu e and mucus were analyzed separately. The range of isotopic values was smal l, but patterns of variation were not random. Tissue carbon and nitrogen is otope ratios were significantly higher in adult than in juvenile A. longa a nd tissue nitrogen isotope ratios tended to increase with increasing biomas s of individuals. Further, carbon and nitrogen isotope ratios were positive ly correlated in both tissue and mucus. Possible causes of the observed pat terns, including physiological effects, body composition and assimilation o f C and N from different plant, soil and microbial sources are discussed. I t is concluded that the causes of natural variability in isotopic compositi on must be understood and validated experimentally before natural abundance stable isotope methods can be used for the analysis of trophic relations a mong detritivorous soil invertebrates.