Selective ingestion contributes to the stoichiometric homeostasis in tissues of the endogeic earthworm Aporrectodea turgida

Most detritivores maintain a stoichiometric homeostasis in their body tissue regardless of the chemical composition of the substrates they ingest, but the mechanisms for stoichiometric regulation in soil-inhabiting detritivores like earthworms are poorly understood. The objectives of this study were (1) to examine whether the endogeic earthworm Aporrectodea turgida (Eisen) exhibits a strict homeostasis in its tissue C:N ratio, (2) to determine if A. turgida controlled its tissue N concentration by changing the quantity of N excreted in epidermal mucus and (3) to consider how the gut transit time, gut load and cast production were related to selective ingestion, which is hypothesized to control the N stoichiometry in A. turgida tissues. Two laboratory experiments were designed to address these objectives. In the first experiment, we evaluated the C and N concentrations, and C:N ratio of A. turgida body tissue and epidermal mucus after the earthworm fed on soil mixed with 15N-labeled plant litter (red clover leaves, wheat leaves, wheat stem) having variable N content and C:N ratios, as well as no litter, for 7 days. The second experiment measured the gut transit time, gut load and cast production of A. turgida fed soil marked with glass beads, either without litter or mixed with plant litter (soybean leaves with high N, wheat stems with low N). The endogeic earthworm A. turgida maintained strict homeostasis in their body tissue, with a C:N ratio of 3.9. The epidermal mucus of A. turgida also showed a strict homeostasis (C:N ratio = 4.6) and constant 15N enrichment, regardless of the N content in plant litter. Therefore, N secretion through epidermal mucus cannot be a mechanism that regulates the N stoichiometry in the body tissue of A. turgida. The gut transit time of ingested substrates was the same, as both N-rich (i.e., soil-soybean mixture) and N-poor (i.e., soil-wheat mixture) substrates took 21 ± 1 h to pass from the mouth to the anus of A. turgida, however, there was significantly (P < 0.05) less material in the gut and less cast production from the N-rich than the N-poor substrate. We conclude that a selective ingestion process controls the intake of organic substrates and likely contributes to the conservation of N stoichiometry in A. turgida body tissues.