Feeding activity of the earthworm Eisenia andrei in artificial soil

Quantitative information on the feeding activity of earthworms is scarce but this information is valuable in many eco(toxico)logical studies. In this study, the feeding activity of the compost worm Eisenia andrei is examined in artificial soil (OECD medium), with and without a high-quality food source (cow manure), and at two temperatures (10 and 20 °C). Methods are provided to estimate the most important parameters: gut load, selection of organic matter (OM), digestion efficiency, compaction, gut retention time, and fraction of manure in the diet. Lanthanides (Lu and Tm) were successfully used as inert markers in soil and manure, and we applied Bayesian statistics to analyse the data and fully capture the compounded uncertainty in the parameter estimates. Results show that the compost worm does not feed on soil indiscriminately but is able to select an OM-enriched diet from apparently homogeneous OECD medium. When manure is present on the soil surface, approximately three-quarters of the diet still consists of soil particles. The gut load of the worms was approximately 10% (dwt gut/wwt empty worm), varying little with the treatments. Unfortunately, the digestion efficiency could only be reliably estimated at 20 °C, and was approximately 40%. Temperature clearly affected feeding as a 10° temperature decrease nearly doubled the gut retention time (from 2.9 to 5.5 h), which corresponds to a two-fold decrease in feeding rate. The present data may be used to interpret toxicity and accumulation studies with E. andrei in OECD medium. However, care must be taken, as it seems possible that feeding is influenced by the size of the worm and subtle differences in experimental set-up.