Assessing daily egestion rates in earthworms: using fungal spores as a natural soil marker to estimate gut transit time

Abstract

Earthworms have an important role in ‘bioturbation’—the mixing of soil due to biological processes. Quantification of earthworm bioturbation relies on estimating earthworm egestion rates which in turn depend on two parameters: the gut content of the worms and the gut transit time (GTT). Gut content can be determined relatively easily, but determining GTT is problematic. The present study aimed at estimating daily soil egestion rates of Aporrectodea caliginosa and Lumbricus terrestris, refining the most common approach for estimating GTT by using fungal spores as natural markers in ingested soil. This approach avoids the use of artificial markers that may adversely affect the earthworms. Gut transit time was estimated by tracking the passage of marked soil through the gut by the appearance of the spores in the egested faeces. Gut transit time was estimated to be 9.6 ± 0.3 h for A. caliginosa and 11.6 ± 0.5 h for L. terrestris. Gut content averaged 465 ± 40(± standard error (SE)) mg dw g−1 dw worm for A. caliginosa and 265 ± 80 mg dw g−1 dw worm for L. terrestris. From these values, daily egestion rates of 1.16 and 0.66 g dw faeces g−1 dw worm d−1 were calculated for A. caliginosa and L. terrestris, respectively. Both values compare well to literature values for each species. The presented method for GTT estimation is inexpensive, rapid and easy to evaluate, with spores being a good alternative to existing markers.