Nest site selection and nutritional provision through excreta: a form of parental care in a tropical endogeic earthworm

Nest construction is a common form of parental care in soil organisms. However, it is unknown whether the tropical earthworm Pontoscolex corethrurus selects sites for nest construction when the nutritional quality of the soil is irregular. Here we studied the reproductive behaviour and nest site selection of P. corethrurus. In tridimensional terrariums we evaluated the combined effect of the food quality (soil only = S, soil+grass = G, soil+legume = L) and soil depth (0-9 cm = Shallow, 10-18 cm = Intermediate, 19-27 cm = Deep) in a factorial 32 design. The number and biomass of cocoons, progeny and the production of internal and external excreta were evaluated. The nutritional quality and depth of soil and their interaction had a significant effect on nest site construction and the deposition of internal excreta. P. corethrurus built a higher amount of nests in the S-Intermediate and G-Intermediate treatments while more internal excreta were found in the L-Intermediate treatment. Offspring biomass was positively associated with internal excreta in the S (soil only) and G (soil + grass) treatments. We conclude that P. corethrurus shows parental care when selecting sites for its offspring in the form of nest construction and excreta deposition. Further research is needed on the ecological conditions that favour the evolution of parental care in earthworms according to their ecological category (anecic, endogeic and anecic).