The influence of Monocystis sp. infection on growth and mating behaviour of the earthworm Lumbricus terrestris

Parasites are capable of behavioural manipulation of their hosts in an effort to complete their life cycle and thus increase their fitness at a cost to the host. Hosts are therefore expected to exhibit mate choice decisions that serve to minimize either their own infection risk or that of their offspring using biological cues as criteria. Skin colour could be one such cue. Lumbricus terrestris is heavily parasitized by the protozoan Monocystis sp.; although infection levels can be highly variable, field samples containing 100% infected individuals are common. We observed earthworms, in pairs, for 4 weeks and recorded growth, copulation number, skin colour, and parasite load. The following predictions were made: (i) growth and copulation frequency should decrease with parasite load and (ii) parasite load correlates with skin colour. Contrary to our expectation, neither growth rate nor copulation number correlated with parasite load, yet skin colour did correlate significantly with parasite load. Mature and subadult individuals were also found to differ significantly in degree of infection. These findings suggest a parasite with a low short-term fitness effect on its host.