Egg grooming in Vallonia snails – anti-parasite defence or trophic behaviour?

During laboratory studies on the reproductive biology of Vallonia pulchella and V. costata we observed an unusual behaviour in adults and juveniles of both species. The snails moved with their heads and radulae over the egg surface as if feeding. The examination of the egg shell revealed the presence of the fungus Arthrobotrys oligospora, which commonly occurs in the soil and has multiple lifestyles: it is known as a nematode pathogen, a saprophyte and a coloniser of plant roots. The extraordinary behaviour of Vallonia may be associated with the presence of mycelium on the egg surface. We hypothesise three possible explanations of egg grooming: (1) parental care, (2) trophic behaviour and (3) the infection can constitute a means of defence against nematodes, which are known to be predators of snails and slugs as well as their eggs. The removal of the fungus is not associated with overcrowding, but may be a selective advantage when combined with feeding.