Do Leucochloridium sporocysts manipulate the behaviour of their snail hosts?

The conspicuous broodsacs of Leucochloridium spp. sporocysts, invading tentacles of their intermediate terrestrial snail hosts, are presented as a classic textbook example of the manipulation of host behaviour by a parasite. However, the conspicuous features indicated as facilitating the transmission of the parasite to its final avian hosts are characteristics of the appearance and behaviour of the parasite and not of its intermediate hosts. The demonstration that the sporocysts also manipulate the behaviour of the snails is still largely missing. In order to find out whether Leucochloridium paradoxum could manipulate the behaviour of its Succinea putris hosts, we compared the behaviour of Leucochloridium‐infected snails with that of control animals (showing no signs of infection) living side by side, in the same habitat patches, in the field (Białowieża National Park, Poland). We had assumed that the ‘moving caterpillar’ display of the broodsacs was addressed to day‐active, visually hunting, insectivorous birds and that the ‘signalling’ parasites should change the behaviour of their hosts to make the broodsacs more visible and/or more accessible to the group of predators mentioned. The infected snails with pulsating broodsacs behaved differently from their apparently non‐infected counterparts. They moved farther, positioned themselves in more exposed and better illuminated places, situated higher in the vegetation. These alterations of behaviour would be beneficial for the parasites, would increase their visibility (detectability) and accessibility to the potential definite hosts. Thus, we demonstrated that, apart from their own phenotypic modifications, L. paradoxum sporocysts also changed the behaviour of their intermediate S. putris hosts. Such combination of modified host behaviour and strikingly visible parasite behaviour is rather unique, it is likely increasing the likelihood of parasite transmission to avian hosts.