Substratum preferences in two notocotylid (Digenea, Notocotylidae) cercariae from Hydrobia ventrosa at the White Sea

Abstract

Notocotylids are digeneans with a two-host life cycle. Their cercariae encyst on underwater substrata, and final hosts (mostly water birds) get infected by consuming encysted metacercariae. The aim of this study was to assess whether notocotylid cercariae have encystment substratum preferences. We used Cercaria Notocotylidae sp. No 11 Deblock, 1980 (presumably Paramonostomum alveatum Mehlis 1846) and C. Notocotylidae sp. No 12 Deblock, 1980 associated with mudsnails Hydrobia ventrosa at the White Sea. Three series of experiments were performed in which distribution of cysts across different combinations of substrata was measured. The results suggest that C. Notocotylidae sp. No 11 cercariae encyst almost exclusively on the leaves of green plants without any plant species preferences. C. Notocotylidae sp. No 12 cercariae use shells of living molluscs and plant substrata equally often but avoid empty shells. These preferences are probably adapted to the feeding habits of the final hosts and this may enhance the transmission of the studied notocotylid species. Mechanisms of the observed preferences might be associated with the ability of cercariae to recognize substrata features: either chemical cues or surface structure. Substratum selectivity in cercariae is considered as a first step towards downward incorporation of the second intermediate host into the life cycle.