Seasonal dynamics of trematode infection in the first and the second intermediate hosts: A long-term study at the subarctic marine intertidal

The purpose of this study was to reveal seasonal reorganizations in groups of parthenitae (rediae or daughter sporocysts) of trematodes in the first intermediate hosts (1IH) and of their larvae (metacercariae) in the second intermediate hosts (2IH) in the nearshore ecosystems of the White Sea. The study involved two trematode species parasitizing seabirds, Himasthla littorinae (Himasthlidae) and Cercaria parvicaudata (Renicolidae), and was based on long-term seasonal monitoring of populations of 1IH (snails Littorina spp.) and 2IH (mussels Mytilus edulis) at two intertidal sites (66°N). We discovered that groups of H. littorinae rediae in the molluscan host were self-sustaining and could function for more than one warm season. Groups of daughter sporocysts of C. parvicaudata caused the death of the molluscan host in the course of a year. Parthenitae groups of both trematode species spend the cold season (7–8 months a year) in the state of developmental arrest. After the water warms up in spring, the surviving mature groups of sporocysts of C. parvicaudata start to emit cercariae, but their molluscan hosts die soon. Most of the mature rediae in groups of H. elongata die in autumn-winter; in spring young overwintered rediae mature and produce cercariae over the next warm period. The maximum prevalence of mature parthenitae groups of H. littorinae and C. parvicaudata in periwinkles was observed in the warmest period, July-August, when they produced and emitted numerous cercariae ensuring mass infection of mussels. Though metacercariae occurred in mussels all year round, the mean abundance usually reached the highest values in summer–autumn. In winter-spring the level of infection decreased because heavily infected molluscs died out and new molluscs were not infected as cercarial emergence from 1IH had stopped. We discuss the components of the transmission success of H. littorinae and C. parvicaudata in the Subarctic and suggest that the parasites’ transmission may intensify owing to the prolongation of the period of functional activity of parthenitae groups under conditions of a warming climate. [Display omitted] •At the White Sea intertidal molluscan hosts are infected with trematodes year-round.•Composition of rediae and daughter sporocyst groups had different seasonal dynamics.•Peaks of parthenitae groups activity and host infection with metacercariae coincided.•Maximal mussel infection with metacercariae was timed for seabirds autumn migration.•Climate wa