Effect of Short-Term Temperature Change on Cercarial Release by Rhipidocotyle fennica (Trematoda, Bucephalidae) from the Freshwater Bivalve Host, Anodonta anatina

Cercarial release from the first intermediate host is an important stage in the transmission of trematode parasites. Besides long-term (seasonal) temperature fluctuations, short-term temperature changes can also influence cercarial emergence. We tested the response of the bucephalid trematode, Rhipidocotyle fennica (R. fennica), acclimatized to 17 °C, to an abrupt temperature change. As the natural cercarial shedding by this parasite takes place annually during the warmest season, we expected a positive effect of temperature increase. Monitoring during one hour after the transfer from 17 °C to 20 °C revealed a significant increase in R. fennica cercarial release compared to the preceding one hour period. In contrast, cercarial release decreased in clams transferred to 14 °C, while no change was observed in control clams transferred from 17 °C to 17 °C. This shows that the cercarial release by R. fennica is sensitive to short-term temperature change, and, as predicted, responds positively to warming and negatively to cooling. The result emphasizes the importance of (i) temperature on the cercarial production of trematodes, and (ii) the need to carefully control temperature conditions when studying factors influencing the cercarial production of trematodes.