Does cooling affect the lipid composition of parasite free-living stages?

Temperature influences physiology and lipid composition of poikilotherms, but its effects on lipids of free-living parasite stages remain poorly understood. Free-living stages of trematodes (cercariae) can play an important role in the nutrition of aquatic organisms due to their high abundance and widespread distribution. Cercariae may experience short-term cooling in nature and during laboratory collection, yet it is unknown whether their lipid composition changes in response to low temperatures. We investigated how cooling (~ 4 h) affects lipids of cercariae of Diplostomum pseudospathaceum, a common freshwater trematode. Cercariae were either cooled (4-7 °C) or uncooled during collection and then analyzed using high-performance thin-layer chromatography and gas chromatography. Contrary to our hypotheses, cooling did not increase the levels of total lipids and polyunsaturated fatty acids. This suggests that cercariae lack the lipid adjustments to low temperatures for maintaining optimal membrane fluidity that characterize many other organisms. However, cold treatment showed some influence on lipid composition, interacting with age of cercariae and number of snails releasing them. Essential fatty acids accounted for 13% of all fatty acids in D. pseudospathaceum cercariae, which is lower than in some other trematodes and zooplankters. Unlike other trematodes, D. pseudospathaceum showed high levels of triacylglycerols and waxes in cercariae (10% and 18% of total lipids, respectively). These results contribute to understanding the potential nutritional role of cercariae in aquatic food webs. Additionally, we provide some of the first data on cercarial dry weight and methodological recommendations for their sampling, facilitating future research on these understudied organisms.