This study examines the distribution and depletion of stored lipids in Gyrodactylus gasterostei Gläser, 1974, migrating off its three-spine stickleback host Gasterosteus aculeatus L., with the prospect that it might prove informative for interpreting the biology of other gyrodactylids species more generally. Nonfeeding life cycle stages, such as the dispersal stages of parasites, are dependent for survival upon finite energy reserves gathered during feeding phases. Thus, those individuals with more limited reserves will die sooner and consequently have less time available to find a new host once detached. At this stage, the principal energy reserves in gyrodactylids are stored as large lipid droplets. Confocal laser scanning microscopy has been used to investigate the distribution of lipid droplets in Gyrodactylus, which have migrated off their fish host, testing the hypothesis that these droplets function as a proxy for the nutritional state. This study demonstrated that the lipid droplets were particularly associated with the gut and that there is a significant variability in the volume of stored lipid carried out by each individual. Transmission electron microscopy showed that gyrodactylids carry lipid droplets at all stages of their life cycle, including at release from the birth pore. It is likely that transferring worms requires stored energy reserves to survive in the event of failure to establish contact with a new host. These reserves could allow the parasite to survive without a host for several days.
Keywords: Ectoparasites; Gasterosteus aculeatus; Monogenean; microanalysis; microscopy.