This postdoctoral project aims at getting insight in the role parasites play in modifying trophic interactions and ecosystem properties.
The fact that parasites can have pronounced effects on their host population is now well established. How this may influence ecosystem properties is however less clear. The importance of predation for community structure and ecosystem productivity through trophic cascades has been extensively investigated. Acknowledging that parasites can have similar effects, comparisons between predator and parasite induced changes in trophic structure has rarely been studied in aquatic ecosystems. The potential of parasite driven cascades depends not only on the parasite-host interaction but also on the parasite-predator interaction. Parasites compete for the same resources as predators and the one who is most competitive will exert the strongest top-down control of the prey population. However, parasites may be at the same time vulnerable to predation when infected hosts are more frequently predated on, i.e. intraguild predation.
A temporal pond system, situated in the region of the Humedales del Este (Eastern Wetlands) in Uruguay, offers an interesting opportunity to study the effect of parasitism on trophic interactions. A peculiarity of these ponds is that some of them contain small killifish (main predators of zooplankton) while others are fishless. This offers the possibility for a natural field experiment where we can quantify if the absence or presence of fish predation affects levels of parasite prevalence in zooplankton and how this may affect the lower trophic level, i.e. phytoplankton.
From previous studies there is knowledge about the distribution of the killifish and their effect on zooplankton community structure. In contrast, nothing is known yet about the zooplankton parasite community. Therefore the first big challenge of the project is of course to identify zooplankton parasites (endoparasites and epibionts) in this system. What makes it even more challenging is the fact that very little (if nothing) is known about subtropical zooplankton endoparasites. Most of our knowledge on zooplankton parasites comes from European and North American temperate lakes and ponds. Therefore this project is also a pioneering study on zooplankton parasitism in subtropical freshwaters.
During a first field survey I observed a quite diverse parasite community. For the screening of endoparasites I concentrated on the larger cladocera species since parasitism and body size is often positively correlated. Based on spore morphology, most of the endoparasites seem to belong to the large group of Microsporidia parasites. Some of them are found in different cladocera species whereas others seem to be host specific, occurring only in one cladocera species. Epibionts were mainly found on cyclopoids. Preliminary qualitative observation indicates a clear effect of fish, with higher epibiont load in the absence of fish.
Endoparasites observed in cladocera species of subtropical temporary ponds:
All pictures are taken with an inverted light microscope. Click picture to enlarge.
Parasite 1 (Microsporidia): infects the carapax of Simocephalus sp.
Parasite 2 (Microsporidia): location of infection unknown, spores look similar to the spores of Parasite 1
Parasite 3: These large spores are found with high prevalence and often high intensity in the gut of Simocephalus sp., Daphnia sp., Diaphanosoma sp. The highest concentration of spores is often found in the hindgut but spores are also present throughout the whole gut. They always occur as single spores.
Parasite 4 (Microsporidia): infects the gut epithelium of Simocephalus sp. and Diaphanosoma sp.
Parasite 5 (Microsporidia): infects the gut epithelium of Daphnia sp.
Parasite 6 (Microsporidia): Infects the hypodermis of Simocephalus sp.
Parasite 7 (Microsporidia): Infects fat body in Simocephalus sp.
Parasite 8: The spore clusters of this parasite look very much like Caullerya mesnili, a unicellular parasite belonging to the Ichthyosporea that infects the gut epithelium of Daphnia sp.