Effects of Chaoborus predation on size of Pasteuria ramosa epidemics in natural Daphnia populations

Background/Question/Methods

Parasites and predators drive population and evolutionary dynamics of their hosts and prey. The ecologically important zooplankter Daphnia are prey to fish and invertebrate predators, as well as hosts to numerous microparasites including the bacterial parasite, Pasteuria ramosa. Daphnia acquire parasites by ingesting infectious spores. The parasite grows within the host and bursts from the host at host death.  Selective predators can reduce transmission of parasites by removing infectious individuals from the environment. In contrast, invertebrate predators in the Chaoborus genus have been shown to increase transmission of Metschnikowia bicuspidata by regurgitating Daphnia carapaces, releasing spores from infected animals in to the water column (Cáceres et al. 2009). P. ramosa transmission could show the same trend, however, it is a slow-growing parasite, and predation may instead decrease its transmission due to fewer spores formed before host death (Auld et al. In review).  Our work asks: does the density of Chaoborus have an effect on epidemic sizes of P. ramosa in Daphnia?  To answer this question we analyzed field data collected from July-November 2013 in which fifteen Michigan lakes were sampled every two weeks.  Chaoborus and Daphnia species were identified and counted, and infected animals were identified by parasite.

Results/Conclusions

Analysis of field data showed that lakes differ in their mean Chaoborus densities (from 1 to 65 Chaoborus per three tows using a 180mm mouth diameter Wisconsin net). The date of peak prevalence of Chaoborus also differed between the lakes from July 22nd to September 19th. Dates of peak prevalence of Chaoborus tended to coincide with the start dates of epidemics of P. ramosa. In the eight lakes where epidemics occurred, four showed the start of a P. ramosa epidemic at the next sampling date.  Analyses of drivers of overall infection levels (which are likely to be influenced by multiple factors, including other predators, habitat structure, and other parasites) are currently ongoing.  Final results may indicate an overall relationship between Chaoborus and P. ramosa.