Despite growing interest in ecological interactions between predators and pathogens, few studies have experimentally examined the consequences of infection for host predation risk or how environmental conditions affect this relationship. Here we combined mesocosm experiments, in situ foraging data, and broad-scale lake surveys to evaluate (1) the effects of chytrid infection (Polycaryum laeve) on susceptibility of Daphnia to fish predators and (2) how environmental characteristics moderate the strength of this interaction. In mesocosms, bluegill preferred infected Daphnia 2-5 times over uninfected individuals. Among infected Daphnia, infection intensity was a positive predictor of predation risk, whereas carapace size and fecundity increased predation on uninfected individuals. Wild-caught yellow perch and bluegill from in situ foraging trials exhibited strong selectivity for infected Daphnia (3-10 times over uninfected individuals). In mesocosms containing water high in dissolved organic carbon (DOC), however, selective predation on infected Daphnia was eliminated. Correspondingly, lakes that supported chytrid infections had higher DOC levels and lower light penetration. Our results emphasize the strength of interactions between parasitism and predation while highlighting the moderating influence of water color. P. laeve increases the conspicuousness and predation risk of Daphnia; as a result, infected Daphnia occur predominantly in environments with characteristics that conceal their elevated visibility.

SUMMARY1. We combined ecological surveys, life table studies, microscopy and molecular sequencing to determine the development, ecology, pathology and phylogeny of Polycaryum laeve, an endoparasite of cladocerans. We report the first records of P. laeve from North America, where we have used a polymerase chain reaction primer and microscopic examination to confirm infections in 14 lakes. Infections are highly pathogenic and caused increased mortality, reduced growth, and reproductive castration in Daphnia pulicaria during life table studies. 2. Biweekly data from Allequash Lake (Wisconsin, U.S.A.) throughout 2003 indicated that fecundity and infection prevalence were inversely correlated. Infection prevalence was highest in late winter and early spring (up to 80%) and lowest during late summer. Epidemics were generally followed by sharp declines in host population density (up to 99%). 3. Within the haemocoel of its host, P. laeve forms thick-walled sporangia, which occur systemically in later stages of infection. Fungal thalli associate closely with muscle fibres and connective tissue, leading to degeneration as the infection becomes advanced. Following death of the host, flagellated zoospores are released through an exit papilla on the sporangium. Based on the infection-induced castration of the host and increases in infection prevalence with Daphnia size, we postulate that transmission is horizontal, but may be indirect through an additional host or free-living stage. 4. Molecular and morphological data clearly indicate that P. laeve belongs in the fungal phylum Chytriodiomycota, order Blastocladiales. Based on ribosomal RNA gene sequences and morphological features, we transfer the genus Polycaryum from the Haplosporidia to the Chytridiomycota, and designate a lectotype and epitype for P. laeve. Considering the high prevalence of P. laeve infection within Daphnia populations, the frequency with which we detected infections among lakes, and the keystone importance of large-bodied Daphnia in aquatic food webs, we suggest that P. laeve may exert a regulatory influence on Daphnia populations in lake ecosystems.

We explored the importance of interactions between parasite infection and predation in driving an emerging phenomenon of conservation importance: amphibian limb malformations. We suggest that injury resulting from intraspecific predation in combination with trematode infection contributes to the frequency and severity of malformations in salamanders. By integrating field surveys and experiments, we evaluated the individual and combined effects of conspecific attack and parasite (Ribeiroia ondatrae) infection on limb development of long-toed salamanders (Ambystoma macrodactylum). In the absence of Ribeiroia, abnormalities involved missing digits, feet, or limbs and were similar to those produced by cannibalistic attack in experimental trials. At field sites that supported Ribeiroia, malformations were dominated by extra limbs and digits. Correspondingly, laboratory exposure of larval salamanders to Ribeiroia cercariae over a 30-day period induced high frequencies of malformations, including extra digits, extra limbs, cutaneous fusion, and micromelia. However, salamander limbs exposed to both injury and infection exhibited 3-5 times more abnormalities than those exposed to either factor alone. Infection also caused significant delays in limb regeneration and time-to-metamorphosis. Taken together, these results help to explain malformation patterns observed in natural salamander populations while emphasizing the importance of interactions between parasitism and predation in driving disease.

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