- University of Guelph
Populations of invasive species tend to have fewer parasites in their introduced ranges than in their native ranges and are also thought to have fewer parasites than native prey. This ‘release’ from parasites has unstudied implications for native predators feeding on exotic prey. In particular, shifts from native to exotic prey should reduce levels of trophically transmitted parasites. We tested this hypothesis in native populations of pumpkinseed sunfish (Lepomis gibbosus) in Lake Opinicon, where fish stomach contents were studied intensively in the 1970s, prior to the appearance of exotic zebra mussels (Dreissena polymorpha) in the mid-1990s. Zebra mussels were common in stomachs of present-day pumpkinseeds, and stable isotopes of carbon and nitrogen confirmed their importance in long-term diets. Because historical parasite data were not available in Lake Opinicon, we also surveyed stomach contents and parasites in pumpkinseed in both Lake Opinicon and an ecologically similar, neighboring lake where zebra mussels were absent. Stomach contents of pumpkinseed in the companion lake did not differ from those of pre-invasion fish from Lake Opinicon. The companion lake, therefore, served as a surrogate “pre-invasion” reference to assess effects of zebra mussel consumption on parasites in pumpkinseed. Trophically transmitted parasites were less species-rich and abundant in Lake Opinicon, where fish fed on zebra mussels, although factors other than zebra mussel consumption may contribute to these differences. Predation on zebra mussels has clearly contributed to a novel trophic coupling between littoral and pelagic food webs in Lake Opinicon.
Between 28 July and 9 August 2010, we collected 99 adult (total length ≥110 mm) L. gibbosus at five locations in Lake Opinicon (n = 64) and four localities in Rock Lake (n = 35), Ontario, Canada (Fig. 1). Lake Opinicon was chosen because of Keast’s (1978) study of L. gibbosus diets prior to invasion of zebra mussels, which were first observed in the early to mid-1990s. In 2010, zebra mussels had not been observed in Rock Lake, which is bathymetrically similar to Lake Opinicon, and has a similar fish community. Rock Lake and Lake Opinicon are expected to have similar parasite communities because they are separated by only 3.5 km (Fellis and Esch 2005; Locke et al. 2012, 2013b).
Parasite and stomach content examination
Upon capture, fish were euthanized by cerebral percussion and kept on ice or in a 4 °C refrigerator for 6–24 h until dissection. We examined the heart, liver, kidneys, gonads, gastro-intestinal tract, and urinary bladder for the presence of trophically transmitted parasites. All parasites were collected and stored in 70 % ethanol, stained in acetocarmine (acanthocephalans, cestodes, digeneans), or cleared in glycerol (nematodes), mounted on slides, and identified to the lowest possible taxonomic level.
Diet items in the gastro-intestinal tract were also preserved in 70 % ethanol. Prey were counted and identified to the lowest taxonomic level. Only singular structures (e.g., head of insects and crustaceans, spire or operculum of snails, left septum of zebra mussels) were used in counts of non-intact prey.