The ecology of Diplostomum phoxini infections in two minnow (Phoxinus phoxinus) populations in Scotland

Data are presented on the infection dynamics of the brain-dwelling metacercariae of Diplostomum phoxini (Digenea: Trematoda) parasitizing European minnow (Phoxinus phoxinus) populations in a lowland river (River Endrick) and a highland loch (Loch Maragan) in central Scotland. Prevalence of the parasite approached 100% in all samples taken from both populations over a 12-month period, and within each population the intensity of infection increased with increasing host fork length. However, the two relationships differed, and size-matched minnows from the Endrick exhibited significantly higher intensities than those from Loch Maragan, suggesting that metacercarial acquisition occurred at different rates in the two populations. Data regarding seasonal trends in the acquisition of D. phoxini indicated that fish in both populations become infected with the parasites mainly during the spring and summer, with negligible infection occurring during winter months. Analysis of size-matched fish from individual samples revealed apparent differences in the distribution of D. phoxini within the two host populations. Whereas D. phoxini metacercariae appeared to be overdispersed amongst length-matched fish from Loch Maragan samples, the parasite was distributed normally amongst fish from the Endrick. Possible reasons for the observed differences in the infection characteristics of D. phoxini in the two minnow populations are discussed, with reference to the parasite's transmission dynamics and local environmental conditions.