Small but diverse: larval trematode communities in the small freshwater planorbids Gyraulus albus and Segmentina nitida (Gastropoda: Pulmonata) from the Ruhr River, Germany.

In contrast to the well-studied trematode fauna of lymnaeid snails, only little is known about the role of small planorbid snails as first intermediate hosts for trematodes in temperate freshwater systems. This study aims at closing this gap by assessing the diversity and composition of larval trematode communities in Gyraulus albus and Segmentina nitida in a Central European reservoir system, and by providing an updated comprehensive review of the published trematode records of these snail hosts. A total of 3691 planorbid snails (3270 G. albus; 421 S. nitida) was collected in three consecutive years from four reservoirs of the River Ruhr catchment area in Germany. Gyraulus albus showed a higher overall trematode prevalence (11.7%) and more diverse trematode fauna (12 species) compared to S. nitida, which harboured three species and showed a lower trematode prevalence (1.7%). Altogether, 13 trematode species belonging to four families were identified in both hosts. Seven trematode species encountered in this study represent novel records for these hosts, and/or constitute first records of these larval stages from Germany. Trematode component communities in G. albus were stable across seasons and years, indicating excellent conditions for trematodes in this snail host and the continuous presence of the final hosts of the most dominant trematode species. Overall, this study reveals the importance of small planorbid snails, in particular G. albus, as first intermediate hosts for a species-rich trematode fauna in European freshwater systems, and highlights the parasites' contribution to the ecosystem's biodiversity.

Parasites in a man-made landscape: contrasting patterns of trematode flow in a fishpond area in Central Europe.

We have explored a large body of novel data focusing on small-scale temporal and spatial patterns in the composition and structure of larval trematode communities in Lymnaea stagnalis (L.) from a typical Central European agricultural landscape. The 5 eutrophic fishponds studied provide excellent environments for the development of species-rich and abundant trematode communities. Nine prevalent species were consistently present in component communities, but had differential contribution to the parasite flow in the 5 ponds resulting in significant contrasting patterns of community similarity and the prevalence of the 3 major transmission guilds driving this similarity. Component communities split into 2 groups: (i) those from the large pond dominated by anatid and larid generalists with active miracidial transmission; and (ii) those from the smaller ponds dominated by 2 plagiorchioideans infecting snails via egg ingestion. We put forward 3 hypotheses for the remarkable differences in larval trematode flow in the similar and closely located eutrophic ponds: (i) species-specific differences in parasite colonization potential displayed by an 'active-passive' dichotomy in miracidial transmission strategies of the species; (ii) top-down effects of pond context on transmission pathways of the trematodes; and (iii) competition as an important mechanism in eutrophic environments with a bottom-up effect on component community structure.