A study was initiated to identify trematode patterns in Cyathura from two key sites in the Mondego Estuary: a Zostera-bed and a sand flat. The two subpopulations of Cyathura differ, as the annual recruitment success is much higher and more regular on the sandflat than on the Zostera-bed.

Counting and sizing of trematode cysts inside the body of preserved Cyathura specimens enabled a description of trematode patterns in space and time. Further identification of trematodes was based on DNA studies of unpreserved cysts as well as on identification of parasites in co-occurring mud snails. Two trematode species dominated in Cyathura: Maritrema subdolum (cysts around 190 μm) and a hitherto unknown Levinseniella species (cysts around 340 μm), the latter being the most frequent one. Generally, the prevalence of both species peaked during winter months, when migratory water birds occur in the estuary. Cyathura from the Zostera bed harboured more infections per specimen than those from the sand flat. A much higher density of mud snails ≥52 mm (which can be host to microphallids) and a low abundance of Cyathura are thought to be the main reasons for this pattern. Field data did indicate a host size-dependent maximum number of cysts in Cyathura that could be a result of enhanced mortality at high cyst intensities. Furthermore, parasites were underrepresented among ovigerous Cyathura specimens, suggesting a negative impact on gametogenesis. As a consequence, microphallid trematodes may be a critical factor controlling recruitment strength in Cyathura, especially at the Zostera-site.