Abstract
The American brine shrimp Artemia franciscana is invasive in the Mediterranean region where it has displaced native species (the sexual A. salina, and the clonal A. parthenogenetica) from many salt pond complexes. Artemia populations are parasitized by numerous avian cestodes whose effects have been studied in native species. We present a study from the Ebro Delta salterns (NE Spain), in a salt pond where both A. franciscana and native A. salina populations coexist, providing a unique opportunity to compare the parasite loads of the two sexual species in syntopy. The native species had consistently higher infection parameters, largely because the dominant cestode in A. salina adults and juveniles (Flamingolepis liguloides) was much rarer in A. franciscana. The most abundant cestodes in the alien species were Eurycestus avoceti (in adults) and Flamingolepis flamingo (in juveniles). The abundance of E. avoceti and F. liguloides was higher in the A. franciscana population syntopic with A. salina than in a population sampled at the same time in another pond where the native brine shrimp was absent, possibly because the native shrimp provides a better reservoir for parasite circulation. Infection by cestodes caused red colouration in adult and juvenile A. salina, and also led to castration in a high proportion of adult females. Both these effects were significantly stronger in the native host than in A. franciscana with the same parasite loads. However, for the first time, significant castration effects (for E. avoceti and F. liguloides) and colour change (for six cestode species) were observed in infected A. franciscana. Avian cestodes are likely to help A. franciscana outcompete native species. At the same time, they are likely to reduce the production of A. franciscana cysts in areas where they are harvested commercially.