Metacercarial aggregation in Digenea (Fasciola hepatica and Paramphistomum daubneyi): environmental or species determinism?

Metacercarial aggregation of Fasciola hepatica and Paramphistomum daubneyi was studied under experimental conditions to determine if the formation of these aggregates was influenced by environmental factors, or it was a characteristic of trematode species. This process was studied using the confinement of infected snails on the bottom of Petri dishes (diameter, 14 cm) for 3 days. The formation of metacercarial aggregates of F. hepatica was not significantly modified by environmental factors such as intensity and duration of lighting, quality and volume of water. Metacercariae of F. hepatica were more numerous on the Petri dish walls and 63.9% of them constituted aggregates. In contrast, most metacercariae of P. daubneyi were found on the Petri dish bottoms and 78.3% of them were isolated or in groups of two metacercariae each. The mean number of metacercariae per aggregate ranged from 6.7 to 12.2 in the case of F. hepatica, and from 2.7 to 4.5 in the case of P. daubneyi. However, these mean numbers were independent of the site of cercarial attachment. The tendency of cercariae to form metacercarial aggregations was a characteristic of F. hepatica and was species determined.