Size-dependent pattern of metacercariae accumulation in Macomona liliana: the threshold for infection in a dead-end host.

While bivalves can acquire trematode metacercariae over their lifetime, the rate at which this accumulation takes place is not necessarily linear. The present study found that the bivalve Macomona liliana acquires very few or no metacercariae until it reaches 30 mm in size, but thereafter the rate at which it becomes infected increases exponentially. It is likely that this ontogenetic change in infection rate is associated with the increased filtration capacity and siphon diameter of larger M. liliana. The echinostome metacercariae that infect M. liliana also infect a much more common sympatric bivalve, Austrovenus stutchburyi, in which they achieve much higher infection intensity. Due to its deeper burying depth, M. liliana most likely represents a dead-end host for the echinostomes: potential definitive hosts preferentially feed upon A. stutchburyi as they are located closer to the sediment surface than M. liliana. However, due to the low infection intensity and population density of M. liliana, its overall impact as a sink for echinostome populations in the ecosystem is probably negligible.