Discussion
Our study confirmed that P. laevis cystacanth size was influenced by intraspecific competition. This is consistent with previous studies on crowding effects on acanthocephalan size [8,14,32]. In particular, Cornet [32] found similar results when co-infecting P. laevis were unrelated (i.e. originated from different clutches). Interestingly, our results indicate that the size achieved by the largest cystacanth is always the same, regardless of the number of competitors. All cystacanths sharing a host may thus not equally suffer from competition. As a consequence, cystacanth size is more variable in hosts harbouring three or more cystacanths. Competition between cystacanths seemed to act on both cystacanth size variability and mean cystacanth size.